[Tristan Lall]

Let's look at this from a slightly different perspective--what work do your motors have to do? If you have, for instance, motors running near their free speeds, you'll tend to waste less energy, and consequently your battery will last longer. Since most of the kit motors don't spin at a very convenient speed on their own, selecting gear (or chain, or lever) ratios that encourage the motor to spin as close to its maximum-efficiency speed as possible can be very useful for keeping power consumption down.

As a concrete example of this, let's compare 188's last two (i.e. 2004 and 2003) robots, Blizzards 5 and 4, respectively. Both employed six drive motors, with very high top speeds (>16 fps and >13 fps, respectively, under ideal conditions). Blizzard 4, with a total of nine motors on board (six of which were used continuously) required a fully-charged battery for any match where significant pushing was undertaken; this is because it forced the motors to operate at too low a speed, and consequently drew more power (refer to one of FIRST's motor spec. sheets, where the power vs. torque curve is vaguely parabolic--this robot was often forced to operate to the right of the parabola's vertex, in a zone where current is high, and consequently, so is electrical power consumption). In fact, power consumption was so high that during on particularly memorable pushing match, the robot controller reset several times due to insufficient voltage (there was no backup battery that year), and the OI was (intermittently) registering less than 6 volts after a 2-minute round, from a previously fully-charged battery. Don't do this; it's fun, but it's bad for the robots (to the tune of over 200 A).

By contrast, Blizzard 5 added a low gear (ideally 4 fps), which enabled pushing contests to be undertaken at lower intended ground speed, while still operating the motors at an efficient speed; this moved the robot's power consumption well to the left side of the vertex (referring to the same graph), and enabled better use of the limited battery capacity, even though Blizzard 5 also had nine motors (including the compressor). We still changed the battery after every match, but the option to prolong their use by an extra match or two was now viable, thanks to the reduced demands on the always-busy drive motors.

So what's all this off-topic lecturing about? Basically, depending on how you selected your mechanical components (and this applies to any motorized apparatus, not just the drivetrain, and excluding the compressor), you'll end up with some level of power consumption, to achieve a given output power level. This is obviously a huge factor in estimating how long a battery will last.

As I hope I've illustrated, there is no one expected value for a robot's endurance--every robot is different. As design criteria, it is helpful to consider several factors in addition to the above: how many batteries can you afford (not just the cost--can you charge 6 batteries at a time in the pit), how much mechanical power does your robot need to use to play the game (and consequently, how much electrical power is drawn), etc.. When you have the luxury of designing from scratch, you can sit down and say, "I want three matches per battery, at a reasonable pace, or one match, flat-out-pushing everything around."

Since your robot probably can't be changed much from here on in, give us some specifications regarding what motors it uses, what reductions it uses, and the intended functions and duty cycles of those motors, and we can give you a more concrete example of what to expect. For example, does it (or perhaps, should it) quickly lift a stack of 4 tetras, 16 feet in the air, three times per match, or does it just flip the corner tetra off of its magnet once in autonomous mode--you can do both with a pair of F-Ps, but you don't have to care quite so much with one of the two implementations.

One other factor: there are different perceptions of when the battery is due for recharging. Some people don't care if their battery is showing 9 V on the OI; others try to put it back on the charger as soon as it drops below 11 V (on the OI). Personally, I tend toward the latter, rather than the former!

So, you should probably design the next robot for two matches without difficulty, with the option of stretching to a third. Fortunately, most robots end up with this sort of performance, without even trying. Since you don't have the benefit of these estimations for the current robot, you should probably attempt to change the battery after every match, replacing it with a fully-charged one. (A good practice, anyway.) This means, figure out how many matches you'll play at a regional, and factor in (partial) charging times, then see if you can afford the number of batteries and chargers that you come up with. Since you're from Canada, I'll assume that you're coming to the Waterloo regional or the Greater Toronto regional. Expect lots of matches in quick succession at Waterloo, because of the small number of teams there--you should therefore bring many batteries. 188 and 1114 both tend to have around 8 batteries at a competition, but these are both relatively well-equipped teams, and they usually have robots with higher-than-average power consumption needs.

If you wanted a short answer, it's buried in there, somewhere.