I currently have a spare phone battery that I am using to power my child's mobile. My question is how can I add a useful way of charging the battery without removing it? Is it possible that I could solder a female USB port to the battery and mount this inside the mobile and then using a typical USB mains charger, charge the battery up that way? The mains unit only outputs 5V at 2.1A.
I'd rather ask instead of blowing a Li-Ion battery!
I did something similar for my kids' rocking swing, only I bypassed the battery. Find one of the 64 old phone chargers you have around that matches the voltage and current requirements of the mobile, and connect that to the battery box. (You did save all those proprietary Motorola and LG flip-phone chargers for hobby projects, right?)
You can either solder directly to the terminals, or solder to a couple metal plates that you hold in place with a battery-sized dowel. This way you can use batteries again later if you like. Consider installing a fuse inline with the positive wire for added safety.
My question is how can I add a useful way of charging the battery without removing it?
You can't. Li-ion batteries require very careful monitoring during both charge and discharge, otherwise it's a fire hazard. Phone battery should be only used in the phone it came with, because it's the phone that has all the circuits for monitoring charge and discharge.
But you should go with isherwood's advice and remove the battery completely, if you don't need the mobility. If you don't want to solder the toy, you can make great battery-to-wire adapter by using a round wooden stick of same diameter, cut to length and with a thumbtack placed on the end to serve as contact.
