Inverter and batteries solutions

Lithium Iron Phosphate Cells (LiFePO4)

LiFeP04 (lithium (Li) iron (Fe) phosphate (P04) has become the most common lithium technology in use today owing to its low cost, low toxicity, well-defined performance, long-term stability.

The life cycle cost of LiFePO4 cells is less than half that of lead acid batteries.

The initial price for a pack varies depending on the application, but a very important to factor in and consider that in 3 to 6 years the lead acid batteries will needs to be replaced and that the LiFePO4 cells can last for up to 20 years depending on the application and pack design.

LiFePO4 cells have quite simply revolutionised the potential and life cycle cost of operating battery based power systems.

With a 10 times longer cell life than that of lead acid batteries.

LiFePO4 cells are available in a variety of sizes to accommodate loads from a couple of amps to over 1000 amps.

They deliver sustained high power without excessive heat generation.

No gasses are released and LiFePO4 cells are thermally stable.

The LiFePO4 batteries can be repeatedly fully charged in less than 60 minutes with no loss in performance.

LiFePO4 Batteries vs Lead Acid

LiFePO4 are taking over as an alternate energy storage.

Used in electric vehicles as well as stationary power applications of all sizes.

Importing in bulk allows for very competitive pricing.

Constant technical backup and design assistance.

Battery warranty of up to 10 years, however, the expected life in off grid systems is more than 10 years, and can be as for as long as 20 years in grid tied back up installations with occasional cycling.

The end of life is defined by the cells containing 80% of their intended capacity.

The capacity deterioration over time does not become increase with extended use, and the cells could therefore be used for much longer periods if a lower end of life capacity is used.

The initial higher cost of installing a LiFePO4 system as compared to lead batteries is dramatically overshadowed by the savings in the total cost, calculated as a cost per kWh delivered by the battery pack during its lifetime.

The lifetime cost per kWh can be as low as 25% of the cost of typical lead acid deep cycle batteries.

The reason for this is that the cells offer up to 10 times the number of cycles than an average deep cycle lead battery and as much as 5 times that of the more robust single cell types.

This is especially apparent in cases of high current discharge and charging scenarios, further contrasted by high ambient temperatures, both of which are not suitable for lead batteries.

The greater efficiency of the LiFePO4 cell, which is typically better than 96%.

A typical efficiency for lead batteries is 65%, although empirical data has demonstrated as low as 55% in a house PV system where the Depth of Discharge (DoD) is limited to 20% as a measure to lengthen the life of the lead acid cells.

In a grid tied back up scenario this results in significant energy savings when recharging the batteries, and in a Photo Voltaic (PV) installation it enables a reduction of the size of the array by as much as 30% with the same usable energy.

When sizing a LiFePO4 pack, the rating of the cells cannot be compared to a typical lead acid rating without making some adjustments.

Owing to the much higher efficiency and the ability to discharge more deeply without rapid capacity deterioration means that a LiFePO4 can be sized to about 70% of the lead acid rating if the same DoD is desired.

This factor originates simply from the fact that only 50% of the rated energy is recommended for a lead acid battery in most backup applications, while 80% of the rated energy is recommended for LiFePO4 cells.

Because it is practical to use a lower DoD in LiFePO4 cells and still achieve an excellent cycle life the designer can reduce the LiFePO4 pack size even further and still provide superior performance over a lead battery pack.

A typical scenario could be 50% DoD for a lead acid pack compared to 70% DoD for a LiFePO4 pack.

This ultimately makes the LiFePO4 pack energy capacity rating 46% of the lead acid rating. The average to work on is 50%.

LiFePO4 cells maintain their rated nominal voltage for about 95% of the discharge, whilst a lead cell voltage drops continually.

When working out the Wh of a lead 12V battery one must use about 11V for the average voltage (1,8V per cell). The nominal voltage for a single LiFePO4 cell is 3.2V.

Larger battery packs generally have a nominal voltage of 48V but can be up to several hundred volts DC. Nominal 24V packs are becoming less common as renewal energy industry gravitates towards 48V.

LiFePO4 cells can be provided in various sizes with the 200Ah cell being the most popular.

These can be connected in parallel to suit the ampere hour requirements and in series to suit the voltage of the system (but the ability and extent to which strings can be paralleled varies by manufacturer).

The LiFePO4 pack either has an onboard Battery Management System (BMS), or must be connected to one that is able to monitor the conditions in each cell and prevent any cell and the pack from exceeding the upper and lower limits. Many of the 12V LiFeP04 batteries can be directly charged by a standard vehicle alternator without the use of a BMS.

The BMS monitors a wide range of parameters within the pack and will shut down the battery if they are exceeded. This protects the battery from damage and also provides a high degree of safety.

System components and monitoring systems will generally need to be programmed to deliver values that conform to the requirements of the battery’s BMS. These settings are provided by the individual manufacturers specific to their batteries.