Regarding the "discharged completely" comments above...
Lead Acid - Don't do it. Fast way to ruin a lead acid is to deep discharge it and worse to leave it flat for any length of time. Even so called deep discharge batteries will live longer if you don't let them go flat. Batteries that aren't going to be used for awhile (eg lawnmower batteries over winter) can benifit from being stored on a float charger to stop them self discharging. This is a constant voltage charge at slightly lower voltage than that used for full charge. If you use the float charge voltage recommended by the manufacturer you can leave them on float charge all winter - just need to check the water level. If the voltage is right they won't boil/fizz much if at all.
NiCAD/NiMH - Depends what you mean by completely. Deep discharging can sometimes rejuvinate these cells BUT beware.. It's not advisable to discharge a battery (containing many cells) down to 0V because some cells in the battery may get reverse charged. For example you might get to a situation where the cells in a four cell battery end up at 0.1V, 0.3V, -0.1V, -0.3V (total adds up to 0V). Best to stop at 1.1V per cell or monitor the voltage of each cell individually.
Lithium Polymer - Don't do it. Never discharge below about 3V per cell on load. Some types should not be recharged if the voltage has been allowed to get too low.