Charging Lithium Batteries with USB: What Determines the Current? - Page 1
Forum section: electronics / general

author: EPAIII
author_rank: Super Contributor
timestamp: January 25, 2024, 07:55:49 am
content: 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?

author: Smokey
author_rank: Super Contributor
timestamp: January 25, 2024, 08:17:21 am
content: 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!

author: RoGeorge
author_rank: Super Contributor
timestamp: January 25, 2024, 08:55:51 am
content: 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-circuitIn 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.

author: kjr18
author_rank: Regular Contributor
timestamp: January 25, 2024, 09:25:03 am
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 25, 2024, 01:20:51 pm
content: 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".

author: Veteran68
author_rank: Frequent Contributor
timestamp: January 25, 2024, 01:36:08 pm
content: Quote from: EPAIII on January 25, 2024, 07:55:49 amStandard USB ports, including chargers, by definition default to 5V @ 100mA max (USB 1.0), 500mA max (USB 2.0), or 900mA max (USB 3.0), until a higher current is negotiated by the connected device (either through a resistor configuration or data signaling). USB2 and USB3 along with power profiles like QC introduced higher current options but again they must be negotiated by both sides before being enabled. So the USB circuit on the device end is responsible for telling the USB source the power it requires.If your Lithium batteries include a USB cable but no charger, then I'm assuming they're the kind with a built-in USB port for plugging the battery directly into a charger. There will be a USB/BMS circuit in the battery that manages the power/charging parameters, like any other USB device.

author: Smokey
author_rank: Super Contributor
timestamp: January 26, 2024, 01:08:28 am
content: Quote from: Veteran68 on January 25, 2024, 01:36:08 pmhttps://www.eevblog.com/forum/chat/what-happens-when-you-draw-too-much-current-from-a-usb-port/... In theory, yes. But as described above, compliance with the "standard" is hit or miss, with way more "misses" than hits.

author: EPAIII
author_rank: Super Contributor
timestamp: January 26, 2024, 11:15:29 am
content: Ahhh!Just when you think it's safe to go back in the water .....As I feared, no real, definite answer.But thanks for all the answers that were provided. I do appreciate it.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 26, 2024, 12:06:44 pm
content: It's the device you are plugging in (the load) who determines the current, not the USB socket (the power source). The USB power source only has to be rated bigger or equal with what the charger will surge.To get an example, a USB socket rated 3A can feed any USB device, no matter the device plugged in that 3A USB socket is a 1A charger, or 0.1A, or 2.5A, doesn't matter, as long as the device is less than the max rated 3A of the USB socket.

author: ebastler
author_rank: Super Contributor
timestamp: January 27, 2024, 11:13:03 am
content: Quote from: EPAIII on January 26, 2024, 11:15:29 amThe most likely situation (unless you bought some nasty bottom-of-the-barrel product) is that there is indeed a charging IC inside which controls the current to the battery, and that you misunderstood the device's specifications.If the spec says "5V, 1A", that does not mean that you are responsible forlimitingthe provided current to 1A. Rather, you need to connect a 5V supply which can provideat least1A without overheating, because that (or close to that) is how much the charging circuit may want to draw.

author: Buriedcode
author_rank: Super Contributor
timestamp: January 28, 2024, 03:12:50 am
content: Quote from: EPAIII on January 26, 2024, 11:15:29 amBut you did get a difinitive answer. USB power supplies are voltage regulated - it is the supply that determines the voltage, but the load that determines the current draw. The supply canlimitthe current by reducing the voltage down to a point where the amount of current that flows is below the limit, but it isn't a particularly accurate limit.So it is your charger that determines the current providing the supply can provide that current at its rated votlage.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 28, 2024, 09:13:01 am
content: Quote from: Buriedcode on January 28, 2024, 03:12:50 amUSB does not support this kind of mode. It's the responsibility of the load to not exceed the available current, which is communicated using one of the many ways, otherwise the load is broken and non-compliant. For broken loads, the source (power supply) may (or may not) implement an electronic fuse functionality, which completely disconnects the power. Using a constant-current mode as you describe, by limiting voltage below the minimum allowed USB voltage, would be rare - and non-compliant, too.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 28, 2024, 09:15:32 am
content: Quote from: EPAIII on January 26, 2024, 11:15:29 amReal, definite answer is this: The charger, which is NOT the power supply, but the device being charged, decides the current, given the limitations it knows:* It knows the maximum charge current the actual li-ion cell can handle;* It knows the maximum current the USB power supply can give, by one of the many ways of communicationand uses the lower value of the two.Hope this helps.