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Custom design and manufacture of state-of-the-art battery chargers, battery packs, UPS, and power
supplies
Battery
Chemistry Tutorial and FAQ
How to Calculate Battery Run Time
Battery Technology Comparison Chart
How to Store Batteries (Lead
Acid, SLA, LiIon, NiMH, NiCad)
Chart of Standard
Cylindrical Battery Sizes
Chart of
Standard Sealed Lead Acid Battery Size
How to Charge Sealed Lead Acid
Batteries
How To Charge NiCad
Batteries
How to
Charge NiMH Batteries
Battery
Connector FAQ
Wire
Gauge and Current Capability Chart
Why use battery
packs?
Cells are like eggs. They come in
fixed voltages and capacities. You can't get half an egg, and you can't get half a cell, at least in voltage. Capacities do
vary, particularly with a supplier like PowerStream that has a great variety of cell sizes available, but voltages don't. A
NiCad or NiMH cells are 1.2 volts nominal, lead acid is 2.0 volts nominal and the various lithium technologies are about 3.6
volts per cell. If you need more voltage you have to add them in series, if you need less voltage you need some kind of voltage
regulator or DC/DC converter.
If you need more current you may need to put cells in parallel. If you need more capacity
you may also put cells in parallel.
Many times the physical configuration makes it more attractive to use many small
cells rather than a few large cells.
Geometry and
Topology Considerations
There are an infinite variety of battery pack combinations. Here are the most
popular:
Ladder, linear, F type, or radial
Note that the straps will both come off the top when there are an even number of cells,
and one off the top, the other off the bottom when there is an odd number of cells. With a connector and heat shrink wrap they
look like this:
The
size of a ladder pack is D x nD x H where D is the
diameter of the cell, n is the number of cells, and H
is the height of the cells.
Multi-row cells
There are two ways to start packing them. One could be called the cubic, and the other face
centered cubic, or nested.
Cubic packing is in neat rows. The size of such a pack is nD x mD x H, where
n is the number of cells in a row, m is the number of rows, D is the cell diameter, and H is
the cell height. 
Face
centered cubic packing is nested to take up less room. Calculating the size takes a little geometry.
The size is L x W x
H where
L = (n +½)D
W = [0.866(p-1)+1] D
H=H
p is the
number of rows wide.
If there are alternating long and short rows, such as the 3,4,3 ten cell pack, the formulas
are
L = mD
W = [0.866(p-1)+1]
D
H = H
Where m is the number of cells in the longest layer, and p is the number of layers. With heat shrink it looks like this:
For a 3 cell pack, you can put the cells in a
tube
The diameter of the outer circle is
2.16 D.
For a four cell pack in a circular tube
The diameter of the circumscribed circle is 2.41
D.
For example, with AA cells the diameter is 14.2 mm, so three would fit into a tube 30.7 mm in diameter,
and four would fit in a tube 34.22 mm in diameter. Of course you will want to expand this slightly to account for up to 0.5 mm
variation in cell diameters.
| Cylindrical batteries in a tube of diameter "D" |
Approximate diameter of the circumscribed circle |
Scheme |
| 2 Cells |
2D |
|
| 3 Cells |
2.16D |
Triangle |
| 4 Cells |
2.414 D |
Square |
| 5 Cells |
2.70 |
Pentagon |
| 5 Cells |
3D |
Square with one inside |
| 6 Cells |
3D |
Hexagon |
| 7 Cells |
3D |
Hexagon with one inside |
| 8 Cells |
3.305D |
Heptagon with one inside |
| 9 Cells |
3.62D |
Octagon with one inside |
| 10 Cells |
4.4D |
Pentagon with 5 more cells symmetrically arranged |
| 10 Cells |
3.92D |
Nonagon with one inside |
| 11 Cells |
4D |
Nonagon with two inside |
| 12 Cells |
4D |
Nonagon with three inside |
| 13 Cells |
4.24D |
Decagon with three inside |
| 14 Cells |
4.4D |
Decagon with four inside |
| 15 Cells |
4.55D |
Undecagon with four inside |
| 16 Cells |
4.9D |
Dodecagon with four inside |
| 19 Cells |
5D |
Hexagonal close packed |
| 37 Cells |
7D |
Hexagonal close packed |
| 61 cells |
9D |
Hexagonal close packed |
| 91 Cells |
11D |
Hexagonal close packed |
Linear, or L-type
This is a stack of cells end to
end.
