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Balancing or Equalizing Lead Acid Batteries

by Chris - April 8th, 2012.
Filed under: Projects. Tagged as: , , .

I came across some very confusing information regarding battery balancing and equalization on the internet. Some of it was correct, some was misleading, and some was outright non-sense. I recently installed a 48 volt battery pack onto my boat as part of my off grid solar system and found that the batteries needed to be equalized. This article shows how I figured out how I needed to balance the batteries and how I went about doing it.


Testing the Battery Pack

First of all, equalization is not needed on batteries mounted in parallel. It is only needed for battery packs that have two or more batteries in series. In my case, I constructed my 48 volt pack out of four 12 volt gel cell batteries and strung them in series like this:

gel batteries wired into a 48 volt pack

Four 12v batteries in series, creating a 48v battery

I’ve had many highly qualified people tell me that lead acid batteries don’t need to be equalized. Due to their low internal resistance, lead acid batteries are said to ‘self balance’. This may be true, but the first time I connected the battery charger up to the pack, the first cell read 13.5 volts while the one next to it read 15.1 volts. Yikes!

Gel batteries are much more sensitive than other lead acid batteries – like AGM or flooded cell. They should never be charged above 14.2 volts, so seeing a voltage of 15.1 volts on one of the cells was a clear indication that these batteries needed to be balanced prior to charging. In the past, I’ve experienced this same scenario – where I trusted the information I received from ‘experts’ and walked away, resulting in blown batteries and a lot of lost money.


Balancing Lead Acid Batteries

I immediately disconnected the charger and disassembled the battery pack. In order to balance a battery pack composed of multiple batteries in series, you need to wire them all up in parallel, like this:

12 volt batteries in parallel

Wiring batteries in parallel in order to balance them

By wiring all the batteries in parallel, it accomplishes the following:

  1. It balances the battery voltages. By wiring all the batteries in parallel, it forces all the batteries to balance out at the exact same voltage level.

  2. It equalizes the charge in the batteries. This is a natural repercussion of the voltage balancing. Electrical charge flows from the batteries that contain more charge to the batteries with less charge. The flow of charge is what causes the voltages to balance out.

I should point out that I knew ahead of time that all the batteries in the pack were near fully charged. You should charge each battery individually before connecting them up in this manner.

equalizing batteries

Equalizing the batteries with a 12 volt battery charger

Disclaimer: Never connect a fully charged battery to a depleted one! The high current surges that occur can seriously stress both batteries.

Once the batteries were wired in parallel, I connected a 12 volt charger and let it charge the pack. Batteries can take a long time to balance out just by being connected to one another. By injecting a little extra current into the pack, it helps speed up the equalization process. Since these were Gel batteries, I was very careful to not let the pack voltage exceed 14.1 volts.


Reassembling the Battery Pack

Once the pack had come up to 14.1 volts, I disconnected the charger and let the pack sit (still wired in parallel) overnight. The next morning, I wired them back in a series configuration and reconnected the 48 volt charger. When I turned on the charger, I checked each cell and verified that the voltage was now evenly distributed across all the batteries in the pack.

Batteries will naturally move out of balance as they age. Now that my batteries are balanced, they should stay that way (according to ‘experts’). However, I will plan on checking them monthly for imbalances and repeat the exercise should they ever get more than a couple hundred millivolts apart.

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