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I have a 3p12s 18650 Li-ion battery pack that I use for my e-bike. I charge it with a balance charger. I know that charging with too high current is bad for battery life. But is it "the lower the better"? If not, is there any recommended minimal charging current? Is charging at 0.1 C safe? My only goal is to prolong the battery life (number of cycles). I charge it only at night so I don't care about it taking long time.

In the data sheet of the cells I use (Sony US18650V3) there is only specified maximum charge current 1 C. It says nothing about minimum charge current. What's more, I've never seen any batteries data sheet mentioning "minimum charge current".

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  • \$\begingroup\$ C/10 is safe but then CV cutoff needs to set to 5% to 10% CC rate minimum but CV must never exceed 4.2 electronics.stackexchange.com/questions/270292/… and lowV cutoff ought to be increased as much as practical for shorter useage but overall longer Ah lifetime. Thermal sense display is best for monitoring ageing rate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 1 '19 at 0:09
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Instead of thinking about what charging regimes will prolong battery life, it's probably best to flip that on its head and say what use regimes will reduce the number of cycles, and then avoid those.

The best way to kill Lithiums is to charge to too high voltage, or discharge to too low voltage. Sacrifice some capacity by charging to less than 4.2v, and stopping before you get to the end point, and you'll avoid killing your cells. Pro-tip, this is what electric car manufacturers do to be able to give an 8 year warranty on the traction battery pack.

The next way to reduce battery life is to run them too hot.

Once you've taken car of voltage limits and temperature, high charge and discharge currents can reduce the life, but not by as much as those first two factors. A low current does not reduce life.

The only way a low charging current might contribute to a reduced life is in the hands of an inexperienced designer who thinks that lithium cells behave like nickel or lead, and that if the current is low enough, then a gentle overcharge is permissible. With lithiums, no overcharge is ever permissible, see the second paragraph.

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  • \$\begingroup\$ Also charging at different rates but to same voltage does not contribute to the same level of charge. Eg: at 1C to 4.2V has a bit less level of charge compared to charging at 0.5C to 4.2V. Therefore charging at too low (eg: 0.1C) to 4.2V can actually do more harm than charging at say 0.5C to 4.2V. \$\endgroup\$ – geeth Sep 22 at 0:25

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