arduino - How to tell a battery's ideal charging voltage with a circuit?

^{simulate this circuit – Schematic created using CircuitLab}

I've been looking to build a universal charger for my laptop and phone, and I'm wondering; is there a viable way to adjust the input voltage to match the battery's ideal charging voltage by measuring A and B across an arbitrary load?

The scenario is as follows;

0) I've got a universal charger that can output anywhere from 5V to 24V

1) I want to charge my phone, so I plug it into the universal charger

2) Without my interaction, the circuit and it's microcontroller adjusts itself to the required voltage (5V)

3) I now unplug my phone and plug my laptop in for charging

4) Without my interaction, the circuit and it's microcontroller adjusts itself to the required voltage (24V)

How can I measure the required voltage without checking the battery label?

Answer

The problem with your scheme is that the battery isn't directly connected to the charging socket. There will be a charging controller in between:

^{simulate this circuit – Schematic created using CircuitLab}

So from the charging port it is not possible to determine what voltage is needed to charge the device.

Only if there is communication between the power adapter and the Charging controller can your scheme work. Qualcomm Quickcharge is an example of that. If the charge controller does not support such a protocol then what you propose cannot be done.

Think about it: USB ports that are designed for 5 V can usually handle 6 V - 6.5 V at most, applying 24 V to such a port will for sure damage it severely.