Connecting 1 x 200Ah Battery in series with with ("2 x 100Ah Batteries in Parallel") - can this work?

Good Day,

I currently have 2 x 12V 100Ah Oliter Gel Batteries connected in series to give me 24v 100Ah for my Axpert 3KVA inverter.

I want to add another 2 x 12V 100Ah Oliter Gel Batteries to my setup to get to 24c 200Ah, but seems like there’s no stock of the 100Ah options.

There is however stock of the 12V 200 Ah model.

If I connect my two existing 12V 100Ah batteries in Parallel with each other and then hook those two up in series with another 12V 200Ah battery it should theoretically and technically be exactly the same as having 2 x 12V 200Ah batteries in series.

Has anyone done this and/or does this sound like a workable solution? (hooking up 1 x 12V 200Ah deep cycle gel battery with another “2 x 12V 100Ah batteries in parallel” as per picture below) ?


Will it work. Yes.

Will one of the 100Ah batteries die causing the other one to be cooked… also yes. It’s a matter of time.

Theoretically it should work. How old is your 12 Volt 100Ah batteries. If they are older than 6 months, I would not recommend adding another battery to the system. Because of the internal resistance difference between the old and new batteries, it will take longer for the one set to charge, over charging the other set in the process.

I have seen sites where people did it and normally it results in the new battery being fried within 3 months of adding them to the old system.

Sorry, for duplicating. I was typing and never saw @plonkster has already answered.

My concern is basically this.

3 Months from now, one of those smaller 100Ah batteries develop a bad cell, which causes the internal resistance to increase.

The 200Ah battery already has a lower internal resistance. One would expect it to be about half of what the 100Ah batteries are. So once one of the 100Ah goes bad, the voltage now divides unevenly over the two remaining good ones (because they literally act like a voltage divider). The 200Ah battery now undercharges, because it never gets up to absorption voltage, while the remaining good 100Ah battery completely overcharges, explodes, and takes the house with it. OK, I’m being overly dramatic there, but I personally would not put this in my house.

Thanks a lot Jaco (and Plonkster),

Yes, the existing 2 x 100Ah batteries are about 1.5 years old, stil working well, but I understand what you’re saying, rather not add new batteries to an existing old batch as new ones would get damaged when the old ones start to “wear out” (higher resistance or if one of them goes etc.).

Thanks, would then rather use them until it’s time and then replace them with 2 x 200Ah batteries only and get rid of the existing ones.


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Thanks Plonkster,

That makes 100% sense, I will rather not add anything to the existing setup and rather replace the batteries completely when the time comes… :sweat_smile:


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Worse if one shorts. But one needs to check cells for this with a Covid thermometer… (routine maintenance!)
Other than that you can add LA batteries in parallel no problem. But yes, don’t buy new dissimilar batteries and start out with this config :frowning: