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October 27, 2014

Dr. Mark's Power Supply and Battery Web Log (Blog)

Friday, 28-Nov-2014 23:44:57 EST
Click Here for the Dr. Mark Blog Archives!

Quality versus Price Dear PowerStream,

I've been researching the reliability of small 5V "wall wart" type power supplies for USB and other applications. In the process I was searching for wire current carrying capacity and found your "Wire Gauge and Current Limits" page. What a helpful page, thanks!

After perusing that I looked around your site further. I have a couple of comments which will no doubt >not< be news to you. These things (small 5V supplies) can be found on Ebay or Amazon very cheaply, and comparatively make your offerings seem expensive. For instance I just bought a product similar to your model PST-AC0520W for $5.29, delivered to my home from a US supplier!
But the reliability of these cheapo supplies is notoriously bad, which is NOT apparent to non-technical buyers. They are known to use electrolytic capacitors which are underrated for the application's ripple current. The capacitors get hot, go bad, and cause the power supplies to have a short lifetime.

The other thing that happens is that the manufacturers out-and-out LIE about the specifications. My cheapo unit is labeled "5V, 2A," but shuts down if you try to draw more than 1.4A from it. And its output cord has undersized 24 AWG wires in it (which led me to your site).

So all of this long-winded discussion leads to a suggestion: on your products' web pages compare the performance and reliability of the cheapo units to yours, and make it clear to potential buyers that they are well worth the few extra dollars. Many many non-technical people have been screwed by your unscrupulous competition.
Regards, Curt

A: Thanks for your analysis, Curt. We try to sort the good from bad suppliers for our customers. It might be fun to make a "tear down" video comparing cheap versus quality power supplies. I'll put it on my to-do list.
--mark
SAM Labs kickstarter photoA KickStarter project worth supporting PowerStream has had fun supporting the SAM project with batteries and advice. You are going to want one of these kits of internet connected gadgets that can be joined wirelessly into any gadget you can conceive. The funding ends on about 29 October 2014
https://www.kickstarter.com/projects/1842650056/sam-the-ultimate-internet-connected-electronics-ki
Low temperature batteries Q: Hi Powerstream people, I have a customer looking for a battery solution for an application in Canada that could get to -20C in the winter. He wants it to solar charge but be able to go at least a week without charging. Is there a lithium-ion battery pack that could work? Thanks, Ed

A: Hi Ed, There is a fundamental problem, lithium ion batteries cannot be charged at less than zero degrees C (some say -10C). At low temperatures the crystal channels get clogged by lithium metal and it kills the batteries.

The best battery for this application is an a appropriately sized sealed lead acid battery. The sulfuric acid in a fully charged battery freezes at about -65 C, and for a half charged battery at about -17C. So if you keep the charge level above 60% the freezing temperature is below about 24 C. As you can calculate yourself, 7 amp hours divided by .6 is 11.7AH, so anything bigger than that is safe as long as the solar panel can recharge it.
Charging a lithium ion cell with a solar cell Q: Hi, I have been reading up on lipo charging.

From what I gather, I see it is charged at a constant current then at some point a constant voltage is put on and the charging is cut off when the current has diminished to about 5% c. ( depending ).

What if I don't mind never fully charging the cell. I directly put on a solar cell array which can not provide more than 1C current and can never push a higher potential than 4.05 volts. Surely this will charge the cell correct ? Slow, but surely ? Or is there some essential reason why a constant current should be provided at the start of the charging sequence ? -Albert.

A: Hi Albert, Yes, you can do that and it will work well. But you will have to live with the fact that the battery will never get to full charge. Here is the explanation including original research we did on the subject http://www.powerstream.com/lithuim-ion-charge-voltage.htm

The "constant current" means that the charger can't put out more than that current, so the voltage is lowered to make sure that no more is drawn. As the battery fills up the voltage raises until it gets to the maximum voltage the battery can take, and then holds at that voltage while the current drops.
best regards
mark
Charging lithium ion cells (and lithium polymer cells) in parallel Q: I have a question about putting two Li-Poly batteries in parallel. I have a 33 x 27 x 8mm space for as much battery capacity as I can get, and stacking two GM382530-PCB batteries in parallel would be perfect. Besides needing to bring them to same potential before initially connecting them in parallel, is there any danger or does it harm the batteries by charging and discharging them while connected in parallel? Thanks! Matt

A: Lithium batteries are unusual in that they like to be charged in parallel, and they don't even have to be at the same charge state or even the same size cell. They are very picky about being put in series, but parallel is OK.
A very audacious fake Ultrafire lithium ion 18650 Here is the youtube video, but the executive summary is that the manufacturer put a 66maH electric cigarette battery in an 18650 package, and filled the rest of the cell with flour.
a fake panasonic 18650 battery https://www.youtube.com/watch?feature=player_embedded&v=eOshOXcSkDA

The difference between a battery charger and a power supply.

There is a problem that a battery charger has to solve that a power supply does not. When you apply 4.6 volts to a 4V lead acid battery it is impossible to know how much current the battery will suck up. A 1000AH battery one would probably take 100s of amps if it was fully discharged. So a battery charger has a "constant current" circuit that cuts back the voltage until the battery is asking for exactly what the charger wants to supply. Then, as the battery fills up, that voltage is increased so that the current stays constant. Eventually the voltage will top out and then the current slowly drops as the battery fills up. Some benchtop power supplies have the constant current limit feature, and if you are trying to charge an odd-voltage lead acid or lithium battery they are the simplest alternative.

Another bad idea for energy harvesting


scoop generatorscoop generator on car
Here is another one from the same source. The idea is to put a couple of scoops in front of the electric or hybrid car to run a generator to charge the battery. Ignoring the fact that squirrel cage fans work on centrifugal force and therefore don't run backwards by pushing air at them, this is another example of putting more load on the car's motor by adding extra drag on the car. Therefore you are going to lose energy rather than harvest some wasted energy.
A bad idea for energy harvesting We get passed a lot of interesting ideas for alternative or free energy generation. I thought about creating a web page based on these, showing the fallacies, but of course most of these are closely held secrets and the inventor would be upset and probably litigious if we divulged them.

However, I just got an e-mail from an invention promotion company that has public links, so I can discuss it. The invention can be found here http://www.sellidea.com/?id=6239
Here is why this is a bad idea. It is usually useful to ask "where is the energy coming from?" In this case he proposes a generator analogous to a linear motor to be run along elevator or conveyer belts to extract energy in the form of electricity. The conveyer belt is the easiest to debunk. Obviously, the only source of energy in the conveyer belt is the motor that drives it. Any energy extracted from the belt's motion will add to the motor's load, and therefore cost more energy than is generated.

The elevator has to deal with gravity, however, which is more mysterious. There are counter-weights in the elevator that compensate for the weight of the elevator and cables, so for an empty elevator the motor is just overcoming friction, no actual potential energy is being generated when the elevator moves to the top floor. (I know that this is a simplification of what must be a lot of counterweight engineering, but it is true enough). When a load is added to the elevator, there is potential energy stored as the elevator moves up, and lost as the elevator moves down. If there is energy to be extracted from the downward motion, the motor is still in the circuit, and could be used for that purpose if practical. There is no energy available for extraction as the elevator moves up, since any generator added will add to the load of the motor, consuming more energy.
Mystery Battery An antique 6V lead acid battery was recently unearthed in Washington State. The finder is looking for more information about this. Take a look.
Shock voltage Q: Hello, I was just reading how to design battery packs on your website, and I just have a few questions. I'm wanting to build some 7.2volt Nicd packs for RC cars. I've read how to do it, but was just wondering if I could get shocked while doing it? I read that you can short the battery if you touch the + and - wires together, but what if you just held in each hand the positive and negative wires at the same time? Would that complete the circuit and I get shocked? I'm asking because with rc packs, there really isn't any insulation for the battery tabs, because we use discharge trays in racing to always keep the cells matched, and the trays have to touch each cell tab.


A: Engineers use the rule-of-thumb that anything under 60 volts isn't likely to shock you.
Replacement lithium polymer cells. Q: I am in search for a Walkman NW-S202F MP3 player replacement battery. Original is a polymer 116mAh 1-756-686-11 manufactured in 2006.

A: All you need is to find one that will fit into the space of that battery. Here is our list, the ones with "add to cart" are the ones we have in stock. You need to find whether the existing battery has a protection circuit board mounted on the flange where the leads come out. It is a small circuit board under the tape, but outside the battery pouch.
http://www.powerstream.com/li-pol.htm
Video-eye Yes, the man with the video-cam eye is using a PowerStream battery in his eye socket. For more information about the eye, plus borgeye video here is a link:
http://www.physorg.com/news/2011-08-eyeborg-vision-future-video.html
We did donate a few cells to that project. Too wild to resist.
http://eyeborgproject.com/partners/

Don't try this at home!
The patent office does another disservice to the investing public I just got an e-mail invitation to invest in a company based on patent number 7615876. Look it up, it is a fairly straightforward perpetual motion machine which purports to use a gear box to increase the torque and horsepower of a motor before it powers a generator, which then powers the motor and a load.

So the lessons to carry away from this is that the attorney, primary examiner, and possibly the inventor should have known better than to let this slip through. And no investor can depend on the patent office to evaluate the technical merit of anything. I can't fault an inventor for trying to raise money on an patented idea that he sincerely believes in, but I can fault the patent attorney, and especially the patent office, which should have technical education enough to pull the plug on this. Investigative reporter alert!
Excess power consumption in home appliances? Q: Recently I've been a bit concerned about the power consumption of my home appliances. I have a notebook power supply that says: Input: 100-240V~ 1.7A 50-60Hz Output: 19V 3.42A Does this mean that it consumes 187 watts if connected to 110 V and 374 watts if connected to 220 volts? Is it possible for it to consume twice the amount of watts if connected to a 220 current? Thanks.

A: No, the current listed is the highest possible, so that would be at 100 volts. As the voltage goes up the current goes down to keep the watts the same. Also, note that the testing agencies (UL) require that you put the highest possible surge current on the label, so you cannot have any possible chance to calculate the efficiency or power consumption of the device based on the label. best regards mark
At what voltage is a 9V battery dead? I'm trying to find info on when a 9 volt battery is sub par for use. I have a voltage meter, but the problem is that I don't know the cut-off point where a battery loses effectiveness. For instance, in a regular, non-rechargeable, Alkaline type battery, does a reading of 8.9 volts mean it's still has over 75% original capacity? What's the point where battery should be replaced--at what voltage reading? I feel like I keep replacing batteries installed in devices for back-up purposes should power fail, when perhaps they would still do the job at a lower reading, but have no idea what the readings really mean.
Thanks, Terri

Sorry Terri, there is no good answer to this question. First of all, the voltage you measure is in large part due to the chemistry voltage. The battery could have very little charge left and still read high voltage when it is "open circuit." You have to measure the battery under some load to get an idea of its state of charge. The load that you would use would be drawing the same power as the gadget under power. Next, it depends on the device the battery is powering. These transistor radio batteries are usually used to power gadgets that don't take much power. Also, some devices are designed to run over a wide range of voltages, so even if the 9 volt battery is only putting out 5 volts they would still operate. Lastly, 9 volt batteries consist of several cells in series. I have seen them use 6 cells in series, and 5 cells in series. So a 9 volt battery could be nominally 7.5 volts. Using 5 cells gives you less voltage, but the cells are bigger, so they give more run time if the device it is running can use the lower voltage.

