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I understand at a basic level that a USB DCP simply advertises that it is a DCP, and then basically puts 5V on Vbus, and then the battery can go to town.

With that in mind this is really a 2 part question:

  1. How does a battery limit it’s current to the 1.5A specified by the USB BC 1.2 spec? Take the example of a charger capable of outputting 2A, advertising as a DCP. Assuming the device being charged is compliant with the spec, how does it limit the current draw to 1.5A given there is no communication between the two devices? Does it modulate some sort of load switch? Does it even limit in the first place?

  2. Are USB portable devices responsible for implementing the typical lithium ion battery Constant-Current/Constant-Voltage charge cycle? If so, does it taper the current by some method described in (1)? Or perhaps it relies on the increasing impedance of the battery as it becomes more fully charged? Perhaps it doesn’t implement a CC/CV charge cycle at all?

I appreciate any feedback on this. It seems like the answer should be fairly straightforward, but it’s been bugging me for a couple days.

Thanks!

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  • \$\begingroup\$ "advertising as a DCP" is done through "communication between the two devices". \$\endgroup\$ Feb 24 at 5:22
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Chargers that use the USB VBUS as power source are very sophisticated mixed-signal processors, with extended analog functions and digital controllers. Below is a typical simplified diagram of a charger IC that takes and limits the intake, charges a Li-Ion cell in accord with source capabilities and follow CC-CV charging algorithm, and provides system voltage output to mobile device processors and other circuitry:

enter image description here

Brief explanation of entire functionality can be found in the datasheet. The IC takes substantial amount of analog information, digitally processes all thresholds, and generates necessary limits, timeouts, and all other sequencing.

In short, modern charging ICs do limit the input current intake by changing switching regulation parameters. The IC will reduce the cell's charging current if it is required to maintain the input limit.

So, to be compliant (and functional), USB devices usually incorporate an IC that implements BC1.2 protocol, so there IS A COMMUNICATION between the DCP (provider) and the consumer (the device). The BC1.2 IC determines the type of port, and this information gets communicated to the charging IC (bq25606 as above). The charging IC then maintains the appropriate input limit.

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Even though DCP has "Charging" in the name it does not incorporate any charging functionality inside it. Think of the USB DCP as a 5V power supply with a maximum output current of 1.5A.

The device on the other side of the cable will have its own charging controller circuitry that will limit the current and voltage that is delivered to the battery.

For Li Ion and Li Polymer batteries the standard way of charging is using constant current (whose value will depend on the battery itself) till the battery voltage reaches 4.2V and then using a constant voltage (4.2V) after that point. The controller takes care of the battery voltage monitoring as well as current and voltage control. It should also make sure that the whole system never draws more than the 1.5A available from the DCP.

For small batteries, a linear controller is typically used, where a simple series transistor is responsible for the regulation. For bigger batteries a switched topology (Buck or Buck/Boost depending on the number of series battery cells) is used in order to reduce the internal power dissipation.

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The simple answer is that often they do not limit the current. If you plug your thing that charges at 2.5A into your dumb wall plug charger that only supplies 1.5A, there is a good chance it just causes the wall plug to burn out (or at least shut down).

Apple has its own system where a certain voltage is present on the data pins to indicate what current the source can supply. The other convention is for the data pins to be shorted which indicates a 'high power' source.

All charging logic is contained in the device being charged.

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