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 (11)

ronexant's picture

I would like to get your expert advise on this. I have an ebike that uses 4pcs 12v/20AH Gel type battery that is connected in series. As far I have read about proper maintenance, its all about charging that will give it a longer lifespan. According to some details charging it connected in series is not the correct one, is this true? and the best way to properly take care of it is to connect them in parallel and charge them that way, is this correct? however doing it this way is time consuming (disconnecting it from series, connecting it in parallel while charging then disconnecting it again from parallel then reconnecting it back in series so I can use my ebike). Is it safe to charge them in parallel? How many amp of charger should I use when batteries connected in parallel? If I decide to charge them in series, can I use 2pcs charger with 24v connection assuming it can charge gel type batteries while all batteries are connected to each other with ebike's controller is connected? or is it better to buy 1 charger for each battery, connected to every battery terminal and charge them while connected in series?

I have read that ebike batteries get easily broken down as early as 3 months, it could be because the way it is charged and the type of charger the ebike came with.

Truly appreciate your help on this.

Debbie Crutcher's picture

Hi Jim47,

You said, "Say I connect a string of 34 watt batteries to a UPS and get 10 minutes run time into a fixed load. Then add a second string of the same battery size. Now I get 24 minutes run time with the same load. That is an increase of 140% capacity with only a 100% increase in rating. "

What you are seeing is Peukerts equation in effect which has to do with the usable capacity of a lead acid battery based on the rate of discharge. By keeping the same load and increasing capacity, you have decreased the discharge rate. That increases the usable capacity. In a nut shell, slower discharge yields more usable capacity. See:

What Michael said is true if you don’t alter the discharge rate.

Larry Crutcher
Starlight Solar Power Systems

Jim47's picture

Hi Debbie,

Interesting that you bring up Peukert. I'm aware of his work and his observations. It's those observations and the non-linear behavior of the battery with added capacity under the same load that has peaked my interest to find out why. The behavior is well known but as far as I have found so far, not well understood. I've written a program that allows me to predict battery run time with extremely good accuracy. When I add parallel strings the result remains highly accurate. Where i am headed with this is trying to predict accurate run time when I parallel battery strings of different capacities. We occasionally have a need to do this and I fell if I can get a better understanding of what is behind the non-linear behavior I may get closer to writing the algorithm I'm looking for.

Thank you for your input.

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

Jim47's picture

While all of this is true I would like to understand parallel operation a little more. When two batteries of any rating are placed in parallel the resulting load run time capacity will be greater than two times the capacity of a single battery. For example a 50 Ah battery will provide 10 minutes of backup to a specific load. Two of that same battery connected in parallel will provide 25 minutes of backup. That's 150% load capacity increase for only 100% total battery rating increase. What is the mechanism that explains this?

Michael Welch's picture
Doubling the batteries also doubles the total watt-hour capacity, it does not more than double the amount of backup available.
Jim47's picture

I beg to differ. I have worked with batteries all my life designing uninterruptible power systems and have experienced this result repeatedly. Say I connect a string of 34 watt batteries to a UPS and get 10 minutes run time into a fixed load. Then add a second string of the same battery size. Now I get 24 minutes run time with the same load. That is an increase of 140% capacity with only a 100% increase in rating. This behavior is supported by the battery data sheet so is fully expected. What I'm looking for is an explanation of what is happening in the battery that causes this. By paralleling the battery we have cut in internal impedance in half. That is simple Ohm's law and if that were the only thing at work here then the capacity would only double. We have also doubled the plate area. Does doubling the plate area of a battery result in more than double the capacity? I suppose that might be an explanation. But I don't know enough about the inner workings of the battery to explain the relationship between plate area changes and battery capacity. I'm looking for an explanation of this behavior.

Ben Root's picture
I'm no expert on batteries, but this interesting to me. If I had to hazard a guess it would be that in a UPS scenario, with only 10 minutes of capacity, the discharge rate ( in relation to total capacity) is quite high. Thus the inverter would actually be shutting down from voltage depression (battery chemical process not keeping up with amperage). Do you get a few more minutes if you let the system rest for a few?. If you double the battery capacity, you'd also be halving the discharge rate, so not only would you be getting twice as long with a given load, but that load would be easier on the rate of chemical reaction needed to keep voltage up. It is documented that battery capacity changes with discharge rate. I don't think this is quite the same issue in RE systems where the discharge rate is so much lower than the C = 0.167 that you are talking about. I'm interested in expert feedback on this.
Jim47's picture


Yes, of course you are correct. The example i used was simply to demonstrate the point I was making. And I'm sure there is some contribution by the heavy load. But let's say instead of a 34 watt battery that only yields 10 minutes run time we use a 200 watt battery with the same load. The run time is now 118 minutes. If I add a second string of the same battery size the run time will increase to 263 minutes. That's still a sizable increase (122%) in capacity for a 100% increase in rating. As another contributor pointed out this behavior is described by Peukert and while it is expected I don't think it is well understood. At least in the literature I have found so far. I'm looking for a better explanation of what mechanism in the battery drives this behavior.


Debbie Crutcher's picture

A battery is an electro-chemical energy storage device. As such, it has a variable efficiency of electron transfer that is affected by the rate of charge or discharge (and other factors). This is known as coulombic efficiency, aka Faraday efficiency. I suggest this for your next course of study.

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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