8.1 Use Case

Battery Monitoring Systems (BMS) ICs are a common occurrence in battery-powered electronic products. At the very basic level, these components continuously measure the battery voltage and the load current. With this information, it is possible to calculate and inform the user when a battery needs charging or replacing before the device becomes unusable.

For cost-sensitive devices, a solution around a voltage divider and an operational amplifier could be the answer (see Figure 8-2). The input to the voltage buffer is a ratio of the battery voltage, while the output of the voltage buffer can be internally routed to an ADC for acquisition and further processing.

$V_{OUT}=\frac{R_2}{R_1+R_2}\times V_{BAT}$

To minimize the current draw, the resistor values are chosen to be high, see Table   1 for typical values. Such a design choice usually poses a challenge for the microcontroller’s ADC. Normally the ADC inputs are not buffered and the input current into the ADC will cause errors in the measurement. This is where the voltage follower/buffer comes in by adapting the high impedance of the voltage divider’s output to the low impedance of the ADC input.

In this design, the battery voltage has an independent voltage from the op amp’s rail supply (which is usually the case). The battery voltage is brought within the amplifier’s common-mode input voltage range by the R1 and R2 voltage divider. For the case of a 2S1P Li-On battery pack, the safe operating voltage level can be anywhere between 5V and 8.4V. However, the supply voltage for the MCU is regulated at 3V. A voltage divider with a ratio of 1:8.66 for the R2 and R1 brings the monitoring voltage between 0.5V and 0.87V (see the voltage transfer function above). The internal 1.024V band gap reference can be selected as the reference voltage for the ADC.

The important aspect is the selection of resistor values for the voltage divider to minimize current draw and reduce its impact on the battery lifetime. Table 8-1 briefly shows the effect of the voltage divider resistor values and does not take into account any potential loads, such as the operating current of the microcontroller . The resulting values for the battery lifetime assume an almost ideal environmental operation of the design, with the Li-On battery in the nominal voltage of 3.7V/cell and a self-discharge of 20% of capacity.