[SOLVED] Cell phone battery charging from 12V DC source

Hi!
I am working on a small project which requires charging a cell phone lithium battery from a 12V source. Input comes form a 12V lead acid battery and output goes into the USB port of the cell phone.
Now, if it was about charging an isolated lithium cell, it would be rather simple to design a CC or CV converter.
However since it is a cell phone battery, it must be having a charge controller on the PCB of the cell phone or perhaps integrated in the battery itself. So I guess, direct connection to battery terminals of the phone will not be available.
My questions are:
1. Are normal cell phone chargers CC or CV devices?
2. How does the cell phone know how much power the charger can deliver? Suppose I have a 1.5A charger, but the battery is capable of upto 2A. Then if the battery tries to pull 2A from the charger, the charger will shut down. But this does not happen. Instead, the battery charges slowly without shutting down the charger.

Thank you!

Normally they are constant voltage chargers but some of the cleverer ones use the USB connection to set the voltage. For example, some Samsung chargers switch between 5V for normal charge and 9V for rapid charge under direction from the phone itself.

Your simplest solution is to make a small 5V constant voltage regulator using an MC33063 or similar. The circuit is simple and has overload protection.

Brian.

The question isn't quite clear.

Old mobile phones have vendor specific supply connectors and supply adapters with defined voltage and current capability. In some cases, there can be a constant current circuit in the power adapter, but usually it's in the mobile phone. End of charge is always controlled inside the mobile phone. For most old mobile phones, 12V car adapters are available.

Recent smart phones are using Micro USB or USB-C connector and standard 5V USB power supplies. The CC/CV charger circuit is always embedded in the mobile phone. Strictly speaking, they should rely on USB Power Delivery specification which has options to signal the current capability by resistor strapping, and in some cases also negotiate a higher supply voltage than 5V. In practice, most devices run well with a dumb 5V/2A DC/DC converter.

Thanks for the reply!
Yes, new phones do rely on USB-Power Delivery for power negotiation. However, I was referring to a very simple charger without any power delivery control.
I will proceed with a simple 5V/2A constant voltage type buck converter design.

Hi,

so the problem is solved now?

If you use an interactive selction guide for buck converters don´t use "12V" as input, but 10.5V ... 14.4V.

Klaus

Hi!
Yes, the problem seems to be solved. I was confused whether "constant current" needs to be implement in the charger or not.
It appears that the charger needs only to be a "constant voltage" device and the rest will be taken care of by the BMS present in the cell phone.

I forgot to mention that a DCP (dedicated charger port) is expected to short D+ and D- and being able to source at least 1.5A, see USB Battery Charging Specification.

Hi,

It appears that the charger needs only to be a "constant voltage" device

Click to expand...

Yes. Indeed it is no charger circuit it is a usual power supply.

Klaus