Lithium Iron Phosphate Cells

Lithium Iron Phosphate (LiFePO4) Cells: Safe, Long-lasting Energy Storage

What are they?

LiFePO4 cells, also known as LFP batteries, are a type of lithium-ion battery prized for their safety, long life, and stability. They use lithium iron phosphate (LiFePO4) as the cathode material, offering several advantages over traditional lithium-ion batteries:

• High Safety: Unlike cobalt-based lithium-ion batteries, LFP cells are inherently more stable and less prone to thermal runaway, a dangerous condition where the battery overheats and catches fire. This makes them ideal for applications where safety is paramount, like electric vehicles and home energy storage.

• Long Cycle Life: LFP cells boast exceptional cycle life, meaning they can be charged and discharged thousands of times before degrading significantly. They typically last 2,000 to 12,000 cycles, compared to 500-1,000 cycles for some other lithium-ion types.

• Good Thermal Stability: LFP cells can operate in a wider temperature range (-4°F to 167°F) compared to other lithium-ion batteries, making them suitable for various environments.

However, they also have some drawbacks:

• Lower Energy Density: Compared to some lithium-ion batteries, LFP cells have a lower energy density, meaning they store less energy per unit weight. This can be a disadvantage in applications where space and weight are critical, like portable electronics.

• Slightly Higher Cost: LFP cells are generally more expensive than lead-acid batteries but offer significantly longer life and improved safety.

Utilizing LiFePO4 Cells:

Here are some practical considerations for using LiFePO4 cells:

• Applications: They are well-suited for electric vehicles, power tools, solar energy storage systems, marine applications, and medical equipment due to their safety and long life.

• Assembly: Individual cells are often combined into battery packs to achieve the desired voltage and capacity. This process requires specialized knowledge and safety precautions due to the potential risks associated with lithium-ion batteries.

• Charging: Use compatible lithium-ion chargers designed specifically for LFP cells to ensure proper charging voltage and current. Improper charging can damage the cells and shorten their lifespan.

• Safety: Always follow safety guidelines when handling and working with lithium-ion batteries. Wear appropriate personal protective equipment and avoid exposing the cells to extreme temperatures, physical damage, or short circuits.

General Specifications

• Cell voltage

  • Minimum discharge voltage = 2.0-2.8 V

  • Working voltage = 3.0 ~ 3.3 V

  • Maximum charge voltage = 3.60-3.65 V

• Volumetric energy density = 220 Wh/L (790 kJ/L)

• Gravimetric energy density > 90 Wh/kg (> 320 J/g). Up to 160 Wh/kg (580 J/g).

• Cycle life from 2,700 to more than 10,000 cycles depending on conditions.

Charging

Because an overvoltage can be applied to the LiFePO4 battery without decomposing the electrolyte, it can be charged by one step of constant current to reach 95% capacity or be charged by constant current combined with constant voltage to get 100% capacity. This is similar to the way lead acid batteries are safely force charged. The minimum total charging time is usually about two hours.

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 a 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 can work but however some lead acid chargers will lower their voltage to 13.8 volts for the float charge, and so will usually terminate before the LiFe pack is at 100%. For these reasons a special LiFe charger is required to reliably get to 100% capacity.

Self balance

Unlike the lead-acid battery, a number of 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. This is why the LiFEPO4 packs need battery management system (BMS) boards.

Here's why a BMS is crucial for LiFePO4 packs:

• Individual Cell Monitoring: Each LiFePO4 cell has a slightly different voltage during the charging and discharging process. Without a BMS, some cells might be overcharged or undercharged, leading to:

  • Reduced lifespan: Overcharging can cause cell degradation and shorten the overall life of the battery pack.

  • Safety concerns: Overcharging can also create the risk of thermal runaway, a dangerous situation where the battery overheats and potentially catches fire.

  • Uneven capacity: Undercharged cells contribute less to the pack's overall capacity, reducing its effectiveness.

The BMS performs several critical functions in a LiFePO4 pack:

• Cell Voltage Monitoring: It continuously monitors the voltage of each individual cell in the series.

• Cell Balancing: During charging, the BMS identifies cells with lower voltages and actively diverts current to them, ensuring all cells reach full charge simultaneously. This prevents overcharging and maintains balanced cell health.

• Overcharge and Over-discharge Protection: The BMS disconnects the battery pack from the charger or load if it detects excessive voltage (overcharge) or low voltage (over-discharge) conditions, protecting the cells from damage.

• Temperature Monitoring: Some BMS options monitor the pack's temperature and take corrective actions, like reducing charging current or stopping the process altogether, if it gets too hot.

While not all LiFePO4 applications require a BMS, it's highly recommended for most situations, especially when:

• Safety is critical: In applications where safety is paramount, like electric vehicles or home energy storage systems, a BMS is essential to mitigate the risks associated with lithium-ion batteries.

• Pack longevity is important: A BMS extends the lifespan of the battery pack by ensuring balanced charging and protecting against overcharge and over-discharge.

• The pack has numerous cells in series: The larger the number of cells, the greater the risk of imbalances if not actively managed by a BMS.

Overall, using a BMS with your LiFePO4 battery pack provides significant benefits in terms of safety, performance, and longevity.

Several key characteristics make lithium iron phosphate (LiFePO4) cells the preferred choice for solar energy storage applications compared to other battery technologies:

1. Safety:

• Inherently Stable: Unlike some other lithium-ion batteries, LiFePO4 cells are chemically stable and less prone to thermal runaway, where the battery overheats and catches fire. This is crucial for home and commercial applications where safety is paramount.

• Wide Operating Temperature: LiFePO4 cells function effectively in a wider temperature range (-4°F to 167°F) compared to other lithium-ion types. This is important for solar storage systems that might experience temperature fluctuations throughout the year.

2. Long Life:

• High Cycle Life: LiFePO4 cells boast a significantly longer cycle life (2,000 to 12,000 cycles) compared to other common battery options used in solar storage, such as lead-acid batteries (500-1,000 cycles). This translates to a longer lifespan and fewer replacements, improving the overall cost-effectiveness of the system.

3. Other Benefits:

• Good Discharge Rate: LiFePO4 cells can deliver their energy quickly, making them suitable for providing power during peak demand periods.

• Environmentally Friendly: Compared to lead-acid batteries, LiFePO4 cells are less toxic and don't contain harmful heavy metals, making them a more environmentally friendly choice.

While LiFePO4 cells have a slightly lower energy density (storing less energy per unit weight) compared to some lithium-ion batteries, this is often not a significant drawback for solar storage applications. The greater safety, longer lifespan, and overall reliability outweigh the slight reduction in energy density for most homeowners and businesses.