# TT or BO gear motor draws too much current and stalls: size the supply and driver

> Check the stall current of a small gear motor and match the driver and battery to it, so the robot does not reset or overheat.

Canonical: https://roboticsengineering.in/fix/tt-motor-draws-too-much-current-stalls-driver/ | Published: 2026-11-03 | Updated: 2026-11-04

**A small gear motor takes far more current when it stalls than when it runs freely.** Size the driver and battery for the stall current.

## What the message means

The current rises as the motor is loaded and is highest when the shaft is blocked. Hobby motors rarely come with exact figures.

## Fix it in this order

1. Measure the free-running and stall current with a multimeter in series, briefly, if you can do it safely.
2. Choose a driver whose continuous current is above the stall current of one motor.
3. Use a battery that can supply the sum of the motor currents without sagging.
4. Add a bulk capacitor near the driver supply.
5. Avoid stalling the motors for long.
6. Use the DC motor power calculator to check your estimates.

## Why it happens

A brushed DC motor draws current in proportion to the load on its shaft. When the shaft is blocked, nothing is turning to oppose the supply, so the current is at its highest. A freely running motor draws much less than that.

Small gear motors rarely come with exact figures, which is why a measurement is more reliable than a guess. Choose the driver and battery for the stall current, or limit the current in the design, so a blocked wheel does not overheat the driver.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Driver hot when pushed | Stall current above the driver rating. |
| Robot resets on load | Battery sag. |

## Related

[DC motor power calculator](https://roboticsengineering.in/dc-motor-power-efficiency-calculator/), [L298N hot](https://roboticsengineering.in/fix/l298n-gets-very-hot/), [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.

## Why this happens

A brushed DC motor behaves like a coil of wire with a back-EMF that grows with speed. At standstill there is no back-EMF, so the only limit on current is the winding resistance, and the current is roughly V divided by R. As the motor speeds up, back-EMF opposes the supply and the current falls. That is why the stall current can be several times the running current.

A driver adds its own voltage drop, and brush contact adds resistance, so the real stall current is somewhat lower than the simple estimate. Hobby TT and BO gear motors seldom come with datasheets that state stall current, which is why measuring beats guessing. The L298N-type driver typically drops a couple of volts across its transistors, which also heats the driver.

## How to confirm the cause

- Measure the winding resistance with a multimeter on the motor terminals. Turn the shaft slightly and measure at a few positions and use an average, since brush contact makes the value vary.
- Estimate stall current as supply voltage divided by that resistance. Use it as an upper bound.
- If you have a current-limited bench supply, stall the motor for no more than a second or two and read the current.
- Watch the battery voltage while one wheel is blocked by hand. A large sag shows the battery cannot supply the stall current.
- Feel the driver after normal use. Heat while pushing against a wall points to current above the driver’s rating.

## Worked example

A stall estimate for two motors with example numbers:

```
Winding resistance measured: 4 ohm, supply 6 V
Estimated stall current per motor = 6 / 4 = 1.5 A
Two motors stalled together = 3.0 A
A 2 A per channel driver is fine for one motor but the battery
and wiring must handle 3.0 A; with a ~2 V driver drop the real value is lower,
but size for the upper bound.
```

## Common mistakes

- Adding the free-running currents and calling it the total.
- Choosing a driver for the average current instead of the stall current.
- Letting the wheels push against an obstacle for several seconds.
- Using a small battery or thin wires that sag under three amps.
- Measuring the current with a meter on the 200 mA range, which can blow its fuse.

## If this did not fix it

- Battery internal resistance: a small or old battery sags under high current and resets the controller.
- Heat sink and ventilation for the driver, especially in a closed chassis.
- Mechanical binding in the gearbox or wheel axle that keeps the motor near stall.