hawk

this is a toughie so I'm putting it here.



a 12 volt car battery has 6 cells that are 2.1 volts each



a 6 volt car battery has 3 cells that are 2.1 volts each



question - is it possible to wire these two dissimilar batteries in SERIES to create an 18 volt (nominal) system, since technically it should show like 9x 2.1 volt cells, or will the 12 volt battery somehow overload the 6 volt battery and destroy both batteries and all the electrical in the car????

Red Raspberry

Yes in series you add the voltage.



The capacity will be limited by the weakest cell.



When charging the smaller sized cells will overcharge and the larger cells will undercharged this is also true if they are in parallel.

pandaturbo

You can't connect a 12v to a 6v battery. If you want 18volts connect 3 6v batteries in parallel.

Red Raspberry

What. You sure can run them in series. You just have to make sure the cell capacities are the close to the same. If you run them in parallel you just get the average voltage of the individual battery configuration. Or in your example 3.6v.



A 12v battery is 6 cells in series, add 3 cells more to get 18v nominal. One cell =~2.1V

pandaturbo

Sorry I meant series not parallel.

I have never heard of this being done, I know the principals of it make sense but you are using 2 different rated batteries so shouldnt the 6v affect the 12v batteries charge rate?

Red Raspberry

You would need a charge regulated at the higher voltage. But assuming the voltage is correct then the individual cell amp hour capacity is what makes the big difference. If you connect a higher amp hour capacity cell with a lower one for charging the lower one will be overcharged and the higher one will be undercharged. This is what happens to parallel arranged cell combos too.