[Al Skierkiewicz]

Jacob,
When you put two batteries in parallel, the higher voltage battery will pass current to lower voltage battery. A 9.001 volt battery will attempt to push current to a 9.0001 volt battery. As the higher passes current to the lower, it's terminal voltage falls and the process reverses. As this occurs the batteries end up draining each other until there is no more current to be had (domino effect). The only way to prevent this is to use diodes in series with one of the leads (usually the positive). The diodes prevent current from entering the battery but will allow both to supply current to the load. As batteries increase their current density this becomes an additional problem. Our robot main battery for instance could self destruct if placed in parallel with another battery. NiCad and NiMh batteries suffer this as well. Batteries are OK to wire in series but not in parallel.