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I just want to charge one lithium ion cell, but don't have the right equipment. I only hear it's possible to charge one if you use 4.2V, but that's only, because a higher voltage means higher current and therefor it could overheat and of course if you charge it for too long it will break it, because you are stressing the capacity.

But if I just make sure that it doesn't overheat and doesn't exceed it's rated capacity, what is the problem? Just limit the current with a resistor and wait till it reaches 4.2V?

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  • \$\begingroup\$ amazon.de/gp/product/B0191EVW0C/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 4 '18 at 22:22
  • \$\begingroup\$ That's how I would do it if I where out in the field with no other options. Just think about what will happen if you forget and the battery does go to 5 V. \$\endgroup\$ – winny Jul 5 '18 at 11:49
  • \$\begingroup\$ yeah, I know it's stupid not to just buy a charger, but I really just want to charge two cells and do some tests. I'll put it outside if I don't find a stone box or so where it would be save to do. \$\endgroup\$ – Cu29p Jul 5 '18 at 11:56
  • \$\begingroup\$ Li-Ion batteries are very sensitive to voltage levels and current. To avoid damage/shortened lifetime/unplanned combustion a specific charging profile should be used. There are relatively cheap (<5USD) off-the-shelf ICs/boards to do this (see e.g. here or for just the ICs here). The charge management IC will make sure that the proper voltage/current curve is followed (and do it much better than one could do by hand). \$\endgroup\$ – Alex Hajnal Mar 6 at 10:14
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Yes, you can do this, use a proper resistor, and then watch your DMM readings until it gets to 4.2 (or 4.35) V. But you need to be really patient, and better use a flame-retardant surroundings... The problem is in your possible lapse of attention since it will likely take several hours, depending on cell capacity and elected charge current. Li-Ion dedicated chargers do it for you automatically and more reliably.

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  • \$\begingroup\$ Alright, I'm not going to risk it in doors. But I'll do it outside and just see how long it roughly takes to go from 3.3 to 4.1 with the same setup so I can set an alarm clock for the next time. \$\endgroup\$ – Cu29p Jul 4 '18 at 22:53
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    \$\begingroup\$ @Cu29p, you better set up some comparator that would sound an alarm. \$\endgroup\$ – Ale..chenski Jul 4 '18 at 23:39
  • \$\begingroup\$ I don't have op amps (only 20 year old ones which are different) but I may be able to just drop 5V down to about 4V with a zener and limit the current so there is no chance of overheating. Put it in a flameproof box and it should be fine. \$\endgroup\$ – Cu29p Jul 5 '18 at 11:51
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Charging a cell requires a higher potential than the battery cell so you can force current into the battery and reverse the internal chemistry that releases power, so every charger has a higher voltage output than the cell it's charging.

The trick is to limit the charging current so you don't destroy the battery during charge. Check the capacity of your battery and charge at 0.2 C with respect to that capacity. You want to be cautious since you're doing things manually, and constantly check the battery voltage!

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