Capacitor smoothing question

Author	Message
Volhout Guru Joined: 05/03/2018 Location: Netherlands Posts: 5919	Posted: 08:05am 10 Mar 2025         lizby said   I'm trying to monitor the capacitor of various 18560 batteries--nominal capacities of 3400, 5000, and 9900!! mWh. The batteries are in an 18650 holder/charger with 5V USB output. My display shows voltage, current, and total mWh. My load is a little Harborfreight 3-AAA LED with 3 on positions--red LEDs, white LED, and red LEDs blinking every second with about 50% duty cycle. I removed the AAAs and directly wired 5V and 0V from the USB output. The white LEDs draw too much and the battery shuts off (even though it nominally has a 5V 3A rating). The red LEDs on solid draw between 360 and 390mW. This is nominally close to C/10 for the "3400mAh" battery. The battery shuts down at about 1080mAh. I'd like to be able to get closer to C/20, which I think is a typical test current draw for the ratings for these batteries. WIth the blinking red LED, the reading fluctuates every second between about 120mA and 300mA. I tried putting a 220uF capacitor across 5V and 0V, but that made no significant difference. My question is, what size capacitor am I likely to need to get the display to approximately half of the 360mW reading--to around 180mW? A 18650 is a huge capacitor itself. There is no-way you can add a smoothing capacitor parallel to it so a pulsed current load (pulsing LED) changes into a constant current load. A big inductor in series may do it. But since the frequency of pulsing is so low, this is not practical (the inductor may weight several kilo's). But in general: capacitors smooth voltage, inductors smooth current. Simplest solution (already suggested here) is to use a resistor to discharge (or accept the pulsing LED as a reference load), and count the time to get from full charge to 80% battery voltage). Volhout @Peter, aha..that is how they do it. A small cell inside a huge shell.