# Why can't you combine different batteries?

#### Bioniclekolacz

Joined Aug 21, 2019
7
I've heard that it doesn't work and will overheat and likely cause damages to combine different batteries together. Such as wiring 2 9v batteries in series, and then in parallel with a AA, outputting 18V and about 3A.

But why does that cause problems? Or does it?

#### djsfantasi

Joined Apr 11, 2010
8,565
I've heard that it doesn't work and will overheat and likely cause damages to combine different batteries together. Such as wiring 2 9v batteries in series, and then in parallel with a AA, outputting 18V and about 3A.

But why does that cause problems? Or does it?
When the batteries are different, wiring them in parallel will cause problems. In your example the 2 - 9V batteries will pump into the AA, quickly depleting the 9V batteries and likely destroying the AA.

If they were all in series, then I don’t know what it will do, but I suspect it will provide 20V, but will only last as long as the lowest mAh rate dictates.

#### dl324

Joined Mar 30, 2015
15,097
Such as wiring 2 9v batteries in series, and then in parallel with a AA, outputting 18V and about 3A.
This doesn't make sense. If you want to put AA batteries in parallel with 2 9V batteries, you should just use AA batteries as they'll have more capacity than 9V batteries.

In your example, you'd need 12 AA batteries in series to get 18V. It might make more sense to use a couple D size batteries in series and step the voltage up to 18V.

Wiring batteries in series and/or parallel is done for convenience and there are always drawbacks.

When batteries are operated in series, they should be matched as closely as possible and be of the same capacity because when the weak one becomes discharged, the output voltage will be decreased.

When batteries are operated in parallel, the batteries should start out as closely matched as possible. Unless a battery becomes shorted, a weak one will only reduce available current. A shorted battery can be bad news and packs using high capacity cells will include fuses. For parallel configurations, the batteries need to be the same voltage.

#### WBahn

Joined Mar 31, 2012
27,387
I've heard that it doesn't work and will overheat and likely cause damages to combine different batteries together. Such as wiring 2 9v batteries in series, and then in parallel with a AA, outputting 18V and about 3A.

But why does that cause problems? Or does it?
If you put two 9 V batteries in series you will have 18 V. If you then put that in parallel with a 1.5 V battery you will have a circuit that has 16.5 V across the low internal resistances of the two batteries resulting in a very high current flow leading to a lot of heat and quite possibly worse as you overcharge the AA cell.

EDIT: Fixed typo. Thanks for catching it, Alec_t.

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#### Alec_t

Joined Sep 17, 2013
13,148
If you put two 9 V batteries in parallel you will have 18 V.
Er, no. Only 9V.

#### WBahn

Joined Mar 31, 2012
27,387