# 20V Max Lithium Battery "Charge Maintainer"

I have a gadget that uses a 20V max, LiIon, DeWalt compatible battery. It's detached for charging. A DeWalt charger is used to fast charge the battery.

The gadget has a POE ethernet connection. I've considered eliminating the battery and powering from a POE PD module, but maximum power requirements (50 watts) exceed the 30W available from the POE input.

However, the maximum power requirement is very low duty cycle. Like 5 seconds every 3 or 4 minutes. Call it 2%.

So, it wouldn't take much constant current (voltage limited) to maintain something like a 70% charge while the device is in use.

The device would be full charged with the external charger when not in use. I think this would eliminate concerns about cell imbalance.

I think a simple, current limited voltage regulator (e.g. 1 amp) should suffice, set to something like 18V.

The regulator wouldn't provide current until the battery voltage drops below 18V. When the battery drops below 18V, the battery current would be decreased by the 1 amp provided by the "charge maintainer" circuit (current limited voltage regulator).

When the power demand decreases (98% of the time), the 1 amp would charge the battery at 1 amp until 18V. Charge current would taper to zero as the SOC of the battery results in 18V with no current demand.

This seems super simple and safe. Issues?

• Using a supercap would be useful for a duty of 2%. Just take the good "value". Commented Dec 4, 2023 at 16:00
• With reduced time on the proper charger, is your series stack going to get enough cell balancing time? What happens if it only charges on your not-balancing charger? Commented Dec 4, 2023 at 16:02
• Capacitance required would far exceed that of a super cap. Call it 2 amps for 2 minutes. That' 240 Coulombs. Allowing a delta V of 2 volts, that would required 120 Farads! Commented Dec 4, 2023 at 17:25
• Yes, time on the charger is 10 hours or more (overnight). Even when completely discharged, the DeWalt charger indicates the battery is ready in 1 to 2 hours. Commented Dec 4, 2023 at 17:28