Lithium Iron Phosphate (LiFeP04) batteries, method of charging and properties

Batterie batteries became very common type in the last 10 years and they are very similar in many ways as lithium-ion battery. Even though these cells having little less energy density then Li-ion, they still can boast with high capacity and high power, which makes them very lucrative storage of energy.

Lithium Iron Phosphate batteries have the highest life time among the rechargeable batteries. Most of manufacturers claims over 2000 charge/discharge cycles. This is due to very resilient crystal structure of the iron phosphate, which can maintain unchanged properties under repeated transport of lithium ions during charging and discharging.

In the last decade Lithium Iron phosphate (LiFePO4) has become one of the comon type of batteries in e-bikes, power tools, electric cars, wheel chairs, and even in larger vehicles.

Typical LiFePO4 cell has the following characteristics:

Nominal voltage 3.2 Volts

Peak voltage 3.65 Volts
Absolute Minimum discharge voltage 2.0 Volts
CV charge voltage 3.65 Volts at100% charge

CV charge voltage 3.5 Volts 95% charge

Charge Temperature 0°-40°C
Discharge Temperature -10°-60°C

They can be charged with a simple charging method with just two stages:
1 Constant current (CC) to reach about 60% State of Charge.

2 At 3.65V per cell (which is the top point of 1st stage) we need to go in to got from CC to constant voltage (CV). Charging current tapers down very rapidly.

To compare, first stage with CC takes about one hour and the second takes about two hours.


This simplified method is very common for many chargers, especially if engineer doesn't want to mess with complex schematics.

More sophisticated charges works with little different way: LiFePO4 cells can take higher voltage across their input electrodes, to reach 95% SOC and charge the rest 10% by CC+CV . This method is used for Lead Acid rapid chargers and allow to achieve less then 2hours charge

In comparison to conventional Li-ion batteries with CoO2 cathode, LiFePO4 battery has a much wider overcharge tolerance to overcharging voltage. Overcharge for 0.1V in LiCoO2 (conventional Li-ion) create about 1600 J/g heat, where as overcharge of 0.7V in LiFePO4 will create only 90J/g. This shows obvious advantage of LiFePO4 as much safer choice for fire or explosion if battery is abused by overcharge.

A LiFePO4 battery can be overcharged to 4.2 volts per cell without drastic outcome. But battery will start deteriorate with performance and capacity if such overcharge is a long term and regular condition. Many users charged LiFePO4 with a conventional 14.4V Lead Acid chargers. This is ok as 4 cells x 3.65v=14.6V and thus the Lead Acid battery charger won't overcharge the LiFePO4 4 cell pack. But at the same time with the Lead Acid 13.8 volts for the float charge and early14.4V termination won't allow to reach 100% charge in LiFePO4 battery. For this reason a special LiFePO4 charger is required to achieve 100% charge state.

This is a big advantage as most of electronic designers prefer to use simplified methods of charging and simplified methods of maintenance. Battery protection methods can be drastically simplified and this brings down the cost of BMS.

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 similar to a lead-acid battery.

In comparison to Lead-acid battery 35Wh/kg energy density, the LiFePO4 has 130Wh/kg, which is almost 4 times higher energy density. This makes them much better choice in many types of applications.

 

LiFePO4 cells in a battery pack in series connection cannot balance each other during charging process. This is because the charge current stops flowing when the cell is full. Because of that the LiFEPO4 packs need Balancing and Management boards.


Unlike the conventional Li-ion ( LiCoO2) battery, the LiFePO4 battery runs better at elevated temperature. At 50...55C it is offering 10% more capacity, due to higher lithium ionic conductivity. Although prolonged state of elevated temperatures is most likely harm the organic parts of the battery. But obviously the LFP batteries has better advantages working at elevated temperature comparing to Li-ion ( LiCoO2) type.

 

 

 

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