I recently came across several old laptop batteries that no longer seemed to work well in their respective computers. Pulling them apart yielded the expected 18650 Li-Ion cells. The exception to this was an external add-on battery for HP laptops. In that case, the battery was comprised of 103450 Li-Ion cells. In all of these supposedly “dead” batteries, I found that the cells were still showing a charge that was well above the safe minimum voltage for a Li-Ion (3.0V – 3.2V depending what source you find online). This meant that while the laptop charging circuits may not have been happy, the cells still likely have a bit of life in them.
With the stockpile of used cells, I wanted to find a way to charge them up using parts I had on hand. Searching around, I found that the best way to charge these cells was to use a CC-CV (Constant Current – Constant Voltage) style charging schematic. Searching around, I found a suitable example:
The simplest way to do this was to wire two LM317 circuits back to back as shown above. The first half is configured as a current limited source with the second configured as a constant voltage source. There are many more complicated circuits out there, but this one fit the bill for what I was looking to do.
Using parts I had on hand, I came up with the following:
The two are functionally equivalent. The current limiting section is set to a limit of approximately 500ma with the 4x 10 ohm 10 watt resistors in parallel:
Rtotal = 1/(1/R1 + 1/R2 + 1/R3 + 1/R4) =
1/(1/10+1/10+1/10+1/10) = 2.5Ω
IOUT = 1.25V / (Rtotal ) = 1.25V / 2.5Ω = .5A
The combined wattage rating of the resistors I had on hand is admittedly overkill (40W total) for this purpose:
P = I2 x R = .5A2 x 2.5Ω = .625W dissipated
The voltage limiting section is configured to provide 4.125V.
VOUT = 1.25V * ( 1 + R2 / R1 ) = 1.25V * (1 + 230Ω / 100Ω) = 4.125V
I’ve charged several cells using the schematic above while monitoring with two multimeters (one in series with the battery to monitor current, and one in parallel to monitor voltage) and the circuit behaves as expected. As the voltage starts reaching the target (4.125V), the current begins to drop off. It can take quite a bit of time to finish up the charge using this method, however. I usually stop when I get to around 50ma charge current and/or around 4.09V.