The answer to your question can only be "When the device can't operate, then the battery needs to be replaced." I know that this isn't much help in your case, where the batteries are used for emergency purposes, sorry again.
best regards
mark
Why can't you charge a lithium ion battery when it is too cold? Mark, I've read (and heard you say) that a LiPo battery should not be charged at low temps (e.g. -20C). Why is this? Is there a safety reason for this or will it just degrade the capacity? -- Damien

The thermodynamics of the lithium-ion intercalation change below about zero degrees C, and make it favorable to deposit lithium metal onto the graphite. This seals the graphite and kills the battery, and lithium dendrites have the chance to short the cells and cause a fire.
best regards, mark
Time to recharge the battery after starting the car. Hi,
There is a lot of discussion in my group about how many miles you have to drive your car to recharge the battery after starting. Can you shed some light?
Tonk

Dear Tonk,
I can do a back-of-the-envelope estimate. Typical car batteries are rated for cold cranking amps, and the one I use has 800 CCA. So being very conservative lets assume that it takes 800 amps. I have three cars, 22 years old, 9 years old, and 10 years old, they all start in less than 3 seconds, but to be conservative let's assume 10 seconds. So 10 seconds times 800 amps is 8000 amp-seconds (8000 coulombs for you physicists) or 2.2 amp-hours.

Now let's look at the charger. All of my cars have alternators greater than 60 amps, so 2.2AH/60A = 0.03 hours, or 2 minutes to recharge the battery.

More realistically, lets say that it takes 300 amps to start the car for 3 seconds, that would take 0.25 AH, and the recharge time is 15 seconds.

Now the Karman Ghia that I had in college usually took 2 minutes to start and had a 65 amp alternator. It would have taken about 10 minutes to recharge.

best regards
mark
Charging lithium ion in parallel Hello Dr. Mark,
I'm glad I found your web log. I have a question about charging Li-ion and Li-polymer cells in parallel. I have searched for and found others who have opinions about this. Some say absolutely don't do it because the cells won't charge equally. I know that is an issue with cell in series, but does it apply to cells in parallel? Others say it's safe because cells in parallel naturally balance because power will flow from the stronger cell to the weaker cell. Can you shed some light in this?
Thanks
Bruce

Dear Bruce,
The answer is that since lithium ion chemistries are charged with voltage control they can be charged in parallel with no problem. The same with lead acid batteries.

The nickel based chemistries are charged with current control and they don't like to be charged in parallel.

I have discussed this with many top battery designers, and I have seen dozens of designs that charge lithium ion in parallel with no problem, so I have high confidence in this answer. However, it is rare for one battery to have enough voltage to charge another, nature just doesn't work that way. What happens is that the battery with the lowest charge state will take most of the current until their equivalent impedances are equal. Since the lithium chemistries use a specific float charge voltage, if you connect a fully charged battery with an uncharged battery very little current will flow between the two. Then putting this combo on a charger the fully charged battery will accept no current while the uncharged battery is being filled.

best regards
mark
Mysterious Instrument A correspondent sent me pictures of something she found in her parent's attic. Any idea what it is?
Mystery meter
Click on the picture for more views.
Switchmode or Switching? Q; What do the words 'Switchmode mean? My experience with switching power supplies is that they only work well if there is some minimum load.
A: There are various topologies used in switchmode aka switching power supplies. Some need to supply some output current in order for the housekeeping circuitry to work, others have a separate bootstrap circuit to make it operate. The former system is used in ATX computer power supplies, the latter in almost everything else. best regards mark
AAA versus AA Capacity Q: Alkaline AA vs AAA amphours comparison question for your battery guru: For Alkaline batteries what is the comparison between AA and AAA batteries 1.5v in the amp hours output for say the 100ma discharge rate (or any other rate for that matter). We want to know how many AAA 's = 1 AA cell? what is the difference in amphours. thanks

A: Depending on which ones you are comparing an AA has about 2 to 3 times an AAA. http://www.powerstream.com/BET.htm
best regards,
mark
Mystery with battery tender Q: I found your website while trying to solve a problem. You seem very knowledgeable and I'm hoping that you wont mind helping me (a newbie) solve a problem... I can't seem to charge a new 12V 18AH sealed lead acid battery. I thought it would be a simple matter to connect my 750mah Deltran Battery Tender however the voltage quickly runs up to 13.5v and the Battery Tender discontinues output. Tried a second new battery tender, same result. Tried a second sealed lead acid battery (12V 7AH), same result. Hmmpf. So I tried a 1 Amp fixed output charging source. Voltage then quickly ran up to 15v after ten seconds so I disconnected it. If I put a moderate load on either battery the voltage quickly drops to 8v. So they definitely are not charging. What the heck am I doing wrong? Thanks for letting me lean on you. -Mick

A: I can't see that you are doing anything wrong. I always assumed that those battery tenders were like ours http://www.powerstream.com/SLA-12V07.htm
and would charge a battery as well as tend it. As for your fixed voltage source, it won't hurt the batteries to be charged at 15 volts until they are near full or full. Sorry, troubleshooting from this distance is tough! Maybe one of our readers has some insight.

best regards mark
Battery Capacity Q: When a battery’s “Rated Capacity” is 170mAh, does that mean it is capable of delivering 170mA continuously for one hour before the entire battery is discharged?
A: Yes, but with limitations. The rating is measured over a specified time. Usually for lead-acid batteries this time is 20 hours. For lithium-ion it is usually 5 hours. When discharging quickly you always loose some capacity.
This is a very strong factor with lead acid, where if you discharge at a 1C rate you only get 50% of the rated capacity. It is not a big factor in lithium ion, NiCad, and NiMH batteries, unless you are discharging at very high rates (above 1C to5C depending on the design of the battery)
Battery Capacity 2 Q: Is a battery’s “Rated Capacity” linearly proportional? In other words, if a battery’s rated capacity is 170mAh, could it be assumed that it can deliver 85mA for two hours, 42.5mA for four hours, etc.?
A: Yes, with the above limitation. mAH are proportional to coulombs. 1 mAH = 3.6 coulombs. Other than the problem discussed above it is linear. The slower the discharge the more linear it is. If you discharge time is a period of months you will start to run into the self-discharge rate of the battery, so you will again get less than the rated amp-hours.

