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We know that a charger is just an adapter that supplies a constant voltage (5 volts usually). The actual charging circuit is in the phone itself. With a bit of soldering and wiring, can I use my old phone as an Universal Li-ion battery charger to charge other phone batteries? Does this have any downsides?

(My old phone uses a 3.7V Li-ion battery and the battery I intend to charge is also a 3.7V Li-ion battery with a different capacity.)

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I would strongly recommend against that. Some charging circuits are designed to charge a battery at a specified current. That current is chosen to be as high as possible without risking damage to the battery. If you try to charge a different battery at that current, a battery which may not be able to handle that much current, there's at least some chance it will heat up and start a fire.

In general, I wouldn't mess with Li-ion batteries unless you're very sure about the specs of both the battery and the charger. And even then, I'd be careful.

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    \$\begingroup\$ The current at which a battery is charged can't exceed the maximum current supplied by the adapter. I notice old phones usually come with low current adapters (range from 500mA to 800mA), while new phones' adapters can have up to 2A. \$\endgroup\$ – Hieu Jun 1 '14 at 1:45
  • \$\begingroup\$ @Hieu That's the power; the current could be increased with a buck converter. Although really old phones are unlikely to contain buck converters, I think. \$\endgroup\$ – immibis Mar 2 '16 at 23:29
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There might be a chip in the battery that authenticates it to the charger so that the phone wouldn't charge any battery but the one it's designed for. At the least, if you see any pins besides + & - on the battery then the charger is probably looking for information on those pins to decide whether or how to charge. Could be only a thermistor to measure temperature or could be a lot more than that going on.

If you built your own charger, or used one designed to charge Li-ion, you might be able to override the authentication, ignore that pin and give the pack charging current anyway. As Stephen said, there are serious safety issues, potentially including fire or explosion of the battery. Having said that, I've recharged a cell phone battery (which I found in the park) simply by connecting it to a voltage- and current-limited bench supply. You want to keep the voltage below where it could be too high for any chemistry that might be in the pack, and the current way below a 1-hour charge, say a 5 hour charge, and disconnect it if it gets warm or goes over the amount of time you expect it to require to charge. Which is to say, charge it slowly and probably not all the way up.

It could still wind up being unsafe, if you overcharge in any way, if the pack is already damaged, if your charger fries some other circuit in the battery pack, etc. If you understand how the appropriate charger would work, you can imitate a very conservative version of that behavior and it will probably be ok. Be sure you don't care if the battery gets ruined and put some protection against fire/explosion around it (concrete blocks) before you start experimenting. Most people on this site will advise against anything which might accidentally cause fire/explosion, and that is sound advice if you don't know what you're doing. However, I've seen videos from laptop makers who deliberately overcharged batteries until they exploded just to find out what would happen, so they could try to design a safer pack. They put it under a Plexiglass (not glass!) box, step far away and let a camera watch it, have remote disable of the charger, and so on. Every engineer has stories of capacitors soldered in backwards that caught on fire when powered up or things like that. If you take the danger seriously it is possible to mitigate the safety risk.

If all you're trying to do is charge your digital camera battery with your flip phone, because you don't want to pay to replace the proper charger you lost somewhere, probably this information is not for you.

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