# Battery protection board cuts power when the motors start

> Why a lithium pack with a protection board shuts off under motor load: over-current limit, voltage sag and what to check.

Canonical: https://roboticsengineering.in/fix/bms-protection-board-cuts-off-under-load/ | Published: 2026-11-03 | Updated: 2026-11-03

**A protection board cuts the output if the current goes above its limit, even for a short time.** A motor start-up can do this.

## What the message means

Motors draw a large current when they start or stall. The protection chip sees the peak and trips, and the robot loses power.

## Fix it in this order

1. Check the current rating of the protection board against the peak current of your motors.
2. Measure the start-up and stall current if you can.
3. Use a board with a higher rating, or a pack made for high discharge.
4. Soften the start with a slow ramp of the PWM.
5. Do not stall motors against a wall for long.
6. After a trip some boards need a charger connection or a short wait to reset.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Cuts off when the motors start | Over-current trip. |
| Cuts off late in the run | Under-voltage cut-off. Charge the pack. |

## Related

[battery management systems](https://roboticsengineering.in/battery-management-system-robotics/), [battery sag](https://roboticsengineering.in/fix/lithium-cell-voltage-sags-robot-resets/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.