See our web page " Engineering Design Notes on Battery Powered Devices "
Conservation of Energy Q: Hello: I have a question. I have a DC power supply that is putting out 14 volts at 14 amps. And I have a motor that is running at 14 volts but needs a higher amp rating. Can a DC-DC converter increase the amp output? If not is there anyway to increase the amp output?

A: Energy conservation means that "watts in" must be more than or equal to "watts out." Watts is volts times amps, so if you increase the amperage you have to decrease the voltage.
best regards
mark
Current limit of AC/DC adapters Q: I have heard that AC adapters can output TWICE the current as stated on the labels. Are there any that limit the current?

A: The old-style transformers could do that and get hot, but the voltage would drop, and eventually they would blow the internal fuse.

Switchmode power supplies can typically supply 120% with some loss in voltage regulation (and they get hot), but start hiccoughing (turning on to see if the problem has been solved, then turning back off again) when the current goes too high.

If you want a current-regulated power supply, those are harder to find, they are what battery chargers typically use.
I don't know of any AC/AC power supplies that are current limited (other than the fuse).

best regards
mark
Battery charging auxiliary batteries Q: I run a computer repair business, and while most of my work is done at the client’s site, sometimes I have to bring their equipment back to my office. I’d like to set up a portable computer lab in my van so I can do more work on-site, and have bought a couple 750AH deep-cycle batteries to power things. I’d like to power the lab off these batteries and not take any chances of running down the vehicle battery. I’ve looked at your product list and wonder if your PST-DC-UPS-1212 is the right product to keep the batteries charged while on the road.


A: For such big batteries I would suggest the http://www.powerstream.com/battery-isolator.htm to charge while driving and then isolate the deep-cycles from the starting battery. The PST-DC-UPS-1212 would take a long time to charge a 1500 AH battery!
Does turning a fluorescent light on-and-off waste energy? Q: How long does a fluorescent light have to stay "off" to pay for the energy required to start it up again?

A: This is an urban myth from my childhood, and it isn't true now and I doubt that it was ever true with the oldest iron ballasts. It does take energy to start a bulb, but very little, insignificant compared with running the lamp for a few seconds.
Comments on the Stanford Nanowire battery This link discusses a breakthrough at Stanford University. http://news-service.stanford.edu/news/2008/january9/nanowire-010908.html
This is interesting, but weird.

Since silicon isn't used in any batteries at the present time, the article is somewhat misleading, evidently the author missed some important facts. Carbon is typically used for the anode material to store the lithium ions, and since silicon doesn't have a graphite-like analog (Si crystallizes in the diamond structure)
Most puzzling is the fact that the best lithium-metal batteries only have 2-3 times the capacity of lithium ion batteries per unit weight or volume. Using lithium-plated silicon nanowires you will have far less density than a solid layer of lithium metal, so how can they have 10x the capacity? You can see from the pictures that there is a lot of empty space in the nanowire tangle.

I can imagine that the cycle life could be 10x because there is no wear-out mechanism, maybe the author got that confused. Or maybe the battery has 10x the power output because so much surface area is in contact with the electrolyte. But it can't have 10x the energy storage as stated in the article.
Replacement batteries for emergency jumper packs Q: Who in US sells replacement batteries for, Jumper Battery packs used for emergency starting? Can't figure it out. Steve

A: These typically use standard-size sealed lead acid batteries, such as those at http://www.powerstream.com/BB.htm. If you find one of the correct dimensions it will work.
best regards
mark
Why are lead acid batteries used in standby applications? Q: Traditionally, lead acid batteries have been used for telecom and off-grid electrical systems. If they can be damaged by deep discharges, or by periods of partial charge, why are they used for these applications? Could you use a NiCd battery in a UPS? Is there a better battery chemistry for a renewable energy system? Thanks! -Tim

A: Their biggest advantage is that they can be float charged, in other words kept at a certain voltage without over charging. This makes them very convenient as batteries that are rarely discharged, but are standing by for most of their lives, such as telecom standby power.
NiCad can be used for UPS systems, but they can't be float charged, so it takes a more sophisticated charge controller. Besides, they are more expensive than the lead acid batteries.

For batteries that are used in off-grid situations nickel-iron batteries are very nice. They are virtually indestructible, can last for 50 years. They have a low voltage and low energy density, and aren't as easy to find, so usually people use lead acid instead.

best regards mark
Battery packs get hot Hi my name is Jerimiah.
I read a lot of your tips online and have run into a situation I can’t figure out. I would appreciate a little insight if possible. I have two battery packs that need to be connected. They hold 4 AA batteries each. Unfortunately the manufacturing company has gone out of business and I have no way of buying a new one. I have connected both the packs exactly the same way they had them connected. When I plug in the batteries they start to heat up. What do you think could be happening? Thank you Jerimiah

You have some polarity backwards, either one or more of the AA cells is backwards or the connection between the batteries is backwards. If both wires from the one pack are connected to the other pack then you should be in parallel, plus to plus and minus to minus. If only one wire from one pack is connected to the other then you are in series and you should connect plus to minus.
best regards mark
At what discharge rate are LiPo batteries measured? Mark,
For LiPo cells, at what discharge rate is the mA-hr rating based? I know for lead gel cells it is based at 20 hr rate, with the A-hr reduced significantly at higher or faster discharge rates. Are there published curves for given size LiPo cells at different discharge rates? Mike

Dear Mike:
Typically it is the 5 hour discharge rate that determines the capacity rating of a lithium polymer cell. Lithium cells don't lose their capacity when discharged quickly as much as lead-acid do. However, when the discharge rate gets to a few minutes they do lose some capacity. Some data sheets will have the capacity versus discharge time.

