# Connecting two similar, equal voltage batteries in parallel

I am trying to increase the range of my son's child electrical vehicle. It came with a 12V 10Ah battery. I bought a second one, which is 12V but 12Ah. It may have other differences too, but it looks very similar, has the exact same size, etc.

Can I put these two in parallel? Some sources say that the voltage has to be the same other sources say everything must be exactly the same. Furthermore, can I also use the built-in charger to charge both at the same time or do I have to modify it somehow? Again I read different opinions about this. Some say it will just take twice as long to charge others say this charger is too basic and it won't work in parallel for charging.

Do I need to put fuses between the batteries?

Here are some photos:

Original battery in the car:

New Battery:

Charging:

• I think you are mixing standard gel battery with AGM deep cycle battery. And we can't possibly know if the charger is meant to charge a battery twice the capacity it was meant for, or if it is compatible with the AGM battery. Orignal battery type is not fully visble either. Commented Mar 29, 2021 at 19:44
• I am charging the new battery with the charger now, seems to be working. The old battery is a gel battery, it says 12V10AH/20HR, the new one says deep cycle. Would that be a problem? Commented Mar 29, 2021 at 20:38

Can I put these two in parallel? Some sources say that the voltage has to be the same other sources say everything must be exactly the same.

Two batteries with the same nominal voltage rating, can easily have different open circuit voltages. When two batteries with different open circuit voltages are connected together in parallel, current will flow from the battery with higher voltage to the battery with lower voltage, until the batteries are equalized.

Hopefully, the total charge transferred from one battery to the other will be small. However, if the batteries have different chemistry, or even different geometry, the charge transferred could be significant. If both batteries are fully charged, and they have different characteristics, it is possible that one battery might become overcharged. That would be bad.

You are safest if the batteries are identical. Identical voltage, identical chemistry, identical geometry, identical manufacturers. If they are not, you take some risk. How much risk? That is impossible for me to say.

can I also use the built-in charger to charge both at the same time or do I have to modify it somehow?

Without knowing the details of the charger, it is impossible to know whether it will charge the batteries correctly, whether it will damage the batteries, whether it will leave them less than fully charged, or even possibly damage the charger, although I think that last possibility is rather unlikely.

Edit: @StainlessSteelRat has provided a useful point of reference regarding the risk associated with using batteries in parallel, and what is considered good practice within his field:

I deal with the marine industry, which uses battery boxes with parallel batteries. A bad battery and we switch out all batteries in box for same batch.

I would also like to concur with his evaluations

Nothing of OP info makes this a good idea. Including a new battery vs used battery.

Different manufactrers, chemistries, ages, electrolytes, capacity = bad idea.

And if everything is exactly the same? Still not a great idea? Id like to stay on the safe side considering this is a child car.

• I would add, that it is probably much safer to use a (high-power) resistor to connect the two batteries. After a few hours it can be shorted. This limits the initial current. Pb batteries can have a very high current capability which can easily lead to overheating of wires or the battery. In the worst case, something is catching fire. I personally would only do it with identical batteries. Even in this case it is somewhat dangerous. If one battery develops a broken cell, there's easily enough voltage difference to cause severe equalization currents.
– GNA
Commented Mar 29, 2021 at 20:00
• I'd vote for this but you are waffling on the repercussions. Nothing of OP info makes this a good idea. Including a new battery vs used battery. Commented Mar 29, 2021 at 20:01
• So what if I just buy another one as my new battery, would it then be considered safe or still dicey? Any workaround to connect the two or maybe have a switch to switch from one to the other? Clearly the chemistries are different and there is a difference in inner resistance, not sure if thats relevant. Commented Mar 29, 2021 at 20:48
• Different manufactrers, chemistries, ages, electrolytes, capacity = bad idea. Connected in parallel with any differences means larger cells will try to charge lower cells. Little resistance means high current and lots of heat. Commented Mar 29, 2021 at 20:50
• And if everything is exactly the same? Still not a great idea? Id like to stay on the safe side considering this is a child car. Probably best to have a manual switch button to switsch between batteries so I don't have to remove the seat and mess around with the wires in the middle of the park. Commented Mar 29, 2021 at 20:51

It should be fine, assuming the terminal voltages don't exceed the limits of either battery; also assuming you connect them under a condition where the voltage difference is small (so that a large current doesn't flow between them).

When charging/discharging, the two batteries will have equal voltages -- one may be more depleted than the other, but neither is outside its range.

The charger is likely a simple one that just charges to a certain top-off voltage. Obviously it will take over twice as long to charge, but if the charger doesn't overheat, it should be fine (it's perfectly OK that each battery will only be charging at half it's original rate).