Lithium battery is classified into primary (nonrechargeable)
lithium battery and secondary (rechargeable) lithium battery. Li-ion,
Li-Po and LiFePO4 are all 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.
LiFePO4 batteries are the high power version of lthium ion
batteries. They use advanced LiFePO4 as the positive electrode material
instead of LiCoO2, which is popularly used in low power lithium ion and
lithium polymer batteries. So they have excellent safety, high energy
density, long service time, good temperature performance, and pollution-free.
Rechargeable lithium 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, Li-ion batteries need
an over discharge protection. The Protection Circuit Module(PCM) is used
to protect lithium battery from over-charging, over-discharge, overcurrent,
short circuit.
Rechargeable lithium 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
