Autonomo - What happens on a low battery

We where looking into cutoff circuits and questioned ourself: What will happen when the battery is low. Will it stop/sleep or will it drain the battery until it is damaged?


The Autonomo schematic is here: Using the schematic, it is visible that the only devices that are powered by VBAT are the XC6220’s. Checking the datasheet delivers that these don’t have a low battery voltage cutoff. However, If you open up a SODAQ battery, you will find a circuit like this between the actual battery and the battery leads:

It is hard to find data about specific circuits, but most commonly, this small PCB cuts the power at a lithium battery voltage of 2.3V - 3V. So your device is always protected. This little PCB of course uses some power: up to 10 μA.

The typical self-discharge of a lithium ion battery is about 1-2% per month.
A quick estimation/comparison:
The 1200 mAh SODAQ battery contains 4.4Wh (1.2 * 3.7). 10 μA at 3V for one month equals 0.0216 Wh, so the protection circuit will use about 0.5% of the battery capacity per month.
An example, using some math, it follows that with a discharge rate of 2.5% a month, a full-charged battery including protection circuit will have half its capacity after 27 months, which is >2 years.

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So the used battery itself has to be protected. Size did not really matter so we used 18650 cells.

Thanks for the answer! :+1:

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