Battery management reference design needs no shunt resistor

April 23, 2014 // By Paul Buckley
A reference design board that measures current to an accuracy of ±1% by monitoring the voltage drop across a copper track on a PCB has been introduced by ams AG.

The development of the technique, which uses the ams AS8510 data-acquisition front end, enables designers of Battery Management Systems (BMS) to reduce bill-of-materials cost by eliminating the precision shunt resistor normally used in current-sensing applications. A precision resistor with low temperature drift can typically cost as much as $1.50 in volume.

Accurate current measurement is an essential function in a BMS, which provides functions including monitoring of a battery’s state of charge and state of health. The new reference design board from ams provides a blueprint for the current measurement function in a BMS, and can be applied in e-bikes, pedelecs and other applications drawing current of up to 40 A. The same design can also readily be adapted to measure currents of up to 100 A using only the resistance of a PCB’s copper track.

The new ams reference design takes advantage of the high sensitivity and precision of the AS8510, an integrated data acquisition front end which provides two measurement channels.

One channel is used to measure current by sensing the voltage drop over a 10mm section of a PCB track with a known resistance value and temperature coefficient. The other, matched channel measures the temperature of the copper track. This temperature measurement can be performed either internally by the AS8510, or by an external temperature sensor. By applying a compensation algorithm developed by ams, the AS8510 can eliminate the effect of the variation in the resistance of the copper track over temperature. This means that it can produce current measurements accurate to ±1% over its entire operating temperature range (-40°C to +125°C) without the normal requirement for a precision shunt resistor with a low temperature coefficient.

"When paired with a precision 100µ resistor, the AS8510 provides extremely high accuracy of ±0.5% over a current range from a few mA up to several kA," explained Bernd Gessner, Vice-President and General Manager of the automotive business unit at ams.