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April 17, 2023

How to use lead acid battery chargers

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Introduction to battery charger use

Engineers hate rules without explanations. We don't want to hear "insert teeth guard while testing," we want to know why you should consider the teeth guard. This is because knowledge is power. But in this modern world rules proceed from the place that platitudes proliferate, and even the technical writers don't understand them. This battery charger tutorial tries to answer all those kind of questions about battery charger use.

This document goes through the safety precautions that are usually included with a battery charger to explain why they are recommended. Using a battery charger is simple. Instructions can be complicated!

Battery Charger Not working

Battery charger troubleshooting can be a pain in the world of "smart chargers." You used to be able to just smack the two electrodes together to see if there was a spark. Now most of the new intelligent chargers need to see a voltage on the battery before they will start up. This is a safety issue, to prevent the charger from the aforementioned arcing when the electrodes are shorted. Thus, my favorite trick of bashing the cables together to see if there is a spark isn't a useful test. Also, if the battery voltage is so low that the charger can't detect it, the charger won't turn on. In this case you need to bootstrap the battery by jumping it to another battery or finding a charger that doesn't have this safety feature, (sorry!). This also means that you can't use these new chargers as power supplies unless you have a battery in parallel with them.

Personal Safety Considerations

Remove all jewelry before working with a battery.

It is hard to conceive how much raw destructive power will be discharged from a battery through a short-circuit. A ring will literally melt on your finger if it finds itself part of a short circuit. Similarly, watches, bracelets, bangles, chains, or balls-of-keys might surprise you by becoming a ball of fire and pain. Even tongue studs could inhibit your cries-for-help when your red hot tungsten ring welds itself (and your finger) to the car's frame. Our intuition isn't always helpful, the battery charger current can be low, but even a mostly-dead battery can still source tons of current.

Always wear complete safety goggles when charging a lead acid battery.

It is generally suggested that safety goggles be used when operating a lead acid battery charger. Exploding batteries are very rare, but so are your two eyes.

So what can go wrong that you would ever need safety goggles, acid aprons, or gloves while charging a lead acid battery? In very rare instances lead acid batteries have been known to explode. Since they have sulfuric acid inside they could spray you with acid, as well as plastic, rubber, glass, and metal if they explode.

What could go wrong that would cause a battery to explode? There are a couple of potential ways. The first is that one of the cells could short, releasing the stored energy in that cell quickly. This is not likely to happen during charging unless the battery is shorted or attached to a high current battery charger backwards. Such a shock on an older battery could cause the plates to break and short each other out. As mentioned above, many modern battery chargers cannot supply current before they detect a battery that is correctly connected. Others are fused to prevent damage to the battery or the charger.

This is very rare and does not usually cause an explosion, but the electrolyte can boil and squirt.

If a cell is already shorted and a high current charger is connected it could also cause the electrolyte to boil.

Still very rare, but more common are hydrogen explosions. Hydrogen is only generated when the battery is overcharged, and this is only when the charge voltage across a cell is above what they call the " gassing voltage ." Hydrogen cannot be generated if the voltage is lower than the gassing voltage. This is temperature dependent (see the gassing voltage link), but at room temperature it is 14.34V for a 12 volt cell. There is a possibility for a tiny bit of hydrogen generated at the end of charge because the 6 cells are jockeying for the last bit of charge. A weaker cell will charge last, and since it has lower resistance the voltage across it will be lower than across the other cells, which have stopped charging. This causes higher voltage to appear across the full cells, and this higher voltage can sometimes be above the gassing voltage. This gas is often recombined, especially in sealed lead acid batteries, but sometimes it can escape, especially in flooded lead acid batteries.

Charge in a well ventilated area.

In order to explode the hydrogen must be over 4% by volume of the air it is mixed with. For example, if you were overcharging at a rate of 30 amps and had a battery box with the air volume of 20 inches cubed (50 cm cubed) you could get an explosive mixture in 8 minutes. To fill a 10 x 10 x 8 foot room with 4% hydrogen at a 30 amp overcharge rate would take 24 hours.
Overcharge rates are typically less than 1 amp, so you can see that it doesn't take much ventilation to keep you in the safe zone.
However we are talking about exploding batteries. An external hydrogen explosion will not typically damage the battery, it is the explosion of gasses inside the battery that will do this. In the air-space within the battery will typically be a stoichiometric mixture of pure hydrogen and oxygen, which is always explosive.

Always charge a flooded battery with the water-fill caps in place.

