Some Technical Data on Lithium Iron Phosphate - LiFePO4
Lithium Iron Phosphate - LiFePO4 is a cathode material used in lithium ion batteries. It has become a favorite of designers of electric vehicles such as electric skateboards due to its inherent safety characteristics and long cycle life.
Other varieties of lithium ion cells include: lithium cobalt oxide - LiCoO2 ; lithium manganese oxide - LiMn2O4 ; and lithium nickel oxide - LiNiO2. All these compounds refer to the cathode material. The electrolyte of a lithium-ion battery can vary. It is typically an aqueous fluidic solution of lithium salts, however it can also be polymer based for easier shaping and safer puncture characteristics. The anode of most cells is made of carbon.
LiFePO4 has many advantages over other types of lithium-ion batteries. The abundance of Iron available makes this material relatively low cost. It is non-toxic making the batteries safer to use as well as better for the environment when producing and recycling them. They have a longer life span both in terms of shelf life and cycle life. They are more stable than other chemistry's and far less likely to suffer from thermal runaway. LiCoO2 cells, for example, can burst into flames if over charged and potentially release hazardous chemicals. Overcharging and overdischarging LiFePO4 cells will damage them, but generally without the explosive side effects. It can shorten cycle life or even result in deformation and a dead cell.
Most LiFePO4 cells operate within a normal or 'nominal' voltage of 3.0 to 3.3 volts, have a maximum operating voltage of around 4.1 to 4.2 volts and a minimum operating voltage of 2.1 to 2.5 volts. Typically a LiFePO4 cell is fully charged at 3.65 - 3.7 volts. In order to get the proper voltage to charge your LiFePO4 battery pack you would multiply 3.65 x the number cells in series. So a 12V nominal pack (3.0V x 4 cells) would have a charging voltage of 14.6V.
Cells may be wired in series (multiply the voltage) or in parrallel (multiply the capacity). Cells are wired in series by connecting the positive(+) terminal of one cell to the negative(-) terminal of another. Cells are wired in parrallel by connecting positive to positive and negative to negative terminals. Wiring in parrallel can be used to increase the discharge rate of the battery (usually refered to as a 'C' rating). Multiplying the C rating by the capacity of the battery will give you its discharge rate. For example, the Headway LiFePO4 cells that we carry have a continous discharge rate of 5C and a peak discharge of 10C. Since the capacity of each cell is 10 amp/hours(Ah) this means each cell can continously output 50 amps (10Ah x 5C) and pulse up to 100 amps for a short amount of time. By wiring two of these cells in parrallel you effectively double the capacity (20Ah) and the discharge rate (100 continuous, 200 peak amps). Higher voltages are achieved by wiring either individual cells or parrallel blocks in series.
Battery Management System - Given the sensitive charge and discharge characteristics of these cells it is important to have a battery management system (BMS) to regulate them, especially when constructing them into packs. Overdischarging a cell below 2.1 volts can cause irrevocable damage to a battery that would otherwise last you for several years. A well designed battery management system includes a low voltage cutoff circuit (LVC) for each cell to keep the cell from discharging beyond the safe operating voltage. It will also have a system to balance the cells in order to utilize the energy more efficiently and to allow all the cells in a pack to recieve a full charge.
The proper charger is also very important. Overcharging cells will damage them and can still be potentially dangerous even with the inherent safety of the LiFePO4 chemistry. A good LiFePO4 charger will simply cut off once the pack has reached the desired voltage. Most standard sealed lead acid chargers will not cut off automatically and should not be used. Also make sure that your charger is intended for LiFePO4 and not simply lithium-ion or LiPO (Lipo, Li-poly, Li-Po) which is a common mistake. Most Li-po (lithium polymer) cells have a charge cutoff voltage of 3.2 or 3.3 volts so a 12V nominal pack would have a charging voltage of around 13.2 volts, well below the 14.6 you need for your LiFePO's to become fully charged.