I did search and read a lot of varying opinions on this site, I'm just trying to get a straight answer.

I have an ordinary 3.8v Lithium cellphone battery. I have a USB charging cord for it, but I understand the typical USB 2.0 port puts out 5 volt.

EDIT: I am charging this battery in the phone. So the question should really be--what voltage limit from the input can the phone handle when charging its 3.8 volt battery? It seems fine with 5-volt USB or a 5-volt cube/transformer. What about 12 volt, or more? This is the crux of my question. The phone clearly has some form of regulation in it and I'm wondering what those limits are, in a general sense.

Quite simply--am I hurting the battery by charging it like this, or should I only charge it with a dedicated wall-charge that specifies 3.8 volt output? The Samsung cube I have --that I thought came with the phone--says 5v output. I could be wrong, this cube could be from another device.

So I guess my question is kind of answered because I've been charging like this for years and never blown up a battery--but I just got a new battery and I want it to last more than a year. It seems after a year I get to the point where a charge will only last about 12 hours of normal cellphone usage, whereas when they are new I can get two full days or more out of a charge.


closed as unclear what you're asking by Daniel Grillo, placeholder, PeterJ, uint128_t, Bimpelrekkie Jun 7 '16 at 6:57

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  • \$\begingroup\$ Where did you get a USB charge cord for a cellphone battery? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 6 '16 at 17:34
  • \$\begingroup\$ Try something like a Max1555 charging IC. maximintegrated.com/en/datasheet/index.mvp/id/4002 \$\endgroup\$ – GisMofx Jun 6 '16 at 17:36
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    \$\begingroup\$ You need a specifically appropriate charge controller circuit to charge a lithium cell. The typical USB-connected "phone charger" is not actually a charger, but rather simply a switch mode power supply intended to power a charge controller within the phone. It must not be directly connected to a lithium cell and doing so would be quite dangerous. \$\endgroup\$ – Chris Stratton Jun 6 '16 at 17:39
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    \$\begingroup\$ @jimo3 - How exactly are you charging your "3.8v Lithium cellphone battery"? Are you leaving that battery in the cellphone and connecting the 5V mains adapter (the "cube" as you call it), or the USB charging cable, to the micro-USB port on the phone - yes? || You said "I did search and read a lot of varying opinions on this site" - the differences you found are likely due to how the questions were asked and what equipment was being used. That is why you need to provide more details of the exact connections which you are making (a photo would be good!), so that you can get a clear answer! \$\endgroup\$ – SamGibson Jun 6 '16 at 17:44
  • \$\begingroup\$ Not the cell alone, you will need a way of cutting off the power once the cell or packs are fully charged, most lithium cells I have used come with that circuitry included. For example, my 14V quadcopter Lipo charges at 16v. So you are not damaging it by charging it this way, but you should try and find some documentation for that battery. \$\endgroup\$ – Alex Jun 6 '16 at 19:35

If you want to properly charge a Li-Ion battery from 5 V DC you should use a charger circuit, there are cheap modules for sale on eBay containing everything you need.

The reason that your battery did not explode yet or has started smoking is because luckily it has a protection circuit build inside it. It disconnects the battery when it gets too hot or the voltage gets too high.

That protection circuit is not designed to make what you did safe in any way, you really mistreated your battery and it will have suffered from how you have been charging it.

What you have learned: NEVER charge batteries straight from a power adapter unless it is designed to connect directly to a battery and charge it. You will not find many of such adapters as almost no-one uses them.

Although mobile phone adapters are called "chargers" they're actually NOT chargers. They just provide power to the phone. The charging circuit is inside the phone.

  • \$\begingroup\$ And then OP mentions he is changing the battery in the phone. Geez... \$\endgroup\$ – Bimpelrekkie Jun 7 '16 at 6:58

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