Lithium iron phosphate (LiFePO4) battery

　　In 1996, the University of Texas found that phosphate could be used as a cathode material for rechargeable lithium batteries. Lithium phosphate has good electrochemical performance and low resistance. This is achieved through nano phosphate cathode materials. The main advantages are high rated current and long cycle life; Good thermal stability, enhanced safety and tolerance to abuse.

　　If it is kept under high voltage for a long time, lithium phosphate is more resistant to all charging conditions and has less stress than other lithium ion systems. The disadvantage is that the low nominal voltage of 3.2V battery makes the specific energy lower than that of cobalt doped lithium ion battery. For most batteries, low temperature will reduce performance, and higher storage temperature will shorten service life, including lithium phosphate. Lithium phosphate has higher self discharge than other lithium-ion batteries, which may cause aging and bring about balance problems. Although it can be compensated by selecting high-quality batteries or using advanced battery management systems, both methods increase the cost of the battery pack. The battery life is very sensitive to impurities in the manufacturing process, and can not withstand water doping. Due to the presence of water impurities, some batteries have a minimum life of only 50 cycles. Figure 7 summarizes the properties of lithium phosphate.

　　Lithium phosphate is commonly used to replace lead acid starting batteries. Four batteries in series produce 12.80V, which is similar to the voltage of six 2V lead-acid batteries in series. The vehicle will charge the lead acid to 14.40V (2.40V/battery) and maintain the floating charge state. Float charging is intended to maintain a full charge level and prevent sulfation of lead acid batteries.

　　By connecting four lithium phosphate batteries in series, the voltage of each battery is 3.60V, which is the correct full charge voltage. At this time, the charging should be disconnected, but the charging should continue while driving. Lithium phosphate tolerates some overcharging; However, as most vehicles maintain the voltage at 14.40V for a long time during long distance travel, the mechanical stress of lithium phosphate battery may be increased. Time will tell us how long the lithium phosphate can withstand overcharging as a substitute for lead-acid batteries. Low temperature will also reduce the performance of lithium ion, which may affect the starting ability in extreme cases.

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