[Al Skierkiewicz]

Jnatt,
The competition docs have a good sheet on battery from the manufacturer. The expected life cycle graphs show distinct functions of battery charge current, load current, and depth of discharge vs. life expectancy. In general, most teams will not deeply discharge the battery since robot controllers reset below 8 volts, so as the battery becomes discharged to the 8 volt level, current demands will cease in normal robot modes. In gelled lead acid batteries, the acid (the gelled part) is heated during charge and discharge and that heat does tend to evaporate the water in the gell with time. The amount of current available from the battery is determined by the surface area of lead plate in contact with the gelled acid. As the acid decreases less current is available and/or less time to discharge. Lead acid batteries also produce conductive particles as part of the charge discharge cycle. In your car battery there is space at the bottom of the battery for this matierial to collect. (gravity carries these particles to the bottom.) When enough of it collects on the bottom, it eventually shorts out the plates and you have a dead battery. In gelled batteries the particles cannot migrate to the bottom and so are held in suspension. When a conductive path finally occurs, you short out the plates. The batteries we use, like your car battery are multi-cell, i.e. 6 cells @ 2 volts=12 volts. A shorted cell will result in a 10 volt battery charged battery instead of 12 volts. Unfortunatelly this is kind of a domino effect. When you have a shorted cell during charging, more current flows in the other cells, raising the internal heat, evaporating more water, producing more particulates, causing more shorted cells.
The bottom line is use old batteries for practice and when they won't hold a charge or have an obvious shorted cell then recycle. Most battery stores will take back batteries to recycle, the lead is recoverable and best not left to accumulate in the environment.
Hope you are still in scouting and working towards Eagle, good luck.