Re: Huge voltage drops and hot battery wires
If the wires past the main breaker aren't becoming unusually hot, there could be a short or a bad connection in the battery connector or the main breaker. Try replacing the battery connector on the robot. If this issue was present with multiple batteries, that probably rules out an issue with the connector on the batteries, but they're still worth a check, though this would cause the batteries to discharge very quickly even when not connected to a robot. As mentioned by others, replacing the main breaker wouldn't be a bad idea.