Overdischarge Protection
Last Updated: 3/6/19
Li-ion cells must also be prevented from discharging below 2.5V, at which point the cells can attain irreversible damage that reduces their capacity. The voltage regulators in the photovoltaic battery charger effectively create an overdischarge protection mechanism.
Overdischarge Protection ✅
Overview
As important as it is to prevent the Li-ion cells from overcharging, they must also be protected from overdischarge past 2.5V per cell. The voltage regulators controlling the battery output to the USB port effectively prevent the cells from discharging past 2.5V per cell, or 5V in series.
How It Works
While I initially planned a similar mechanism to the overcharge protection circuit to disconnect the batteries when their voltage fell below the 5V threshold, I recently realized that the two voltage regulators in the USB part of the circuit would effectively accomplish the same thing. A 5V regulator needs at least 5V to operate, and this coincidentally happens to be the threshold voltage I was looking to disconnect the batteries at. So, since the 5V regulator is the only component the batteries are connected to, the circuit will open if the batteries fall below 5V as the regulator will turn off and produce no output. Much easier than the overcharge protection problem!
Difficulties
None to report here as the voltage regulators essentially do the entire job for me.
Future Improvements
It would certainly be interesting to build an overdischarge protection circuit for a more general use case; however, I am more intrigued by the SoC and SoH estimation challenge and will be devoting my time towards that (below).
Last updated 3/6/19