4QD-TEC Electronics Circuits Reference Archive 
Battery Current Sensor: Part 1

This was developed for a customer who insisted on having a battery current limit fitted to a controller. The principle is to measure the battery current and if that exceeds a preset value, use it to turn down the demand speed.

The circuit was developed for use with the 4QD series controllers specifications. These controllers have an inhibit input and the effect of shorting this down to ground is to turn down the internal demand speed. So with the battery current limiter fitted, the internal demand speed can only ramp up to a value which engages the limit.

The basic circuit is as shown. The 4QD-TEC members area information gives a circuit board layout and details of all components and the slotted ferrite ring used to sense the current.

Bcl

Constant power throttle

Some enthusiasts of electrically assisted cycles believe constant power acceleration is the ideal response. Although personally I am not convinced that constant power acceleration is the ideal, limiting the battery current gives constant power acceleration whenever the limit is engaged.

Controllers are very nearly 100% efficient, motors rather less. So there is an inaccuracy due to motor losses. However, ignoring these, all the power taken from the battery is delivered to the motor. For a particular system, the battery voltage is essentially constant, so limiting the battery current drain during acceleration forces the system to take a constant power from the battery, and thus to deliver a constant power to the motor. Certainly there is no simpler way of incorporating a constant power throttle system! 

Other pages of information relevant to this:

Part two of this article in 4QD-TEC Members Area Information