These are usually constructed by standing
two cells side-by-side, and welding a nickel strip across the terminals, as in the ladder pack. then the cells are bent end to
end by bending the nickel strip in a "U" shape. Allow a thickness increase of ½ to 1 mm per junction for this.
Thermistor
The industry standard
thermistor is NTC 10K at 25°C and B=3950.
Pack
assembly
Solder versus
Weld
Most battery packs are spot welded together using nickel strip for contacts. Soldering directly to
the cells is dangerous for the cells. It is easy to melt or disturb the safety vent, thwack the seals, or cause internal
shorting if the heat is too high. This damage might not be noticeable until later.
However, not everyone has a
capacitive discharge spot welder, so PowerStream has instituted a program where we can spot weld nickel solder tabs at the
factory. This is cheap, and allows the end user to solder together custom packs easily without fear of damaging the
battery.
For information about the resistance of the nickel foil weld strips click here:
PowerStream now
sells an inexpensive spot welder suitable for welding battery
packs.
Heat Shrink Tubing
The most common way to hold the pack
together is to use heat shrink tubing. This has sufficient strength for small packs, but as the weight increases more structure
is necessary. This is done by adding a sheet of structural material, usually plastic, to the top and the bottom of the pack. If
the battery is to be put into another structure, either a plastic case, or the system box, it is still important to tie it
together with heat shrink for ease of handling.
When using battery packs be careful not to inadvertently short the
cells. A pack of cells wired in series will become shorted if the cases of adjacent batteries touch, since the outer case is a
terminal. This can happen if the cells are shrink wrapped, film wrapped or painted and the batteries rub against each other.
Brittle shrink wrap is known to shred under stress, leaving the bare cell walls to touch.
Battery Holders
When using or designing battery holders make sure there is adequate provision
for short cells, long cells, or wide cells. Keep sharp clip edges from touching the cell where they could cut the film or
paint, causing a short between cells held by the same clip.
Potting?
2. Batteries expand and contract during charge and
discharge. Potting a battery is not a good idea, unless there is some provision made for this dimension change. There is also a
problem with venting unless the potting is such that the seals are not covered. You don't want the pressure to build up to the
point it blows the potting material apart.
Case and Cabinets?
Over the course of life most batteries release hydrogen, and sometimes oxygen. Take this into
account if you are designing a closed system, such as waterproof lights, weatherproof installations, etc. Some method of
releasing or absorbing the hydrogen, flooding with air or inert gas should be used. In closed cabinets some provision for
ventilation is necessary to prevent hydrogen gas from accumulating.
Electrical
Considerations
How many amp-hours do I
need?
Cell capacity is rated in amp-hours or milliamp hours. The symbol
for capacity is C. This is amps times hours. Divide by hours and you get amps, divide by amps and you get hours. For example a
5 amp hour battery is the same as a 5000 milliamp-hour battery. If you want to discharge in 10 hours, you can get a current of
5/10 = 0.5 amps. If you need 100 milliamps current, then you can run for 5000/100 = 50 hours.
Often a discharge or
charge rate is given proportional to C. So a discharge rate of C/5 means C/(5 hours), or the constant current to fully
discharge the battery in 5 hours.
The calculation of run time versus current is a rough estimate, but is accurate under
the right conditions. The faster you discharge, the lower the capacity of a battery. This trade-off depends on the battery
chemistry and construction. Usually the capacity of a battery is quoted at a C/20 discharge rate. So an 12 amp hour battery
sealed lead acid battery will actually put out a steady 0.6 amps for 20 hours. However, if you discharge the same battery at 12
amps, you would expect to run an hour, but you will only last for 22 minutes. Also, if you wan to run at 10 milliampere you
will get less than the expected 1200 days, since self-discharge of the battery will limit your run time.
