2.2 Battery Monitoring

Keeping track of the battery voltage is battery monitoring in its simplest form. The system either powers off or the application notifies a battery replacement or recharging the battery if its voltage drops below a given value. The lowest acceptable voltage for the battery may be decided by the specified battery voltage range or a component in the system which cannot operate below a certain voltage level.

Depending on the system, the voltage over different components may vary based on more factors than the battery voltage. For example, in systems monitoring the current, the voltage drop across the shunt resistors is proportional to the current that passes through them. In those cases, the voltage may be measured in multiple places or combined with current measurement to ensure that every component is supplied enough voltage.

In most batteries, the discharged capacity is not directly proportional to the voltage. See also the figure below.

When estimating the remaining battery life, this relationship between voltage and capacity must be taken into account. If the ADC reads 3V and the maximum voltage for the battery is 6V, the remaining capacity is not necessarily 50%.

Finding the discharge curve for a specific battery can be done in different ways depending on the desired accuracy. If there is no need for high accuracy, use the discharge curve of a battery of the same type to approximate its behavior. If higher precision is needed, test the battery by applying a constant expected load and monitoring the voltage as it discharges to produce the discharge curve for the specific battery used.

Here, we discuss two methods for monitoring the battery state-of-charge (SoC).

The first method is to use the voltage to estimate SoC. This method would use the measured voltage to access a lookup table (LUT) to get the SoC. This method is simple but will give a poor estimation of the SoC if the mid-section of the discharge curve is flat.

The second method uses voltage monitoring in combination with current monitoring. A widely used method for current monitoring is Coulomb counting, which involves integrating over the current drawn to get the SoC. You can achieve a high accuracy regarding the SoC when using voltage monitoring for the discharge curve’s upper, lower, and non-flat parts and current monitoring for the flat mid-section of the discharge curve. However, this method is more complicated to implement than voltage monitoring.

Battery properties change with usage and over time and will therefore need periodical re-calibration. Different capacity estimation methods might require various calibration methods. Running the battery through a complete charge cycle (depleted to fully charged) and then a discharge cycle (fully charged to depleted) is a simple calibration method. Let the battery rest and cool down after each cycle to achieve a more accurate calibration. By doing this, you get new values for updating the discharge curve.