How to Charge Lithium Iron Phosphate Rechargeable Batteries

Lithium Iron Phosphate (LiFePO₄) batteries are safer than Lithium-ion cells and are available in a range of huge cell sizes between 5 and 100 AH with much longer cycle life than conventional batteries:

Cylindrical LiFePO₄ cells are one of the hottest products among all series, they have many great features:

• High energy density, 270 to 340 Wh/L; this means long working time
• Stable discharge voltage
• Good consistency between different cells in the same order
• Long cycle life, 2000 times with 80% capacity left
• Fast charge, they can be charged within one hour
• Safe and high temperature resistant performance

Lithium Iron Phosphate is a type of Lithium-Ion battery, since the energy is stored in the same way, moving and storing Lithium ions instead of Lithium metal. These cells and batteries not only have high capacity, but they can deliver high power. High-power Lithium Iron Phosphate batteries are now a reality. They can be used as storage cells or power sources.

In addition, Lithium Iron Phosphate batteries are among the longest-lived batteries ever developed. Test data in the laboratory shows up to 2000 charge/discharge cycles. This is due to the extremely robust crystal structure of the iron phosphate, which does not break down under repeated packing and unpacking of the lithium ions during charging and discharging.



LiFePO₄ Battery Charging/Discharging Main Parameters

Although small capacity Li-ion (polymer) batteries containing Lithium Cobalt Oxide (LiCoO₂) offers the best mass-energy density and volume-energy density available, Lithium Cobalt Oxide (LiCoO₂) is very expensive and unsafe for large scale Li-ion Batteries. Recently Lithium Iron Phosphate (LiFePO₄) has been becoming the "best-choice" of materials in commercial Li-ion (and polymer) batteries for large capacity and high-power applications, such as laptops, power tools, wheel chairs, e-bikes, e-cars and e-buses.

The LiFePO₄ battery has hybrid characters: it is as safe as the Lead-Acid battery and as powerful as the Lithium Ion battery. The advantages of large format Li-Ion (and polymer) batteries containing Lithium Iron Phosphate (LiFePO₄) are listed below:

A. Conventional charging

During the conventional Lithium Ion charging process, a conventional Li-Ion Battery containing lithium iron phosphate (LiFePO₄) needs two steps to be fully charged: Step 1 uses constant current (CC) to reach about 60% -70% State of Charge (SoC); Step 2 takes place when charge voltage reaches 3.65V per cell, which is the upper limit of effective charging voltage. Turning from constant current (CC) to constant voltage (CV) means that the charge current is limited by what the battery will accept at that voltage, so the charging current tapers down asymptotically, just as a capacitor charged through a resistor will reach the final voltage asymptotically.

To put a clock to the process, Step 1 (60%-70% SOC) needs about one to two hours and the Step 2 (30%-40% SoC) needs another two hours.

Because an overvoltage can be applied to the LiFePO₄ battery without decomposing the electrolyte, it can be charged by only one step of CC to reach 95%SoC or be charged by CC+CV to get 100%SoC. This is like the way lead acid batteries are safely force-charged. The minimum total charging time will be about two hours.