Also, lithium polymer cells differ in their design. Some are designed for rapid discharge for the RC industry and some are designed for slow discharge for more sane applications. In the case of fast-discharge (10C to 20C or 3 to 6 minutes) batteries the trend is to give the capacity at fast discharge, since the 5 hour discharge rate capacity is irrelevant to the users.

best regards mark
What is the truth about silicone batteries? There has been a bit of flurry about a new lead battery, the Silicone Power Battery. It is advertized as new, green, and cool. To quote it uses a "silicate salt electrolyte that replaces the sulfuric acid electrolyte of a normal lead acid battery. This produces a battery that is environmentally friendly."

This excited the materials scientist in me, since I know that 99% of silicon compounds are non-soluble, there must be something cool going on. Besides, they say it is still a 2 V per cell chemistry and is compatible with automobile charging systems.

Well, it turns out that all this is just plain (mostly?) hype, or hyperbole whichever you prefer. What they do is to gel the sulfuric acid with sodium silicates (a common way to do it). So it is just a typical gel cell. They may be making a better gel cell, I don't know yet, but it is nothing more or less than a lead acid gel cell.

If you want to look up the patent application it is USA application 20040175623.
Smaller rechargeable lithium ion coin cells Many engineers have been asking for smaller rechargeable lithium ion coin-cells. We are happy to announce the new additions to our line of Li-ion LIR series cells, the LIR1220, which is 12 mm in diameter and 2 mm thick, and the LIR1024, which is 10 mm in diameter and 2.5 mm thick.
Why are non-rectifying contacts important in CMOS? Dear Dr. Mark,

I know you are a former JFET designer, but I have a question in general about metal contacts to semiconductors.

I've read that depending on the n well process or the p well process it is essential to diffuse p+ in a p-region or n+ in a n-region so that an ohmic contact is achieved. Then, it is said that metal-semiconductor contacts are rectifying, and that it is not "good" for the operation of CMOS. Why is that? why do you need an ohmic contact so that current flows both ways from metal to semiconductor?
What is so "bad" about rectifying contacts?

Thanks. Andres.

A: Dear Andres,
Rectifying junctions not only prevent the current from flowing both ways, but also present a voltage drop for forward biassed operation. This wastes power for simple circuits and makes complicated circuits impossible to make. So the p+ or n+ implants or diffusions are done to make a metallic-like silicon to contact to, which more closely matches the fermi levels of the aluminum contacts.
Best regards
mark
6 to 12 volt converters for Volkswagens and Porsches We finally have released a 6 to 12 volt converter that is 200 watts and fully isolated, so it can be used in either positive or negative ground cars. This opens the 12 volt world up to vintage cars that use 6 volt negative ground systems such as VWs and Porsches.

Take a look at http://www.powerstream.com/dc6-12.htm
Using car batteries for deep discharge Q: Car batteries are the cheapest because of the high quantity manufactured. Why does everyone want to steer me toward an expensive, special purpose, marine battery for deep discharge applications?

A: Car batteries are mainly there for starting the car. This takes a surprisingly high current, up to 400 amps for a big engine on a cold day. The way they accomplish this is by having a lot of very thin plates so there is a lot of surface area. This allows them to discharge quickly, but they are easily damaged by deep discharging. Car batteries are normally rated for only 5-10 deep discharges, sustaining damage with every one of them.
Deep discharge batteries are designed to put out less peak current, but they have thick plates that can be charged and discharged many times without corroding or falling to pieces.
Model helicopter charging problems Hello, I'm Looking for advice on charging NiMH batteries after having problems, I came across your web page - very informative and helpful and answered some of my questions - many thanks for that.
I do however still have some queries and I wonder if you could find the time to give me further advice on this matter? My problem concerns charging batteries for my model helicopter which uses seven AA NiMH batteries (8.4v).

When I first purchased the model, the instructions said to charge the batteries for three hours with the supplied charger. This I dutifully did (using a timer), though the batteries became increasingly hotter after each charge, causing me to reduce the charging time.

Within a very short time, the batteries simply wouldn't power the helicopter and very little or no lift was generated. I purchased new batteries, but the same thing is happening again. I can't help but feel that the supplied charger is just not right for these batteries, it is marked 12vdc and 300ma output and furthermore it says on the side of the charger, (if that's what it is!) 12v adapter.

I would be happy to buy a 'proper' charger, but I can't find anything that will charge the seven cells altogether. Any help or advice that you can give would be much appreciated, Regards, Phil

Dear Phil, It looks like you just have a power supply that is overcharging the batteries. I have used supplies like this to charge NiMH, but you have to be careful to make sure that the current is less than 1/10 of the capacity when the battery is full. So for a 1000 mAH pack the current should be less than 100 mA when the battery is full. You don't say what the capacity of your batteries is, but I would be surprised if a 12 volt power supply would work well for this. We have a smart charger at http://www.powerstream.com/NiMHWMm.htm

that will do it.
best regards
mark
"Paper" batteries from Rensselaer Polytechnic Institute There has been a lot of press lately about the Rensselaer Polytechnic Institute's battery made of cellulose and carbon nanotubes I thought I would weigh in.

This is a great technical achievement, however don't look for these in your cell phone soon, the cost of carbon nanotubes is tremendous. Refined nanotubes sell for $300 per gram, unrefined soot containing nanotubes $20 per gram. A 8.5 x 11 inch sheet would take $36 worth of nanotubes. Due to the nature of nanotubes it is difficult to imagine that they will ever be cheap enough for a consumer battery.

I haven't been able to find the original paper to find out how many amp-hours per square meter. I'll weigh in when I do.
Aircraft batteries are they 24 volts or 28 volts? Q: I am confused about the 28 volt airplane systems. Do they use 28 volt batteries?

A: Cars and trucks and other ground-based equipment traditionally use the "2 volt per cell" terminology. For example a 6 cell battery is a 12 volt battery even though the open circuit voltage is 12.9 volts and the charging voltage can be above 14.4 volts. This is because the "average" voltage as a lead acid battery is discharging is about 2 volts per cell.

Aircraft have used a different standard. They typically quote the alternator voltage rather than the battery voltage. This make sense because (even for ground equipment) most of the time the system is running at the alternator voltage, the battery itself never gets discharged very much. The alternator voltage is nominally 13.8 volts for a "6 cell" system and 27.8 volts for a "12 cell" volt system. This is typically rounded to 14V and 28V.

