General > General Technical Chat

Charging Lithium Batteries with USB: What Determines the Current?

<< < (3/3)

Buriedcode:

--- Quote from: EPAIII on January 26, 2024, 11:15:29 am ---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.

--- End quote ---

But 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 can limit the 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.

Siwastaja:

--- Quote from: Buriedcode on January 28, 2024, 03:12:50 am ---The supply can limit the 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.

--- End quote ---

USB 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.

Siwastaja:

--- Quote from: EPAIII on January 26, 2024, 11:15:29 am ---As I feared, no real, definite answer.

--- End quote ---

Real, 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 communication
and uses the lower value of the two.

Hope this helps.

Navigation

[0] Message Index

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod