While traveling, charging of the mobile phone, cellphone battery is a big problem as power supply source is not generally accessible. Battery will go flat within five to six hours if you keep your cellphone switched on continuously, making the cellphone useless.
When your distance from the nearest relay station increases, a fully charged battery becomes necessary. Below is simple charger that replenishes the cellphone battery within two to three hours. Basically, the charger is a current-limited voltage source. Generally, 3.6-6V DC and 180-200mA current is required by cellphone battery packs for charging. These usually contain three NiCd cells, each having 1.2V rating.
At slow rate, 100mA current is sufficient for charging the cellphone battery. A 12V battery containing eight pen cells provides sufficient current (1.8A) to charge the battery connected across the output terminals. The voltage level of the battery can be monitored by this circuit. When its output terminal voltage increases above the predetermined voltage level, it automatically cuts off the charging process.