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I get some 18650 cells from old laptop battery. Cell voltages are good and seems healthy. I want to use this cell as a battery pack for some of projects. I add protection circuit to prevent short-circuit undervoltage. But i dont have proper charger for this.

Now is it ok to by charge from other sources. I have buck converter with CC/CV pot. Can i charge 18650 cell by set th CC(~4.2V) and CC(0.4C ~1000mA)?

Do i have to take extra safety steps while charging or is it ok if i can just control Voltage and Current?

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  • \$\begingroup\$ An easy way to do this "well enough" is to us eg CV set to about 4.0V. Set current limit to Imax_allowed - usually C or C/2 depending on cell. The cell will charge to 4.0V and current will then truckle to zero. As long as voltage is well below 4.2V (say 4.0V or less) this is "safe enough". Doing this at 4.2V will destroy the cell in far fewer cycles than usual. Some capacity will be lost compared to a full charge BUT cycle lifetime of cells will increase for a net increase in mAh x cycle_lifetime. ... \$\endgroup\$ – Russell McMahon Oct 29 '17 at 22:52
  • \$\begingroup\$ ... Setting Vmax to about 3.7V gives maximum overall lifetime capacity and about 50% of full capacity per cycle. Lower charging current increases capacity achieved per cycle in this mode. \$\endgroup\$ – Russell McMahon Oct 29 '17 at 22:53
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It should work, if the settings are accurate enough. You can set it to 4.1V for extra safety, you won't lose much capacity. Check voltage and current with a multimeter...

You will need to monitor the voltage to know when the charge is finished though. It is not recommended to let the battery charge at 4.2V for too long (most charge control ICs include a timer and various other safety checks).

From mkeith: The standard method of charge termination for lithium ion (and lithium polymer) chargers is by current. During the CV stage, if the current drops to some low level, the charger stops charging. The timer would be a backup termination method in case the primary termination condition never occurs. For a single 18650, the charger might terminate at like 50 mA or something.

Also make sure the cell is within the acceptable temperature range. If this happens indoors, it should be.

This starts to sound like a lot of work.

You can buy a 2-cell 18650 charger for little money these days, so that would be a better and safer solution.

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  • \$\begingroup\$ Regulating the voltage is not enough: The current also should be limited. batteryuniversity.com/learn/article/… ineltro.ch/media/downloads/SAAItem/45/45958/… \$\endgroup\$ – Solomon Slow Oct 29 '17 at 16:06
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    \$\begingroup\$ The standard method of charge termination for lithium ion (and lithium polymer) chargers is by current. During the CV stage, if the current drops to some low level, the charger stops charging. The timer would be a backup termination method in case the primary termination condition never occurs. For a single 18650, the charger might terminate at like 50 mA or something. Feel free to add this to the answer. \$\endgroup\$ – mkeith Oct 29 '17 at 16:12
  • \$\begingroup\$ @jameslarge the asker mentions CC/CV potentiometers on his supply so I assume it has an adjustable current limit as well as voltage... Personally I wouldn't trust a pot that much (especially one in a cheap ebay junk product) due to aging etc, I would wonder if the circuit guarantees voltage and current drop to 0 (and not to the max value) in case the wiper has a bad contact, etc... \$\endgroup\$ – peufeu Oct 29 '17 at 16:36

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