[Kit Gerhart]

Those of us who have been around FIRST long enough to remember when our robots used NiCads really appreciate how good the currently used sealed lead-acid batteries are.

Before '98, the robots used two NiCad drill batteries in parallel. We had much less run time than with the current batteries, and it was not uncommon for the batteries to fail within a few months of robot use. It is true that NiCads have very low internal resistance and, thus, provide very high current for their size, but there are shortcomings. Most important is that they do not like being reverse charged, and to avoid this happening, a multi-cell pack cannot be completely discharged. Also, the power density is not all that high, though it is higher than lead-acid. For the robots, using a fairly big lead-acid battery that we have seems to work very well. Out batteries are big enough to provide the current we need without too much voltage drop. You just need several batteries and chargers in order to conduct all day practice sessions.

Nicads definitely have their place, and are hard to beat for running the radio equipment in my R/C airplanes. You never run them completely down, avoiding the reverse charge problem. Also, even though lightness is a virtue for anything in an airplane, the fact that you only need 600mah batteries for the R/C equipment, NiCads' having low power density is not a problem. In our robots, battery weight is not a problem at all, as long as we are all building to the same weight rules. If I remember correctly, FIRST went for 120 to 130# max when they switched to the current batteries.

I have been surprised at how well the current batteries hold up to deep discharges, knowing that only a few total discharges will destroy a car battery.