# Can I replace rechargable batteries with (super)capacitors?

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?

• A LiPo charger may refuse to charge a "dead" (substantially undervoltage) cell, so you may need to replace the charger with something (resistor?) if there isn't enough leakage to get the voltage to the point where the charger considers injecting current safe. Commented Oct 23, 2018 at 16:14
• Keep in mind that, unlike a battery, the voltage on the capacitor will begin to drop the instant you start discharging it. You will need to see how much headroom you have voltage-wise, and also calculate the amount of energy you will need to see if you have enough capacitance to keep the thing up long enough. Commented Oct 23, 2018 at 16:20
• When contemplating caps instead of batteries, always compare the energy stored vs. volume & weight, because that's most often the deal killer. A 1-farad, 5V capacitor can store 12.5J at 5V. That's roughly the energy equivalent of a LiPo battery with a capacity of 1mAh. You can't find LiPo cells that small! Commented Oct 23, 2018 at 18:21
• @TimWescott Note that I am explicitly not trying to achieve the same runtime I would get with LiPo batteries. I'm looking for < 60s of runtime, for which I think the caps should be sufficient. Commented Oct 23, 2018 at 18:29
• @larsks: if you've done the math, all well and good. Commented Oct 23, 2018 at 20:01