Why is this 4V battery not charging my mobile phone directly?

 

Do NOT try and recharge a lithium polymer ("mobile phone") battery with a 4V lead acid! You WILL blow up something and start a fire. At the very least, you will damage the charging battery.

There is a reason they put those scary warnings on batteries. See here.

Besides, you need 4.2V (+/-0.05V) to charge a LiPo. 4V will not charge them.

 

Actually, a 4v lead-acid battery should measure 4.23v-4.28v when fully charged.
However, it would be dangerous to attempt charging a Li-Po battery directly from a lead-acid battery with no current limiting or terminal voltage regulation.

 

Whatever kind of battery technology the OP has in his phone, this can't be a good idea. Maybe he has avoided having an accident because the lead/acid battery was not too well charged?

I also note that the rated output voltages on my own phone chargers are all greater than their battery voltages, even allowing for a reasonable rise of voltage on charge. Does this imply that the phones have something between the charging input socket and the batteries, like for instance a diode to protect against discharge if the input were shorted?

 

The charger circuit that limits the voltage to 4.2V is inside the phone, it is not inside the 5V wall-wart power supply. Maybe the charger circuit will not work when its input is less than 5.0VDC.

 

It's a trick question. Look at the picture closely. That battery isn't charging the cell phone because it is not connected to it!

 

The charger circuit that limits the voltage to 4.2V is inside the phone, it is not inside the 5V wall-wart power supply. Maybe the charger circuit will not work when its input is less than 5.0VDC.

It's usually a buck regulator which steps voltage down. Also, they integrate temperature monitoring, SoC sense and current monitoring - almost like a life-support machine for the battery. Plugging 4V straight into the cell will not work; it could even be dangerous.

 

A charger IC is used inside the phone to charge its lithium battery.