Batteries can be connected in series to increase their voltage. For example, two 1.25V NiCd batteries in series can provide 2.5V.

With difficulty, batteries can be put in parallel to increase their current rating. For example, two 1200 mAHr batteries in parallel can provide 2400 mAHr.

Rule number one: Never directly connect batteries in parallel! Paralleled packs or cells need to be isolated from each other and charged independently. If you do not follow this rule, the batteries will neither be charged nor discharged evenly, and the pack will quickly die a gruesome death.

I've never seen a pack in any commercial product that contains paralleled cells. The primary problem is charging, since when in parallel, there's no way to control which batteries get how much current. You can easily fry a single battery by pushing too much current through it, where its neighbour isn't getting charged because it's getting no current.

If you connect one charged and one discharged battery in parallel, they will have uncontrolled amounts of current flowing between them. It's roughly equivalent to shorting out a battery, and you will quickly set yourself on fire that way.

Don't charge batteries when they're hot. Hot batteries cause the liquid inside to turn to vapour. The vapour is vented outside the battery, never to return. Therefore, you just permanently lost some of your battery capacity.

One workaround is to treat each battery module separately and isolate them from each other with diodes. Diodes will allow current to flow in only one direction, and in this case, only out from the battery. You will still have to charge each battery module separately. This is what most folks using camcorder batteries use.

The circuit diagram might look something like this:

       +|----|>|---------*-----|+
Battery1|               /      | Power Input to Device
       -|--------------/---*---|-
                      /   /
       +|----|>|-----/   /
Battery2|               /
       -|--------------/

Key: 
Diode: --|>|--

Diodes allow electricity to flow in one direction only, but it comes at a cost. Standard diodes reduce the voltage by 0.7?? volts. Shottky diodes only reduce the voltage by 0.3?? volts, and are thus much more desirable. Be sure to get diodes that can handle at least twice your maximum rated current, otherwise the diodes will burn out, sometimes spectacularly.

-- KevinWang - 15 Feb 2002