Backup and Off Grid Battery Packs Using Superior Lithium Iron Phosphate (LiFePO4) Cells – the next Generation of Energy Storage

By: Antony J English, Co-founder of Freedom Won Pty Ltd

The rate of adoption of Lithium Iron Phosphate (LifePO4) batteries as a mainstream storage solution for alternative energy systems in Europe and some other parts of the world is not surprizing due to the improved performance of LiFePO4 batteries when being compared to the more traditionally used deep cycle battery technologies in alternative energy storage solutions.

The LiFePO4 expected life in off grid systems could easily reach 10 years, with some manufacturers claiming life and cycling expectations as long as 20 years under specific conditions (i.e. grid tied back-up installations with occasional cycling). The end of life is defined by cells containing 80% of their Beginning of Life (BoL) capacity. The capacity deterioration over time is linear (the deterioration does not become substantially more rapid with extended use), and the cells could therefore be used for much longer periods if a lower end of life capacity is acceptable.

Higher costs of installing a LiFePO4 system compared to Lead Acid batteries are dramatically overshadowed by the savings in the total life cycle cost which is calculated as a cost per kWh delivered by the battery pack during its lifetime. Not even Lead crystal batteries can compete on life cycle cost. The main reason for lifetime cost per kWh being as low as 25% of the cost of typical lead acid deep cycle batteries is that the cells offer up to 10 times the number of cycles of an average deep cycle lead acid battery and as much as 5 times that of the more robust single cell types. This is more 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.)

Another top benefit to the customer is the far greater efficiency of the LiFePO4 cell, which is typically higher than 96%. When compared to lead acid batteries being around 65% Empirical data has demonstrated efficiencies as low as 55% in a residential PV system where the Depth of Discharge (DoD) is limited to 20% as a measure to lengthen the life of the lead acid cells.

The advantages of LiFePO4 cells over lead cells are extensive so a full elaboration is not included in this article. A summary is however provided in the table below, and further questions posed to Freedom Won will be welcomed.

Table 1: Summary of Benefits of Using LiFePO4 Cells in Stationary Power Applications compared to lead acid (this table can be viewed in the complete article

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 65% of the lead acid rating if the same DoD is desired. This factor originates simply from the fact that only 65% of the rated energy is available from a lead acid battery in most backup applications, whilst 100% of the rated energy is available from LiFePO4 cells (LiFePO4 cells can deliver more than the rated capacity for moderate current applications hence the use of 100% instead of the 96% efficiency mentioned earlier). 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 acid 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. It is a rule of thumb to work on 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 LiFePO4 is 3.2V.

An example comparing a 200Ah LiFePO4 pack to a 520Ah lead pack is included in the below table. The theoretical energy capacity for the lead acid battery is reduced to 65% of the rated capacity in line 8. In line 9 the capacities are adjusted to the typical DoD expected in the design. For this scenario the LiFePO4 pack rating is 45% (just less than half) of the lead acid battery rating. The LiFePO4 pack costs 65% more than the Lead battery, however as shown in line 15, after taking into account the cycle life and the kWh produced in the lifetime of the packs it is clear that LiFePO4 costs only 30% of the Lead batteries used in this example.

Table 2: Cost Comparison – Example of Lead Battery vs. LiFePO4 This table has been included in the complete article

The Freedom Won LiFePO4 cells can be provided in various sizes. The 200Ah cell is the most popular. They can be connected in parallel strings to suit the ampere hour requirements and in series to suit the voltage of the system. The LiFePO4 pack must be connected to a Battery Management System (BMS) that is able to monitor the voltage of each cell and prevent any cell from exceeding the upper and lower limits. The BMS must also balance the cells to ensure that the pack can perform at its best. LiFePO4 cells do not naturally balance themselves.

Freedom Won supplies the BMS configured for each application and will also supply plug and play battery packs with the BMS pre-connected if requested. These cells have been used locally in electric vehicles conversions for nearly four years already and the company is now providing this superior technology for stationary power applications for all sizes.

Freedom Won provides competitive prices made possible by bulk imports as well as comprehensive technical backup and design assistance. Freedom Won has also now extended the battery warranty in most applications to 5 years spurred by our confidence gained from our own test programme in electric vehicles, which is far more demanding on the batteries than stationary power systems.

For further information, assistance and sales of LiFePO4 cells please contact Antony English at Freedom Won Pty Ltd