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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

    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.

B. Large overcharge tolerance and safer performance

A LiCoO2 battery has a very narrow overcharge tolerance, about 0.1V over the 4.2V per cell charging voltage plateau, which also the upper limit of the charge voltage. Continuous charging over 4.3V would either damage the battery performance, such as cycle life, or result in fire or explosion.

A LiFePO4 battery has a much wider overcharge tolerance of about 0.7V from its charging voltage plateau of 3.5V per cell. A LiFePO4 battery can be safely overcharged to 4.2 volts per cell , but higher voltages will start to break down the organic electrolytes. Nevertheless, it is common to charge a 12 volt, 4-cell series pack with a Lead Acid battery charger. The maximum voltage of these chargers, whether AC powered, or using a car's alternator, is 14.4 volts. This works fine but Lead Acid chargers will lower their voltage to 13.8 volts for the float charge, and so will usually terminate before the LiFePO4 pack is at 100%. For this reason, a special LiFePO4 charger is required to reliably get to 100% capacity.

Due to the added safety factor, these packs are preferred for large capacity and high- power applications. From the viewpoint of large overcharge tolerance and safety performance, a LiFePO4 battery is like a lead-acid battery.

C. Four times higher energy density than a Lead-acid battery

A Lead-acid battery is an aqueous system. The single cell voltage is nominally 2V during discharge. Lead is a heavy metal; its specific capacity is only 44Ah/kg. In comparison, the Lithium Iron Phosphate (LiFePO4 ) cell is a non-aqueous system, having 3.2V as its nominal voltage during discharge. Its specific capacity is more than 145Ah/kg. Therefore, the gravimetric energy density of LiFePO4 battery is 130Wh/kg, four times higher than that of Lead-acid battery, 35Wh/kg. Four-times higher density makes LiFePO4 cells much more appreciable in technical usage than Lead-acid cells.

C. Four times higher energy density than a Lead-acid battery

Large overcharge tolerance and self-balance characteristic of a LiFePO4 battery can simplify the battery protection and balance circuit boards, lowering their cost. The one-step charging process allows the use of a simpler conventional power supply to charge LiFePO4 battery instead of using an expensive professional Li-ion battery charger.

E. Longer cycle life

In comparison with LiCoO2 battery which has a cycle life of 400 cycles, LiFePO4 battery extends its cycle life up to 2000 cycles.

F. High temperature performance

It is detrimental to have a LiCoO2 battery working at elevated temperature, such as 60o C. However, a LiFePO4 battery runs better at elevated temperature, offering 10% more capacity, due to higher Lithium ionic conductivity.

Lithium Iron Phosphate Batteries Main Advantages:

  • Like nickel-based rechargeable batteries (and unlike other Lithium Ion batteries), LiFePO4 batteries have a very constant discharge voltage. Voltage stays close to 3.2V during discharge until the cell is exhausted.
  • This allows the cell to deliver virtually full power until it is discharged.
  • It can greatly simplify or even eliminate the need for voltage regulation circuitry.
  • Lithium Iron Phosphate cells are much harder to ignite in the event of mishandling (especially during charge) although any fully charged battery can only dissipate overcharge energy as heat.

Therefore, failure of the battery through misuse is still possible. It is commonly accepted that the LiFePO4 battery does not decompose at high temperatures.

  • LiFePO4 cells experience a slower rate of capacity loss (or greater calendar-life) than Lithium-Ion battery chemistries such as LiCoO2 Cobalt or LiMn2O4 Manganese.
  • After one year on the shelf, a LiFePO4 cell typically has approximately the same energy density as a LiCoO2 Li-ion cell, because of the LiFePO4 's slower decline of energy density.

3 comments on “How to Charge Lithium Iron Phosphate Rechargeable Batteries

  1. SaraDVL
    January 23, 2019

    I'm a content manager in Adsy company (adsy.com).

    I wrote the same article about Lithium Iron Phosphate Rechargeable Batteries.

    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.

  2. adok3812
    November 10, 2019

    Hello,
    Very good, detailed paper!
    I bought 2 x 25.6V 100Ah AKKUTECH battery sets and connected to AXPERT 1kVA charger/inverter control with 4 x 150W PVs plus mains input (to replace 8 x TROJAN T-105 batteries).
    Interesting how the charging characteristics very closely match your predictions/writing. In particular the very steady terminal voltage during/after charge.
    My supplier could not provide spec sheets of the AKKUTECH and neither could I find any in Google.
    Label reads : Product 25.6V 100Ah, Cut-off 28.4V +-0.2V, Cut-off 21.6V.
    Question : What would you set the bulk charge voltage? And the float charge setting?
    (The question lies in real capacity of this battery : when can I expect 100Ah? To reach the “100% charge” I should need to reach 29.2V.)

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