I need a very small UPS just to give an embedded device time to shut down when power is lost. There are a variety of solutions that use LiPo batteries to provide extended runtime; they typically take USB input and manage both the charging and failover to battery on power loss.
Can I replace a single-cell LiPo (nominal voltage 3.7v) in a charging circuit with a pair of 2.7v supercapacitors wired in series and have it mostly "just work" assuming that (a) there is already some sort of current limiting in place on the charging circuit and (b) I include a passive balancing circuit between the capacitors?
In this situation, I would be under-charging the capacitors (to 3.7v, vs the combined 5.4v capacity in this example). What impact does that have on their behavior?
Best Answer
You could do this, but will it work? The charging circuit will be confused as it is built to charge/discharge at certain voltages. Because lithium ion batteries are considered dead below a certain threshold (like 3.0V) the UPS charging circuit may not switch on at all because a supercapacitor will have no voltage initially.
As far as the capacitor goes, it doesn't matter if it is charged below a threshold, the main thing to worry about is the breakdown voltage (max voltage) and that you don't exceed that rating. It can hold any value between the max voltage and zero.
Source: https://www.tecategroup.com/ultracapacitors-supercapacitors/ultracapacitor-FAQ.php