which batteries can be operated in Parallel?

Can any of the following batteries be run in parallel?
- NiMH
- NiCd
- Lead-Acide Gel Cel
- Alkaline

I thought all batteries could be run in parallel.

Matched batteries can be discharged in parallel. Batteries of different voltages cannot be placed in parallel, and care needs to be taken when mixing different chemistries (say a high energy density one and a high power density one to get good pulse output.)

It is generally bad to _charge_ NiCd and NiMH in parallel. Lead acide can be charged in parallel, and alkaline should not be charged.

-Jon

I thought some battery types go low impedance when discharged thereby effectively shorting the battery they are in parallel with. Are you saying as long as they start at the same voltage, their discharge will be even and therefore one battery will never short the other?

As cells are discharged, their voltage drops. If you have two cells in parallel and one of them 'uses up the chemicals first', then its output voltage will drop, and the other cell will start carrying the load current. The net result is that you can't have one cell discharge to a lower voltage than the other.

I suppose that if a particular chemistry had the characteristic that the cell fails as a short circuit when it is drained, then two cells in parallel would have the issue that one could discharge and short, and the other one would still have capacity, but I don't believe that this would be an issue with any chemistry if both cells had the same capacity at the start, since both cells would discharge at pretty much the same time.

-Jon