Therefore a 28 volt aircraft electrical bus will have a 24 volt lead acid battery in it.

So the answer is "No, 28 volt aircraft use 24 volt batteries."
Parallel and Series in pallet jack batteries Q: Hello, I have a quick question regarding pallet jack batteries. We use Hyster, 24 volt systems. When some of these came in, they contained 4, six volt batteries, wired in series. Some have 4, twelve volt batteries, wired parallel.

How do you determine the amp - hour capacity for the correct charger. Wired parallel, do you add the capacity of all four batteries or is it divided by two? I have asked this question to two people who work in the battery industry and actually got two different answers.


Dear David, When in series the voltages add, but the amp hours stay the same. When in parallel the voltages stay the same but the amp hours add.

If you have a 24 volt system with four 12 volt batteries then you have two in parallel and two in series, so the pack would have twice the voltage and twice the AH of a single 12V battery.
best regards mark
What is happening to Lead Acid Battery Prices? Q: Why are lead acid battery prices going ballistic?

A: You are right, prices for lead acid batteries are going up faster than oil. There are a couple of issues that have caused it. The first was that China has taken the VAT tax exemption for exports of lead acid batteries off because lead acid batteries are no longer a preferred export.

Secondly, the price of lead has been skyrocketing. Here is a chart of the price of lead from August 2000 to August 2007. You can see that the prices were fairly stable for several years, then took a 100% increase for a year or so. Now lead is trading at 7 times what it did 4 years ago.
7 year price chart of lead metal
The next chart is over one year between August 2006 and August 2007.
1 year chart of lead metal prices.
This shows that over the last year the price has gone up about 300%, this has really played havoc in those industries that use lead, particularly for lead acid batteries.

We estimate that for every $100 per metric ton increase you can expect a 4% change in the cost of a lead acid battery. This alone would say that the price should have increased 80% over the last year.

This, of course, plays havoc with those of us in that industry. Some factories have lead stockpiles that allow them to smooth out the price increases, but this means that even if lead plateaus in price they will eventually run out and need to raise their prices. Unless the price of lead drops soon there will continue to be increases in prices.

I can also vouch for the fact that no-one stockpiles raw materials these days.

best regards
mark
24 volt batteries Q: Hello, I'm just curious why 24V batteries are not more popular or even used, or available. Thanks, Mark H.

A: Dear Mark,
24 volt batteries are very popular, they are the standard for wheelchairs, used on heavy earth moving equipment, the standard for Semi's in Europe, many electrical bicycles, yachts etc.

But they are never, ever sold as 24 volt. They are sold as 12 volt batteries, then connected together in series by the user.

best regards
mark
Parallel and Series lead acid batteries Q: Dear Sir or Ma'am, I am considering purchasing one of your sealed lead acid battery chargers, but I have some questions that I'd like to ask to make sure that I am buying the right charger.

The batteries that I am trying to charge are 6V 12AH sealed lead acid batteries. Is is possible and/or safe to connect two of these 6V batteries in series (essentially creating one 12V 12AH battery) with either your 12V 6A car battery charger or your 12V 35A car battery charger?

Assuming that is both possible and safe, would it also be safe to charge two of those composite 12V 12AH batteries with one of those car battery chargers by connecting the two 12V 12AH batteries to the charger in parallel?

Is it safe to place a load (12V, up to 10 Amp load) on the batteries while they are being charged with one of those car battery chargers? Is there a formula to determine how long it will take for a sealed lead acid battery to charge based on the current output of the charger and the amp-hour rating of the battery? Thank you very much for your assistance. Sincerely, Daniel

A: Dear Daniel, Thanks for your letter. Yes, it is appropriate to charge two 6 volt batteries in series. It would be best if they were at the same state of charge (or discharge) before being connected in series and either discharged or charged. You can also charge two sets in parallel. Again, if they aren't approximately at the same state of charge it might confuse a smart charger into slowing down the charge prematurely.

A 10 amp load might confuse a smart charger. Adding a load in parallel is done all the time, but for best results you need a charger that is designed for continuous duty, most car chargers are not, they will overheat if left on a 10 amp charge continuously. Also, smart chargers might time out, or cycle on and off under these conditions.
These chargers were designed for this kind of application: http://www.powerstream.com/charger-low-noise-high-power.htm

Amp-hours are just that, amps times hours. So a 7 AH battery should take 7 hours to charge if 1 amp is applied to it. However, it typically takes more time to get the last 5 or 10% into the battery because the voltage of the battery goes up and unless the charger has an accurate way of determining the end-of-charge it isn't safe for the battery to keep up the constant current until it is full. So it takes a little longer than you would expect. Also there are some charge inefficiencies that make the amp-hour law a little inaccurate.

best regards
mark
Running LCD TVs off RV Power A: Hello, I have a 15" flat panel TV that I usually plug into an AC wall outlet in my home. Instead, I want to run this TV from a 12v battery in my RV when I am camping in an area where AC power is not available. I know that I can purchase a DC>AC inverter that will do this but, is it really necessary since the power adapter that came with the TV shows that the set runs on 12v DC 4.16 amp. So, if my TV ultimately requires 12v DC 4.16 amp input, why wouldn't I simply run it directly from the 12 volt RV battery instead of converting DC>AC and then back to DC? Is there a simple solution to this problem? Thanks, Frank

A: If the TV can handle the range between 10 and 15 volts you can run it directly off the RV battery. If not you need to stabilize it to 12 volts. http://www.powerstream.com/dc-buck-boost.htm has the PST-DC/2812-8 which would do the stabilizing.
Capacity at end-of-rated-life for a battery Hi, I am writing a report on batteries, and I had a question regarding charging cycles. I see that the battery comparison chart specifies how deeply the battery is discharged, and how many times that battery can be charged. But what exactly does it mean that the battery can no longer be charged?

