Battery Life, 50A load discharge test method/results

[KenWittlief]

This is great work. Will be interesting to see the graphs. Usually a battery is 'dead' when the slope of the graph takes a turn downwards (the knee).

Im thinking that not many teams will be happy starting a match off with a battery that is down to 9 or 10 volts at the start of a match.

One of the reasons that batteries seem to discharge faster than the A-Hr rating is that batteries go dead by increasing their internal resistance. At any given state of discharge a higher current will cause more voltage drop than a lower current.

Therefore, if you discharge a battery at 50 amps the under-load terminal voltage will drop to 9V sooner (A-Hr sooner) than if you had discharged it at 10 amps. The way you can see this best is to disconnect the load, the voltage will shoot back up.

I noticed one thing about your typical robot-loading numbers. Reading the stall currents on motors separately can be misleading. The current through the wires and connectors on the robot will cause a voltage drop in the system, so having 4 motors in parallel on the battery circuit will cause more voltage drop, therefore the stall current will be less.

also, the motors are on 40A breakers, and the entire robot is on a 120A breaker, so if your calculations are saying the robot is drawing an average of 160A while stalled, you have not taken the voltage drop in the wiring into consideration, or the fact that breakers will start kicking off.