Usb battery charging

Question

I am designing a system which has a mcu (with usb peripheral). The whole system is powered by a 18650 Li ion battery. Now, imagine I am charging the battery by plugging the usb to a computer usb socket or even a wall adapter. The max current via the usb 2.0 is 500mamps. But this is a 3400mAH battery and so would take hours.

I then saw som articles on DCP (dedicated charging port). So basically I will have to short the d+ and d- via a very small resistor. By doing so it goes into a high charging current mode.

Now imaging I am using a Li ion charger like the rt9511. It can charge the battery using only a max of 500mAmps. How can it take more than that leave alone 1.5Amps. Or do we have different Li ion charger ic that can manage charging up to 1.5Amps ?

Answer 1

This is not a simple matter. There are several types of charging ports in USB space. USB ports can support Battery Charging specification (which detects port charging capabilities via a special handshake on D+/D- wires). There is Apple-style charging "signature", using certain voltage levels on D+/D-. There is "chinese charger" signature, where D+ and D- are connected via a resistor less than 200 Ohms (the one you mentioned). Finally, there is a newer and insanely complicated "Power Delivery" specification. You need to decide which type(s) of chargers your device must support. If you choose several chargers, you device needs to solve a challenge of proper detection of all of them. Hint: this is usually done in sequential manner, and is not simple. One complexity comes when the port you are trying to connect to also tries to exhibit own intelligence to determine what kind of device you are. I don't think this problem has a solution.

And yes, there are plenty of charger ICs that can be configured to take whatever current you want in accord with optimal battery charging requirements. Brief look at Digi-Key shows about 400 ICs for charging single Li-Ion cell. Good luck to you.