Does that mean the battery no longer charges beyond a certain capacity? For example, if a battery has a nominal 1500mA-hr and is said to be able to go through 700 cycles, then after the 700 cycles what is the new amp-hour rating? Please let me know what you know about this topic, or if there's any good literature. Thanks a lot > > ~James

Dear James,
The industry standard is that the battery is at its end of life when the recoverable capacity reaches 80%. So a 10 AH battery would only have 8 AH when considered to be at its end of life. Of course the batteries can still be used way beyond this 80% mark.

I used to have a cell phone that would only last for 2 calls per day, but that was enough to get a call from my wife to remind me to come home from the lab. :)
Voltage of a generator Hi, I found your website to be very helpful.
Is there any way I could convince you to offer the same level of explanation to this question: when I spin a 12V DC motor backwards to generate electricity, how is it possible to generate voltage greater than 12V?

Thanks, Dave

A 12 volt motor isn't really a "12 volt" motor. It is just rated to rotate at a certain speed when 12 volts is applied.

If more voltage is applied it will spin faster, and if less is applied it will spin slower.

The voltage generated when used backwards is proportional to how many wire loops are cutting how many magnetic flux lines per second.

Best regards mark
Are silver oxide cells "alkalines?" Hi. I was just looking at some batteries on your website. I'm writing because I have a basic question as to how to categorize silver oxide button cell batteries, and you seem very knowledgeable. I would really appreciate any clarification you can offer.

My question is this: I understand that a silver-oxide button cell battery (like the SR1120W and other SR batteries) has a silver oxide cathode and uses an alkaline electrolyte. Does the use of the alkaline electrolyte (potassium hydroxide or sodium) make these batteries "alkaline batteries?" Or are they not "alkaline batteries" because they have a silver oxide cathode? Also, what does the "W" at the end of a battery designation (like SR1120W) mean, as opposed to say an "X"? Thank you so much for any help you can give me! -
-- Andrew

Hi Andrew, A battery engineer would say "this is an alkaline battery" because it uses a KOH electrolyte.

But a consumer thinks that a manganese dioxide-zinc battery with a KOH electrolyte is what is meant by "alkaline" due to the perpetual dumbing down of the populace by marketing types.

This opens up a confusion, because there are zinc alkaline button cells with about the same voltage and size as the silver variety. But you can pretty much count on the fact that when button cells are advertised as "alkaline" they are the lower capacity manganese-zinc alkaline and not the silver-zinc alkaline. I know about the prefix, S means silver-oxide-zinc and R means round, but I don't know what the suffixes mean.
best regards mark
How low can a NiCad or NiMH go? Hi,
Can you discharge NiMH/NiCad cells to <1.0 volts and get away with it? Say down to .3V-.4V..
Scott

Dear Scott,

A single cell can be discharged to zero volts without damaging it. However in a pack there is always the danger of reverse charging one of the weaker cells when the pack voltage gets too low. This causes damage to the cell, which makes it liable to be reverse charged even deeper the next cycle, etc.

best regards mark
Corrosion from NiMH Dear Dr Mark,
A considerable number of circuit boards fitted with NI-MH batteries (not purchased from PowerStream) are being returned with severe corrosion of the copper tracks and components within a 2” radius of the battery, (gold plated pins have a blue powdery corrosion).

The corrosion also softens the plastic PCB and causes capacitive leakage/breakdown between the copper tracks. The batteries are 3.6V 150mAh with an average charge voltage of 4.1V and retain their full charge even when badly corroded. Most of the boards come from petrol pumps but the problem also exists on computer boards and POS circuits. The corrosion effect continues after cleaning the board and replacing the battery.

Q ...Assuming the corrosion is caused by gas coming from the battery can you tell me what chemical is causing the damage and recommend a treatment I can apply to the circuit board to remove/stop the corrosion process prior to replacing the battery?

Thanks Tom
PS .. Brilliant site

A: The electrolyte is primarily potassium hydroxide solution KOH, otherwise know as potash. It is corrosive, but quickly reacts with atmospheric CO2 to form potassium carbonate which isn't corrosive, and neither are poisonous.

The batteries will only vent if they are being over charged, which builds up gas pressure in the cell. If this pressure gets too high it is released by the safety vent in the cell to prevent explosions. Such pressure relief does not kill the battery, it only reduces the capacity slightly.

With the gas some of the liquid comes out, which contains the KOH. I wouldn't have expected the corrosion to continue after cleaning the board. KOH is extremely soluble, so deionized water should take it off.
Potassium carbonate is harder to wash off, it isn't as soluble, but it isn't corrosive either.
Charging the starting battery with a "house" battery Q: Dear Dr. Lund:
I was wondering if the PST-BC1212-15 DCDC Charger would be appropriate for the following use: If I am carrying a house battery on board my van being charged with a battery to battery charger from the start battery, and then the start battery is inadvertently run down (say I leave the headlights on)… can I then reverse the wiring to use the battery to battery charger to charge the start battery from the reserve in the house battery? I would use sufficient gauge cable to prevent more than 2-3% voltage drop. I am hesitant to connect the house battery directly to the start battery… is this scenario a feasible solution? Am I being too careful in not wanting to make a “jumper” type connection, instead using the battery to battery charger?

A: Yes, this would work, and not a bad idea at all. Usually the house battery isn't designed to put out enough current to start the van, it is designed instead to deep cycle. If it is large enough to even give a partial charge to the starting battery it could get the van started.
You need more voltage to charge a battery than another battery would supply, which is why the PST-BC1212-15 would be needed to do the charging. It has a DC/DC converter to boost the voltage.

