Battery Knowledge

Lithium battery is classified into primary (nonrechargeable) lithium battery and secondary (rechargeable) lithium battery. Lithium ion/polymer battery is a secondary lithium battery composed of lithium metallic oxide in its positive electrode and carbon material in its negative electrode. Lithium, however, is an active metal. The lithium ions inside the battery transfer between the two electrodes during charge or discharge.

The characteristics and performance of the commercial polymer and traditional lithium ion cells are very similar. Their main difference is packaging. Unlike traditional lithium ion technology using rigid metal case to tie the chemical materials together, lithium polymer uses flexible and aluminum foil type case, by which the cells can be made typically lighter, wafer-thinner and easily shaped to fit the device it will power.

The high power batteries use advanced LiFePO4 or Li-NCM as the positive electrode material instead of LiCoO2, which is popularly used in low power lithium ion/polymer batteries. They have excellent high charge/discharge rate performance, great safety, high capacity, good temperature performance, and pollution-free. Such batteries are widely used for electric vehicle, energy storage, medical equipment, electric tool etc.

Lithium ion/polymer batteries need protections in operation. The battery's negative electrode consists of, mainly, carbon or graphite that can store or release Li ions. Once there's an over discharge voltage, the Li ions turn to be Li metals and deposit on the carbon or graphite surfaces. Because the Li metal is very active, it can burn by any flush or other reasons. The electrolyte contains flammable organic solvent. Therefore, once the used charger goes wrong and there is an overcharge voltage existing, the voltage goes above the limited, the battery will be overheated and then to be burned. In order to prevent deep discharge that leads to shorten the battery's cycle life, it needs over discharge protection. The Protection Circuit Module(PCM) is used to protect the battery from over charging, over discharge, over current, short circuit.

Lithium ion/polymer batteries must use "Constant Current / Constant Voltage (CC/CV)" charging method. First the battery is charged till it reaches a limited voltage by using a set constant current and then charged with this constant voltage till the current is below 0.02C. It requires high precise charging circuit and protective circuit to control the charge. Over current discharge will damage the battery. It will cause high internal temperature of the battery and decrease capacity.

Comparison table of VRLA, Ni-Cd, Ni-Mh, Li-ion, Li-Po and LiFePO4