VESC: Adjust motor and battery current limits without damaging anything

After motor detection, setting the current limits is the most crucial step. These limits determine the power output of the motor, the demands placed on the battery, and the safety margin of the controller. An overly conservative setting will restrict the motor's performance; an overly aggressive setting can trigger the Battery Management System (BMS), cause the motor to overheat, or damage the controller. This guide explains each limit, using the names they are known by in VESC Tool.

Where can I find these settings?

In VESC Tool, the limits are found in the motor configuration, under the General section, on the Current tab for current values, and on the adjacent tabs for voltage, temperature, and power. To connect to the controller, see our guide for installing VESC Tool. Our VESC guide provides an overview of the entire tool.

Motor current and battery current

This is the distinction that raises the most questions.

• `Motor Current Max` is the maximum current flowing through the motor windings. It determines the torque, which affects the force during startup and acceleration.
• `Maximum Battery Current` is the maximum current drawn from the battery. It determines the power drawn from the battery.

The two are not equal. At low speeds, the controller converts a low voltage applied to the motor into a high motor current, drawing a much lower battery current. This is why the battery current is always less than or equal to the motor current, as stated in the VESC Tool documentation. A high motor current with a moderate battery current provides strong acceleration without putting excessive strain on the battery.

The battery current must never exceed what the battery and its BMS (Battery Management System) can handle; otherwise, the BMS will cut power during acceleration. See our guide on batteries that cut out during acceleration. On a scooter with two controllers, the battery powers both: it's the sum of the two battery currents that matters.

Braking limitations

• Motor Current Max Brake sets the braking current in the motor, which determines the strength of the electric brake. It is entered as a negative value.
• Maximum Regen Current (Battery) sets the current that is sent back to the battery during braking. This value is also expressed as a negative number.

The charging limit is crucial: a battery generally accepts significantly less current when charging than when discharging, and a fully charged battery accepts almost none. See our guide on regenerative braking.

Absolute current

Absolute Maximum Current is a safety measure. If the current exceeds this value, even for a moment, the controller cuts off all its outputs and records the ABS_OVER_CURRENT fault. The control loops normally keep the current below the limits; the absolute current value serves to stop a runaway situation. Therefore, it must remain above the maximum motor current, with a safety margin, while still staying within the controller's capabilities. See our guide on VESC faults and error codes.

Protections that limit current

• `Battery Voltage Cutoff Start and Battery Voltage Cutoff End: When the battery voltage drops below the first value, the controller gradually reduces the current, until it reaches zero at the second value. These settings protect the battery cells at the end of the discharge cycle.`
• MOSFET Temperature Cutoff Start / End and Motor Temperature Cutoff Start / End: The same principle applies to the controller and motor temperatures. The motor limits only function if a temperature sensor is connected and recognized.
• Maximum Wattage: A power limit, useful for capping the power output of a machine regardless of the voltage.

These gradual reductions prevent a sudden cut-off: the motor loses power instead of abruptly stopping.

The method

1. Start with the values you already know: the specifications of the battery (continuous and peak discharge current), the motor specifications, and the controller specifications.
2. Adjust the battery current to be below the BMS (Battery Management System) limit, and set the charging current to be within the battery's charging specifications.
3. Start by setting the low motor current, then gradually increase it in small increments.
4. After each stage, ride the vehicle and then check the real-time data in the VESC Tool to record the temperatures, the minimum battery voltage, and the currents reached.
5. Stop riding immediately if the battery voltage drops significantly, the motor overheats, or the controller reaches its temperature limits.

If the battery is the limiting factor, our scooter batteries in the Performance version deliver significantly more power than the original batteries.