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Charging Lithium Batteries with USB: What Determines the Current?
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EPAIII:
Being a careful person, I actually read the directions. And more than one lithium battery powered device I have purchased recently has had a 1 Amp charging current limit. They came with a USB cable but no charger.

I have a number of chargers including a charging station with multiple USB outlets and a power strip on one of my electronic benches. Some of these outlets are labeled 1 A and others 2 or 3 A.

Both the battery powered devices and the charging stations are not very expensive so price tells me nothing. But I am wondering what would determine the actual charging current if a 1 A device were plugged into a 2 A or 3 A USB socket? In other words, is that a safe thing to do or should it be avoided like the plague? Among other things, I am thinking about the tendency of lithium batteries to combust but there could also be lifetime considerations.

I would hope that with the plethora of USB devices available today that someone would have thought of this and set some standard. Anybody have a definite answer to this?
Smokey:
There is hopefully a charge controller IC somewhere in the chain there.  That's it's job to do constant current, then constant voltage charging and not break the cell.  It doesn't matter what source you are plugged into, a good charge controller will keep the cell happy at the required voltages and currents. 
The good news is that these charge controller chips are pretty inexpensive, and also pretty mandatory.  So the chances of something who's job it is to charge a cell not having a charge controller is pretty slim. 

Plus if you jack a cell expecting a max voltage of 4.2V directly into a 5V source bad stuff will happen, if not immediately then eventually when the voltage exceeds the cell maximum.  This is actually an interesting experiment that I haven't tried yet.  Might have to move a power supply out into the driveway tomorrow and do some science!
RoGeorge:
The charging current is limited by the charging IC/schematic inside the charger.  Preferably, charge Li batteries with their own charger (the one that came with the appliance), or with a compatible charger specified by the battery/appliance producer.

Apart from that, most Li-Ion cells (i.e. mobile phones batteries) have a small protection board inside the battery itself, usually on the same PCB where the contacts are, with a dedicated IC and a (thermal) fusible.  The inside protection circuit is supposed to disconnect the battery against damage conditions:
- at over discharge (less than 2.8V or so)
- at over charge (more than 4.3V or so)
- at over current/short-circuit

In normal use, the charger IC is the one that limits the max voltage and the max charging current applied, while the appliance is supposed to limit the max discharging and shut down before the minimum voltage is reached.

In fact, it is not unusual to have a smaller charging current at lower voltage (less than 3.5V or so), than a nominal charging current until the top voltage is achieved (4.1-4.3V or so, depending on the cell type), then a 3rd phase of constant voltage for a limited time (it is not good to keep Li-Ion at their max voltage for long periods of time).

Sometimes, after the appliance shut down, the battery might keep going discharged slowly, and after a while the internal battery protection will disconnect it (has a series MOSFET inside, between others), and the battery will measure 0V.  If you put it in the charger, after a couple of minutes the protection circuit will disarm and the battery will start charging normally.  So, if a battery shows 0V, don't abuse it by connecting the battery to higher voltage, as sometimes advised online.  Just let it charge normally, with its own charger, and be patient.
kjr18:
While charging ic's are indeed cheap and easy to use, you can still find products that skip them. While it might sound strange, really cheap electronics use a single silicone diode like 1N4007 to limit current. And some others skip this and use very thin cables to use their resistance as current limit resistor. So if you want to be sure you can use any usb charger, it won't hurt to take your device apart to learn what type of charge circuit it uses, of course if it is possible without damaging it.
Siwastaja:
The charger decides. USB power supply is not a "charger", even if mainsteam media calls it that. It's just a 5V power supply. Charger is usually a small integrated solution within that "battery-powered device".
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