I have a BLDC 72v 3000watt motor&controller..

[Couchmike101]

Unsure of my next steps.
When I apply the throttle, my ebike looses power and the volt meter shows the voltage going down to 0.
Even when throttle is released, the voltage stays at 0. It resets and acts like it will work normal after pulling the ground and reattaching but the bike does the same again. Wiring issue or battery issue?

 

[Sierratim]

Where are you measuring the voltage, at the output of the battery management system or the motor input after the controller?

 

[Couchmike101]

From the battery before the controller.

When I give the bike throttle, it revs up for a split second and then power descends to 0 no matter what I do or try.
Also, turning the key off and in doesnt reset it. I have to disconnect a battery wire and reconnect to get it to work again.

 

[funnel]

Sounds like some sort of overcurrent protection of the BMS. Or possibly the battery voltage sags so much on high power that it kicks in an undervoltage protection of the BMS and shuts it down.

 

[Sierratim]

From the battery before the controller.

When I give the bike throttle, it revs up for a split second and then power descends to 0 no matter what I do or try.
Also, turning the key off and in doesnt reset it. I have to disconnect a battery wire and reconnect to get it to work again.

Sounds like the BMS is detecting a low voltage on one of the cells as soon as you start to draw power. It then shuts off the pack to protect the weak cell. If you open up the pack you might be able to find and replace the weak cell.