Mobile phones run on batteries. Battery is needed to keep the mobiles work . To make a call, to keep in touch with the network the mobiles must have a battery power. Normally mobile phone batteries are lithium based and  designed to be small and light. mAh is the term to measure how much power battery hold.The unit mAh stands for Milliamp Hour.

The batteries are different types like Lithium Ion and Lithium Polimer. The Lithium batteries are expensive rechargeable batteries. The battery includes electronic charge and discharge control circuitry. They have an excellent power/weight ratio, and do not need to be discharged at all. 

Lithium ion: Lithium ion batteries (Li ion) offer twice the energy per charge of NiMH and approximately 500 charge-discharge cycles. They cannot sustain high currents at temperatures below 0°C and are relatively expensive.There is no memory and no scheduled cycling is required to prolong the battery's life.self-discharge is lesser.It requires protection circuit to maintain voltage and current within safe limits and metals and chemicals are changing on a continuing basis.

Lithium polymer: Lithium polymer batteries can be made in thin, flat or shape fitting forms and their biggest plus is that they won't leak corrosive electrolyte. Lithium polymer batteries are not suitable for high-power applications and they are limited to the operating temperature range 0 - 65° and are relatively expensive. It has a less chance for electrolyte leakage.