A lot of things could be the problem. I agree with Vic. Battery voltage should not be dropping below 80 to 85%% of whatever the voltage the truck is when you are trying to accelerate. So if it a 36 volt truck, voltage should not be below 28 or 29 volts when accelerating.
Accelerator voltage should be 0.5 volts to 0.0 volts with the pedal pushed to the floor. The contol card has 6 screw terminals. The top left termianal is accelerator voltage. Measure between battery negative and this terminal.
Other than that, it would be a bad flyback diode (3 rec), bad drive motor, brakes dragging, a bad conrol card, an improperly wired card, etc,.
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