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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:

  1. A BMS board with a suitable current rating, that gives me 12 or 13.8 V out
  2. Using linear voltage regulators (can I parallel them to give 15 A?)
  3. Switching regulator, like a buck converter.

I believe:

  • 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?

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    \$\begingroup\$ Think of it this way... A reverse-charged lithium cell has a good chance of killing you, burning down your house, or both. Using 2nd hand cells in a pack is very risky - especially given what you have disclosed so far about your research. \$\endgroup\$ – Sean Houlihane Sep 16 '16 at 11:39
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The short answer: Don't even try

Using Li-Ion batteries in a pack requires a BMS at least, using parallel cells even more so. Connecting cells in parallel wil result in very high currents from one cell to another, resulting in (more) damage to both cells, even when the voltage differs only a few mV.

Constructing a working pack of different, possibly damaged, cells is very dangerous, even an almost depleted lithium cell can generate enough heat to get in thermal runaway. This can in turn heat the other cells enough for them to explode.

If you want to do something useful with these cells I suggest limiting yourself to a single cell at a time, and even then be careful.

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