I need to gauge a 9V alkaline battery to display using a MCU and a LED when the voltage drop below a certain treshold.
I checked on TI website most of gas gauge are for lithium battery. We have supposed that we could use an ADC on our MCU to do this with a few Resistor but we are worry to discharge the battery with resistor due to constant heat dissipation. What can I do to gauage this 9V battery efficiently ?
Battery voltage doesn't change very fast compared to the time of a A/D reading. One solution is therefore to enable the voltage divider only for just long enough to take the A/D reading, then do this infrequently.
For example, turning on the divider for 10 µs every second makes it draw 100,000 times less than when left on all the time. Now you can use a lower impedance divider too. Let's say the divider draws 10 mA (a lot) when on. The average divider current is now 100 nA, which is well below what the micro draws likely the self-discharge current of the battery.
To switch the divider when the input voltage is higher than the micro's power voltage, you have to switch the top side. A P-channel FET is one way.
All batteries have an equivalent capacitance, ESR and chemical cell voltage from which SoC can be computed by cumulative discharge current or floating cell voltage or loaded cell voltage much higher than the ESR of the battery. There is a secondary high ESR and larger C value as well due to absorption characteristics. (2nd order effects)
The most common method for Alkaline is float voltage or pre-determined load measuring V drop due to ESR rise as SoC drops and for LiPo's the cumulative I*t discharge is more popular.
Actual numbers depend on the battery quality and Ah ratings which affect ESR, C values as well as the hidden absorption RC value when the resistive voltage climbs back up when load is removed.
Try this quick N dirty design I did in Java https://goo.gl/EYl7ci
The LED current can be adjust with 470R, the series diode is not needed for comparators (open collector) and the diode can be any Silicon diode or even an LED with R adjustments used as a reference voltage.
Ignore the circuit used to test it with a slow sweep, but there are two sliders, Vbat and Threshold adjust. The 100k to 7.5V is shown in scope as mV above 7.5V to show hysteresis and LED ON thresholds for low battery. Using a 0.01Hz sweep.
