ASK THE EXPERTS: Batteries in Series & Parallel

Two options for wiring the cells of a battery bank together.

What happens to the battery bank capacity, and the charging and discharging characteristics, when you connect a 12-volt (V), 100 amp-hour (AH) battery and a 12 V, 200 AH battery in series? Conversely, what happens when they are connected in parallel?

Sid Baxter • Pocatello, Idaho


Connecting two battery banks of different amp-hour capacity together in series is a bad idea. The problem is that the battery charging controls will operate based on the average battery voltage and the two batteries will have very different voltages because their capacities are different. The 100 AH battery will become fully charged long before the larger one. The combined voltage will rise, but by the time the controller turns off the charging sources, the 100 AH battery will be overcharged. Meanwhile, the 200 AH battery will not get fully charged. When the bank is being discharged, the 100 AH battery will go flat and its voltage will fall well before the 200 AH battery. The inverter will eventually cut out but not before the 100 AH battery is excessively drained.

Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age. For example, avoid combining a sealed (gel or absorbed glass mat) battery with a flooded (conventional) battery because they have different charging setpoints. Broadly speaking, you can parallel batteries without problems, and the charge controller will look after them. Just make sure you give them plenty of charge. If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated.

Hugh Piggott • Scoraig Wind Electric

Comments (2)

Debbie Crutcher's picture


When you parallel any 2 healthy batteries (same type and voltage), current will flow from the battery with the higher voltage (or SoC) to the lower due to the difference in voltage and resistance between them. This happens if the batteries are the same capacity or quite different. Current will decrease until the voltage of both is identical. At that point, essentially no more energy will pass between them except the tiny difference in self discharge rates.

While charging or discharging, the equilibrium between them will be maintained. More current will come from the larger battery but voltage between them will stay constant. Because voltage is related to SoC, they will both have the same SoC.

Example of a 30 amp load 200AH and 100AH battery in parallel: If 20 amps is flowing out of the 200AH pack, then only 10 amps will flow from the 100AH pack. This is automatically balanced as voltage remains constant.

Larry Crutcher
Starlight Solar Power Systems

GumBoocho's picture

As to connecting batteries rated at different voltages together in series:

1) I know that the conventional wisdom is to only mix & match identical cells (batteries in common Eng).
2) I have seen a researcher's advise with an appliance to add a 3.6V battery in series with a number of 1.5V's.
3) I have never read any proof on this subject nor read experiments & their results.
4) All I have read are pontifications with no proof.

As to parallel connections:

I do not believe in adding batteries or cells in parallel, batteries with different capacities, that is, different internal resistances. That is because Ohm's Law tells us that the stronger battery will discharge through the weaker battery -- you will always have current flowing, wasted current that does nothing but warm batteries. And as this happens, there will be battery discharge.

No 2 batteries will have identical internal resistances, the stonger will always discharge through the weaker. Notwithstanding, I had a diesel engine vehicle which had per manufacturer 2 identical batteries in parallel.

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