Different
battery chemistries differ in this respect. Lead acid batteries are probably the worst at the rapid discharge end of the scale.
NiCads and NiMH are much better.
How do you determine the current that your system
draws?
The best way is to measure with a current meter and an adjustable power supply (usually the
current meter is built into the power supply). Set the power supply to the highest voltage that the system is rated at and
measure the current, then set the power supply to the lowest voltage that the system is rated at and record that current.
Adding a measurement half way between the two will give you an idea of where the lowest power consumption point is (power is
voltage times current). The idea is that you want to design your pack so that the voltage swing of the batteries (see below) is
adequate, and where the power consumption is the least. Some systems will show approximately constant power consumption no
matter what the battery voltage is, and some will have a sweet spot where the power is lowest.
If a variable power
supply is not available, chart the current versus the voltage of the battery during a discharge cycle.
If making actual
measurements is not possible, use the system data sheet, or the "boilerplate" sticker on the back to find the rated wattage or
input current. This will usually give you a high estimate, or a peak value.
Series and Parallel.
Generally batteries are best used in series, not in parallel. This is
because keeping the battery pack equally yoked during repeated charge and discharge conditions can be a problem. So a good
approach is to choose the cells that will give you the capacity and current that you need and put them in series to get the
voltage you need.
However, this is not always possible, and parallel and even series + parallel packs are made every
day.
Series
The first question to answer is "how much voltage do I
need?" The second is "how many cells in series do I need?"
The voltage of any cell is a moving target. When fully
charged the voltage will be higher than nominal, and at the end of capacity the voltage will be lower than nominal. The
following table shows the range of the various chemistries:
| Chemistry |
Type |
Nominal
Voltage |
Fully charged
voltage |
Fully discharged
voltage |
Minimum charge
voltage |
| NiMH |
Secondary |
1.2 V |
1.4 V |
1.0 V |
1.55 V |
| NiCad |
Secondary |
1.2 V |
1.4 V |
1.0 V |
1.50 V |
| Lead Acid |
Secondary |
2.0 V |
2.1 V |
1.75 V |
2.3 - 2.35 V |
So a 10 cell pack of NiMH cells would have 14 Volts when fully charged, and run down to 10 volts when
fully discharged. Your system must be able to tolerate this voltage range.
Furthermore, if you want to be able to charge
while your system is running, the system must be able to accept the charging voltage, which is always higher than the nominal
or the fully charged voltage. Work with the charger manufacturer to make sure that you have this problem solved.
Matching Cells in a Pack
Be careful to match the cells in a battery pack. When a
battery pack is near zero volts under load the weaker cells will go into reversal, and suffer damage and perhaps venting.
Resistance of the Nickel Strip
Nickel foil is used to spot
weld packs together. Nickel is fairly low resistance, yet has enough resistivity to be spot welded. It is strong, has very good
corrosion resistance, and will not oxidize easily.
The resistivity of nickel is 6.9 x 10-6Ohm-cm. The formula to
be used to calculate the resistance for a nickel strip is R = { L / ( w*t } rho,
where L is the length of the strip, w is
the width, and t is the thickness, all in cm. The length L can be estimated as the diameter of the cells. The following table gives typical values.
This is a conservative estimate, since in many cases the spot welds are closer to the edge of the than we have assumed.
| Cell Size |
Foil Thickness |
Strip Width |
Strip length |
Resistance |
| AA |
0.018cm |
0.5cm |
1.4 cm |
1.0milliOhms |
| AA |
0.025 |
0.5 |
1.4 |
0.76 |
| Sub C |
0.025 |
0.5 |
2.3 |
1.2 |
| Sub C |
0.025 |
1.0 |
2.3 |
0.6 |
| Sub C |
0.018 |
0.5 |
2.3 |
1.7 |
| D |
0.018 |
1.0 |
3.3 |
1.2 |
| D |
0.025 |
1.0 |
3.3 |
0.9 |
| D |
0.025 |
2.0 |
3.3 |
0.4 |
This table gives examples of the calculation of the resistance of nickel spot weld
strips.
Connectors
To look at some connectors, take a look at
our connector web page
Other connectors are available!
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