I have read articles here and there about charging Li-Ion batteries. Most of them try to solve the problem of maximizing capacity and also maximizing cycles. The most recommended method is charging with around .5C current until the battery reaches 4.2v and then switching to the constant voltage until the current gets less than a threshold and then stopping it.

I don't need the battery to be fully charged, even half the capacity of 250mAh battery can run my circuit for days, and the circuit is almost always using external power source. I want to see if using no control circuit, but offering a lower voltage, for example 3.7v and limiting the current to for example 50mAh will hurt the battery in long run. Minimum parts in my circuit is essential for me, so I would be glad if I could remove as much as possible! Thanks in advance.

  • \$\begingroup\$ Pulse charging to 4.0V. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 23 '15 at 18:34
  • \$\begingroup\$ Ok. But I still don't get that can I run this method constantly while the external power is on or should I terminate it at a point. I prefer to have method that doesn't need to be stopped. And I want to know if such a method would damage the battery in long run. \$\endgroup\$ – Arash Jun 23 '15 at 18:40
  • \$\begingroup\$ Once the battery reaches 4.0V you stop. You can use a comparator (during the off time) to determine this. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 23 '15 at 18:41

Are you familiar with simple modules like this ? It is basically a simple TP4056 board, for sure 1A is too much for you but a simple exchange of a resistor fixes that.

Although the suggested pulse charging to 4 V will work without doubt and I also think that that is a method that is safe enough. If you need to make the pulse and compare voltage then maybe a TP4056 (or any other similar IC) will be less complicated. The IC takes care of everything so you don't have to :-) And for the price, I know what I would choose.

  • \$\begingroup\$ Thanks for this hint, although I would wait for alex's experiment and my own, but I didn't know about TP4056 IC, and the circuit is pretty easy and straightforward. Thanks. \$\endgroup\$ – Arash Jun 24 '15 at 19:06

Li-Ion batteries do not have memory; so charging it at 3.7V should not hurt it. I am currently testing what you described on some of my batteries, to see the effect. The picture that I have thus far looks like the following. Depending on the manufacturer and on capacity of the battery, levels as high as 4V at C/50 seem to be sustainable long-term without hurting the battery. But due to the amount of time it takes to do one test (an order of couple months), I do not have very many tests completed to give you a more precise answer.

Edit: I have completed my testing. What the commenters have said is correct. After prolonged charging at 4V, the battery reaches 4V, and the consumed current decreases to a minimum. Some factors:

  • Keeping the battery charged at a higher voltage makes it consume more current. Since this also reduces the life of the battery, I only recommend keeping batteries charged at the level which you need, and not higher.

  • Batteries which are better at holding current consume less current, probably due to lower internal leakage.

Batteries tend to get relatively little damage by staying charged at 4V; however, keeping them charged still contributes to their lifespan. For example, keeping the battery charging at 4V 1mA for a month means the battery has consumed 720mAh of power. It would take too long to test with 1 charger, whether that is comparable with just regular consumption of 720mAh, or if it's more or it's less harmful to the battery.

  • \$\begingroup\$ The thing is that about 75-85% of the energy in a Li-ion is contained between 3.4V and 4.0V, so charging only to 3.7V reduces the uptime of the device to about 25% vs. charging to 4.0V and 20% vs. charging to 4.2V. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 24 '15 at 1:21
  • \$\begingroup\$ Thank you for pointing this out; however, in my answer I said that charging them as high as 4V does not hurt the batteries. The reason I put the 3.7V there is, because that's the number that the OP used, and that's a number that was "good enough" for him: he said he didn't need full capacity anyways. \$\endgroup\$ – Alex Jun 24 '15 at 13:40
  • \$\begingroup\$ Ok, what alex says is much closer to what i was looking for. There is a thing though, as far as I have checked with my power supply, when such condition is applied to the battery. after a while, the battery reaches the highest capacity it can get in that condition, maybe 50-60 percent of the total capacity. After that there is a constant current usage, which I guess is wasted on battery internal resistor. It is a low current, about 10mA in my case, so another form of the original question would be would that heat or current consumption damage the battery? \$\endgroup\$ – Arash Jun 24 '15 at 19:03
  • \$\begingroup\$ I don't think the battery will keep consuming a current of like 10 mA. I think that if you for example connect it via an external resistor to a 4.0 V supply, the battery will reach 4.0 V after some time and the current will decrease to some micro Amps (I guess due to leakage). It's not like NiMh batteries which you can charge for ever with a small current and where the excess energy is indeed dissipated by the internal resistance. \$\endgroup\$ – Bimpelrekkie Jun 25 '15 at 7:23
  • \$\begingroup\$ After a long charge, using a 120r resistor and 4v, the current has reached a little less than 0.001A. \$\endgroup\$ – Arash Jun 25 '15 at 12:52

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