[Venkatesh]

Lets take a look at the datasheet for the battery for a minute...

The battery we use in competetion is the Exide EX18-12, which is very similar to the commonly available Exide ES18-12. I couldn't find the datasheet for the EX18-12, so the ES18-12 will have to suffice for now.

The ES18-12 is billed as an 18 amp-hour battery. This is true, but only at relatively low discharge rates. In FIRST robots, current tends to be very high, in the highest band of data available for the battery. The datasheet tells us that we can only get 9.0 amp-hours from the battery if our discharge rate is 18.0 amps and has a maximum current output of 230 amps. The usable energy at such high current levels will be lower than 9.0 amp-hours.

Each robot is limited to drawing 120 amps by its main breaker. Even though there have been reports of the main breaker not tripping at notably higher levels for long periods of time, we should ignore those to make these calculations simpler. Let us estimate a robot uses an average of 100 amps throughout an entire match. A match is 2 minutes. This is equal to roughly 3.3 amp-hours per match.

This would lead us to think that a battery can be used for two matches without any issues at all.

However there are more factors to think about - not all robots will drain an average of 100 amps per match. If your robot has a very undemanding drivetrain, a limited number of motors, and never runs motors near stall, you will be able to use one battery for 3 matches (or more) without any issues at all, as more energy will be available at lower current levels. Big, burly robots or ones which put motors in stall conditions often will have to deal with less available time from a battery.

The estimate of 100 amps continuous is probably very high, but can be used as a guide. If you want to swap batteries every two matches, having three batteries would be fine. A fourth could act as a spare when matches are too close together.

Another thing to keep in mind - the charger. Most teams use the kit-provided 6 amps charger, which is relatively quick at charging. We used the 2001 4 amp charger, which took longer somewhat longer. If any teams are using 2 amp automotive chargers or the 2002 1 amp trickle charger, think about having more batteries on hand.

The loads that we place on these batteries are large. Do not count on old batteries to provide as much energy as modern ones, particularly at high current levels.

Draining an entire battery in one match is relatively unlikely. We would need to drain at 200 amps (an entire match stalled, with all motors on, and the main breaker well-cooled) to take us down to those levels of performance. Batteries abused in such a manner will not last more than one or two seasons.

And too many batteries can never hurt.