You can charge your mobile battery without carry your mobile charger by a simple universal mobile battery charge solution circuit.The Li-ion battery can charge by simpler way with this circuit.you can charge the battery from usb or any 5-8v source by a basic circuit , this contains IC7805 and a 3.3 ohm or 6.8 ohm resistor.the only 2 components are sufficient for safely charge your battery.
use 3.3ohm(or 6.8 ohm for safe circuit) ,it can be selected by expresion as:
R = V(s) – V(l) / I
where V(s) = source voltage
and V(l) = load voltage
as the load vlotage of li-ion battery is near above 3.2-3.7v ..fully charged may have potential of 4.2v . the mobile phone device usually needs 200-300 m.A of current as pe varied device by devie.Thus a battery of suitable mAh should be assembled by manufacturers.
the cahrging voltages for li-ion battey are from 3.8 to 4.2 and must not be exeeced than this limit.Normally it takes 150 min to charge a battey at 4.0 volts.But i suggest to charge for only 2 hours,not more than it as you are charging your battery manually,there isn auto cut down circuit .thus it may harm your battery if more time than 2 hours is spent on it.For more information regarding battery charging visit batteryuniversity.com.