Depends on the charger design, the battery type and the circuitry around the battery. There are dozens of possibilities.
For example the charger I use for my GPS generates 5 volts at up to 1 amp. The circuitry in the GPS unit controls the charge current to correctly charge the internal battery to avoid damage. So the charger box has nothing to do except supply the 5 volts, the unit does the rest.
IF by "mobile" you mean mobile phone and not mobile charger, as it seems to read (the latter), it would be the same as my GPS unit.