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Types and performance parameters of lithium batteries
Lithium iron phosphate (LiFePO4)
In 1996, the University of Texas discovered that phosphate could be used as a positive electrode material for rechargeable lithium batteries. Lithium phosphate has good electrochemical performance and low resistance. This is achieved through nanoscale phosphate cathode materials. The main advantages are high rated current and long cycle life; Good thermal stability enhances safety and tolerance for abuse.
If kept at high voltage for a long time, lithium phosphate has stronger tolerance to all charging conditions and less stress than other lithium-ion systems. The disadvantage is that the lower nominal voltage of the 3.2V battery results in a lower specific energy than cobalt doped lithium-ion batteries. For most batteries, low temperature will reduce performance, and increasing storage temperature will shorten service life, and lithium phosphate is no exception. Lithium phosphate has higher self discharge than other lithium-ion batteries, which may cause aging and balance issues. Although this can be compensated for by selecting high-quality batteries or using advanced battery management systems, both of these methods increase the cost of the battery pack. The battery life is very sensitive to impurities in the manufacturing process and cannot withstand the doping of moisture. Due to the presence of moisture impurities, some batteries have a minimum life of only 50 cycles. Figure 7 summarizes the properties of lithium phosphate.
Figure 7 Spider diagram of a typical lithium phosphate battery
Lithium phosphate is commonly used instead of lead-acid starting batteries. Four connected batteries generate 12.80V, which is similar to the voltage of six 2V lead-acid batteries connected in series. The vehicle will charge lead-acid to 14.40V (2.40V/battery) and maintain a float charge state. The purpose of float charging is to maintain a fully charged level and prevent sulfation of lead-acid batteries.
By connecting four lithium phosphate batteries in series, each battery has a voltage of 3.60V, which is the correct full charge voltage. At this point, the charging should be disconnected, but continued while driving. Lithium phosphate can tolerate some overcharging; However, due to the fact that most vehicles maintain a voltage of 14.40V for a long time during long-distance travel, it may increase the mechanical stress on lithium phosphate batteries. Time will tell us how long lithium phosphate as a substitute for lead-acid batteries can withstand overcharging. Low temperature can also reduce the performance of lithium ions, which may affect the starting ability in extreme situations.
By connecting four lithium phosphate batteries in series, each battery has a voltage of 3.60V, which is the correct full charge voltage. At this point, the charging should be disconnected, but continued while driving. Lithium phosphate can tolerate some overcharging; However, due to the fact that most vehicles maintain a voltage of 14.40V for a long time during long-distance travel, it may increase the mechanical stress on lithium phosphate batteries. Time will tell us how long lithium phosphate as a substitute for lead-acid batteries can withstand overcharging. Low temperature can also reduce the performance of lithium ions, which may affect the starting ability in extreme situations.
