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Optimise your Stationary Power by using LiFePO4

Lithium Iron Phosphate Cells (LiFePO4) – No one can afford to be outside of this revolution!

It is not a case of LiFePO4 being affordable, it is a case of can one afford to use lead acid? The life cycle cost of LiFePO4 cells is a quarter of lead acid batteries. The upfront costs for a pack vary from similar to double depending on the application, but one must consider that in 3 to 6 years’ time the lead acid batteries must be replaced and the LiFePO4 cells will last 13 to 20 years depending on the application and pack design.

LiFePO4 cells have revolutionised the potential and life cycle cost of operating battery based power systems. These cells have a cycle life 10 times that of typical deep cycle lead acid batteries. They only cost approximately 50% more to purchase upfront, however the saving on life cycle cost is substantial.

LiFePO4 cells are available in a wide range of sizes to accommodate loads of a few amps to over 1000 amps. They can deliver sustained high power without excessive heat generation. There are no gases released and LiFePO4 cells are also thermally stable. They can be charged repeatedly to full capacity in less than 60 minutes with no appreciable loss in performance.

LiFePO4 Batteries  vs Lead Acid

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

Antony John English
Co-founder Freedom Won Pty Ltd

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

Lithium Iron Phosphate cells,  or LiFePO4 for short,  are beginning to dominate the alternate energy storage sphere amongst the more discerning designers and customers in Europe and some other parts of the world. Freedom Won has been using these cells in electric vehicles for nearly four years already and the company is now also providing this superior technology for stationary power applications or 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. The expected life in off grid systems is more than 10 years, and as much 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 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.

The initial higher cost of installing a LiFePO4 system as compared to Lead batteries is dramatically overshadowed by the savings in the total life cycle cost, calculated as a cost per kWh delivered by the battery pack during its lifetime. Even Lead Crystal batteries cannot compete on life cycle cost. The lifetime cost per kWh can be as low as 25% of the cost of typical lead acid deep cycle batteries. The main reason for this is that the cells offer up to 10 times the number of cycles than your 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.

Another top benefit to the customer is the far 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.

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.

Comparison Aspect Lead Acid LiFePO4
Cycle Life (50% DoD with 80% remaining capacity, 30 deg C ambient temperature) 500 to 1300 cycles depending on manufacturer and model Up to 5000 cycles
Calendar Life Average (poor in high temperature or partial/full discharge condition or infrequent cycling) Excellent – no sulphation, partial charge storage is no problem, regular cycling is not required, heat tolerant
Charge – discharge efficiency 60-70% typical depending on current. Typically rated capacity is based on 10 hour discharge (C10) 96%, consistent throughout current range. Rated capacity is based on 20 minute discharge (3C), a one hour or longer discharge will actually give 10% more than the rated capacity.
Temperature resilience Poor – temperatures above 25 deg C significantly reduce the calendar life Excellent – ambient temperatures up to 45 deg C will not affect the life of the cell at all.
Up Front Cost Cheaper 20 to 50% more expensive up front than Lead Acid depending on what lead acid cells are used for comparison
Life Cycle Cost per kWh R2.50 to R6 depending on battery type and model R1,50 (approx.)
Quick Charge Time Typically should not be done in less than 5 hours 1 hour standard, 30 min quick
Discharge Current Higher discharge than C10 (10 hours), or 0.1xC rating causes substantial loss in efficiency and affects life C1 (one hour discharge) is standard, higher currents are also acceptable up to 3C (3 x Ah rating) continuous with negligible loss in efficiency and cell life
Gravimetric Energy Density Poor Weigh 3 to 4 times less – reduced transport costs and installation effort.

Volumetric density more than 2 times higher – less than half the space required

Pack Capacity Loss of 30% in heat (70% pack efficiency) means pack must be larger to meet a specific output objective

Max practical DoD is 50%, which requires a larger pack to stay above this DoD to prevent rapid life deterioration

Pack can be sized to 60% of the “rated” capacity of a lead acid pack because of 96% efficiency and ability to discharge on regular occasion to 80% DoD with much lower effect on life reduction
Charging Energy Source Size The charging energy source must provide an additional 30 to 40% energy to overcome the inefficiency of the pack at substantial cost Only about 4% of the energy is lost to heat – big savings in charging energy and capital on PV installations etc

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 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 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 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: Comparison Example of Lead Battery vs. LiFePO4

Line Lead LiFePO4
1 4 No. of Typical Solar Deep Cycle Batteries 8 LiFePO4 cells
2 260 Ah each 200 Ah each
3 12 V per battery (6 cells) 3.2 V nom per cell during discharge
4 520 Ah total (two strings in parallel, two per string) 200 Ah total (one string of eight cells)
5 22 V nom during discharge total 25.6 V nom during discharge total (3.2VX8)
6 11440 kWh rated (520AhX22V)
7 65% efficiency of lead acid battery
8 7436 Wh available for 100% DOD (11440kWhX65%) 5120 Wh available and rated for 100%DOD (200AhX3.2V)
9 3718 Wh available for 50% DOD 4096 Wh available for 80% DOD
10 45% Percent of LiFePO4 pack rated capacity to equivalent Lead Acid with optimal CAPEX DoD (50% vs 80% DoD)
11  R 16 000 Pack Cost  R 26 400 Pack Cost including delivery
12 1.65 ratio for upfront cost
13 500 cycle life (50% DoD) 2500 cycle life (80% DoD)
14 1859 Kwh in lifetime 10240 Kwh in lifetime
15 R 8.61 cost per kWh R 2.58 cost per kWh
16 30% Percent cost per kWh

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 parallel 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.

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