And our charger isolates the house battery from the van so that it won't run down if you leave the headlights on.
New Product, 24V to 48V DC/DC converter Announcement
We just released the new DC/DC converter for running 48 volt equipment in 24 volt vehicles. This is useful for wireless routers and bridges on low voltage systems. This compliments the 12V to 48V converter we have sold for a couple of years.
Circular Mils Q : For transformers the current carrying capability of wire is often listed in "circular mils" What is a circular mil, and how do I relate it to the real world? Thanks a lot

A: The unit "circular mils" is a strange unit. It is a measure of area--the area of a circle 0.001 inches in diameter, which turns out to be 7.85 x 10^-7 square inches, or 0.000507 square mm. So if the criterion is 500 circular mils per amp and you need 7 amperes you would multiply 500 * 7 = 3500 circular mils, multiply by 0.000507 to get 1.771 square mm, or a wire with diameter 1.5 mm, or about 14 AWG.

Free power from the Phone Company!


Are shallow discharges equivalent to deep discharges?
Pee-powered battery, hype or hoax?
How much voltage is required to charge a lead acid battery?

Can you really steal power from the telephone company?

There is an infamous company (that I won't link to for the world) that sells appliances that run off of a telephone outlet (even vacuum cleaners). Is this real? Can you really “stick it to the man” by tapping power off of the greedy, faceless phone company?

Let’s do the numbers:

It turns out that there really is 48 volts on your telephone outlet. The current is limited to at most about 50 mA, and most phones run on 20-30 mA. If you draw more than 50 mA on a the phone company will probably put you down for a short and cut off your phone service until the short is fixed. When you start to draw any current at all from the phone line the 48 volts sags considerably, but we will use 48 volts for our calculations just to be conservative.

(Don't forget that the ring tone is 90 volts, 20 Hz, and it can go up to 120 volts, 20 Hz, so don't get shocked while messing around.)

So, say you wanted to get free power. The 20 mA at 48 volts is about one watt. Here in Utah we pay about $.07 per kilowatt-hour. So by tapping power from the phone line you could save seven cents in 1000 hours, or 41 days. Now you are sticking-it-to-the-man to the tune of 61 cents per year.

The company that makes these so-called products sells a little voltage converter for $70.00 to get you started in the free-energy business. You can pay for it with your savings, breaking even in just 114 years. After that it is all 100% pure savings.

Now, just who is that “man” that is getting the stick stuck to?


Are shallow discharges equivalent to deep discharges?

A recent blog posted on Zdnet implies that you can wear out your laptop battery by leaving your battery in the laptop and letting it recharge every time you plug it in. This is misleading. To paraphrase the poster, he said that the battery is only good for 300 charge-discharge cycles, so leaving your battery installed every time you boot up from AC power will wear it out after only 300 boot-ups.

The truth is that a lithium-ion battery that is rated for 300 full discharge cycles is also rated for thousands of shallow discharge cycles. A rule of thumb in battery design is that the life of the battery is related to the total number of electrons drawn from it, therefore 300 full discharge cycles is equivalent to 600 half discharge cycles, or 3000 1/10 discharge cycles.

As with all rules of thumb the relationship is only approximate, in this case the cycle life actually gets better than you would expect for shallower discharges.

You can verify this from almost any cell data sheet, but I'll just list one other witness to this fact, this one from Motorola :

The relationship between DOD [depth of discharge] and cycle life is logarithmic. In other words, the number of cycles yielded by a battery goes up exponentially the lower the DOD. Research studies have shown that the typical cellular phone user depletes their battery about 25 to 30 percent before recharging. Testing has shown that at this low level of DOD a lithium-ion battery can expect between 5 and 6 times the cycle numbers of a battery discharged to the one hundred percent DOD level continuously.
A recent science news story that I really need to comment on:

The first urine-powered paper battery has been created by physicists in Singapore. The credit-card sized unit could be a useful power source for cheap healthcare test kits for diseases like diabetes, and could even be used in emergency situations to power a cellphone, they say. . .

The same story was repeated all over, look it up on Google if you want to read it. I am reluctant to dignify it with a link.

The sad thing about this is that the battery does not take its power from urine, it is just a dry-charged battery that needs some water to activate it. I have some 30+ year old army surplus batteries somewhere squirreled away in my "neat-science-stuff" collection that work the same way, using the exact same chemistry. The battery is completely inert until you add water to it, and it isn't too picky about what is also dissolved in that water, or where (or who) it came from.

Luckily the article goes on to explain the chemistry to us technical types:

The battery is made of a layer of filter paper steeped in copper chloride, sandwiched between strips of magnesium and copper. This "sandwich" is then laminated in plastic to hold the whole package together.

The resulting battery is just 1 millimetre thick and 60 by 30 mm across – slightly smaller than a credit card. To activate the battery, a drop of urine is added and soaks through the sandwiched filter paper. The chemicals dissolve and react to produce electricity. The magnesium layer acts as the anode, losing its electrons. And the copper chloride acts as the cathode, mopping up the electrons.


So it is just a magnesium/copper chloride battery just like my army surplus one. (Not rechargeable, though the article from New Scientist implies that another drop will reactivate the battery). The claim of "first" mentioned above is hard to understand.

To learn more about reserve batteries, go to http://www.powerstream.com/BatteryFAQ.html#MgCu
Q: I am trying to charge batteries with "alternative energy" which does not give a steady voltage. How much voltage is required to charge a lead acid battery?

A: The minimum voltage necessary to charge a lead acid battery is 2.15 volts per cell, or 12.9 volts for a 12 volt battery. This is what is needed to get the battery chemistry moving in the right direction. Anything less than this will not do anything.

At this voltage the battery won't accept very much current, or in other words it won't charge very fast. There is a band of voltage between 2.15 and the 2.39 volts gassing voltage when the water starts to disassociate (12.9 and 14.4 volts for a 12 volt battery) which is the charging range. The higher the voltage the more current you can stuff in the battery.

Some schemes go way above 14.4 volts to get a rapid charge, but drop their voltage when the battery nears full charge.

Depending on the voltage range of your energy source, you may need a DC converter to efficiently use the power generated, and possibly a full buck-boost converter.



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Disclaimer: Information presented here is the opinion of the author. No warranties regarding the correctness or value of this information are implied or should be assumed.


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