I've dumpster dived and hamfested a whole load of duff laptop battery packs.
I'm going to discharge them all then charge them to get the capacity of all the cells, triage the dead ones and sort into capacity groups.
I'd like to construct a 8000 mAh battery from the 18650 cells.
I thought about a 4S4P configuration, but say a fully charged unloaded 18650 has a voltage of 4.2 V, then 4 times this is 16.8 volts. This is too high for my radio (Elecraft KX3) which has a maximum specified voltage of 15 V.
The radio draws a few amps on transmit, but I'd also like to run a linear amplifier from the same battery that might draw something like 10 A
I guess there are several possibilities:
- A BMS board with a suitable current rating, that gives me 12 or 13.8 V out
- Using linear voltage regulators (can I parallel them to give 15 A?)
- Switching regulator, like a buck converter.
- Linear voltage regulators are very inefficient
- Switching regulators generate a lot of RF noise which needs to be filtered for RF power supply applications
I guess I could go for 3S4P, the max unloaded battery voltage is then 12.6 V, but I guess when loaded it's going to drop, my radio does have quite a wide operating voltage, so I'd get some use out of it.
So what's the best way to construct a 12-13.8 V battery from 18650s?