Modern batteries have spark-arrestors, essentially Faraday shields to prevent external flames from reaching the air space. They also try to minimize the amount of hydrogen that is stored. Charging with the water-fill caps off defeats these safety features. Again, sealed lead acid batteries always have gas recycling features, very little room for gas storage, and no way for a flame to reach this gas.

Always fill the battery with water before charging.

The most dangerous lead acid battery would be a flooded battery with most of the water gone, which maximizes the amount of stored gas possible. When the battery is filled with water there isn't much volume to store the hydrogen. And, of course, the battery will be happier and give more capacity when topped off.

Most of the reported lead acid battery explosions are caused by charging a flooded automotive battery outside the car with an unregulated battery charger. In this case the battery can be left on charge for a long time, and the unregulated charger can run it out of water. Chargers that are designed for battery tending or maintenance , as well as most of the PowerStream lead acid chargers can be left on the battery forever without causing them to gas. Simple, old fashioned, or unregulated transformer chargers should not be left on the battery after it is charged.

Place a wet cloth over the caps before charging

For those caps that don't have a Faraday shield, this prevents any flame to get into the battery's interior.

Fire Precautions

Smoking, cooking, welding, brazing, etc. are a bad idea because of the possibility of igniting any potential hydrogen.

Use the battery charger away from gasoline, kerosine, propane, or other flammable materials.

They are worried about any sparks igniting the flammable liquids or vapors. Sparks can happen when the battery is connected or disconnected.

Connect the second terminal away from the battery.

For example, many instruction books say to connect the negative terminal second, and on the engine block and not on the battery. This is because the second connection may create a spark, which could ignite any hydrogen stored in or near the battery. Connecting the negative terminal on another grounded part of the car would cause the spark to be away from the battery.

Always plug the battery charger into the AC after all other connections are made.

Again, they want to prevent sparks from happening near the battery. Many modern chargers won't start charging until they detect a battery, which means that for these chargers this admonition is redundant. This calls to mind my friends splashing sparks off the terminals when connecting up jumper cables, just to make sure that everything is connecting.

When you remove a battery from the vehicle make sure that all car appliances are turned off.

Again, they want to minimize the spark that will happen when the first terminal is removed. If the headlights are on there will be a bigger current interrupted when the terminal is disconnected.

Don't drop a tool on a battery

A metal tool that lies across the two terminals of a battery can cause a huge spark, and damage the tool as well as the battery terminals in milliseconds.

Always disconnect the negative terminal first.

This is a good rule-of-thumb for clumsy people like me. If the wrench that you are using on the battery terminal should accidentally touch the car's frame you can short out the battery if you are working on the positive terminal first, but if you disconnect the negative terminal first, no power can flow to the frame when you disconnect the positive terminal. When you are in your friend's garage looking at a half-inch open-ended wrench with a big burn mark taken out of it you know that he disconnected the positive terminal first.

Charging details

Rock the charging clips back and forth on the battery terminals.

This helps break any insulating corrosion or oxidation from preventing a poor connection. Also it makes sure a low resistance connection is made, it would be bad to have a high-resistance connection that would heat up, and which also would lower the charging voltage.

Charger protection

Don't put the battery-under-charge on top of the charger.

Aside from the potential of the battery to fall off and land on your person, the charger generates a lot of heat. The battery could block some of the cooling vents. It could also heat up, which is always bad for battery life.

Always match up the polarity correctly.

Imagine the destructive potential of a battery capable of sourcing 300 amps and a charger capable of sourcing 50 amps getting together in the wrong way. Many modern battery chargers, including ours can detect reverse polarity safely and will just flash an error LED, but not all do this, so be careful.

Q. After being stored for the winter my 24V boat wouldn't start. It has two 12V batteries in series, but when I measured the voltages, one had +11.2V and the other had -6V! How could this happen?

A: Before storage the two batteries were not balanced. One battery had more capacity than the other, either because it was charged more, or because the other had lost some capacity. This usually happens because the self-discharge rate of one is less than the other. Or, worse, you are using one of the 12V batteries to run a radio or other 12V appliance.

Then, during storage the boat was slowly draining the batteries, maybe through the clock, or maybe through corrosion pathways. When the weakest one ran out of charge, the stronger one started reverse-charging it. Not much, because there isn't enough voltage in one battery to move the chemistry to charge the other, but enough that electrons were stored on the plates, like a capacitor. So you measured a negative voltage on the weaker battery.

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