Hi Andy

Mark's reply above is correct, but is only half the answer.

You need to know what your running current (amps) for each motor, at full throttle - you can usually find this on the spec sheet that came with the motor, or by doing a web search.

For instance, let's assume that each motor takes 3 Amps - 3 Amps x 2 motors = 6 Amps from your battery. If your battery is 5 A.hrs (or 5000 A.hrs) capacity this will give you around 5/6 hours run time, or about 50 minutes. You can substitute any motor running Amps and any battery capacity into this equation.

Note though that this is only a very rough approximation. It depends on how well the battery is charged in the first place, and how low you want to discharge it. The lower you discharge it, the less overall working life the battery will give, and the sooner you will need to replace it. Most battery suppliers would recommend not going below about 50% "depth of discharge", so your 50 minutes above would become 25 minutes. (As RC modellers, we might go as low as 75% depth of discharge, as we use our models so infrequently that reduced battery life is not much of a consideration.)

Some ESCs have a low battery voltage cutout - this is not necessarily a good thing. Imagine your boat out there on the water and the ESC cuts out - you have to swim out there to get your boat.

Des.