FIG. 14 shows a typical function of maximum torque plotted against the battery voltage. As can be seen, at low battery terminal voltages where it can be considered that the battery is partially depleted it is beneficial to greatly limit the current drawn by the motor. When the battery is fully charged, in this case above its nominal 12 volts, the current drawn can be held at a constant high limit. This is typically chosen as a function of the maximum rate at which the alternator can replenish the drawn charges, and ensures that over time the battery does not become depleted. Other factors may be relevant in the setting of this maximum limit.

FIG. 15 shows the effect of the limit on the torque demand over time. The current that is demanded from the motor, or generated by the motor, is capped at the limit. Since the current demand is a function of the torque demand, this means that the torque demand can be limited to achieve the required cap provided that the algorithm is aware of the required limit and the operational parameters of the motor.

Prior to limiting the torque, the torque demand limiter in a first block shown in FIG. 6 calculates a set of current limits. As shown there is a limit calculated for the motor battery when the motor is in a motoring operation and drawing current. A limit is set for the battery current when the motor is generating. Limits are also set for the motor drive stage bridge when motoring and the drive stage bridge when the motor is generating. The limits may include a motor battery current limit 29, a generator battery current limits 30, a motor electric power limit 31 and a generator electric power limit 32.