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.
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
Lithium Iron Phosphate cells, or LiFePO4 for short, are now dominating the alternate energy storage sphere amongst the more discerning designers and customers in the more advanced markets including Europe, Australia and South Africa.
Freedom Won has been using these cells in electric vehicles since 2010 and in stationary storage since 2014.
Freedom Won provides competitive prices made possible by high volume manufacturing in South Africa and guarantees market leading technical backup and design assistance. Freedom Won has the market leading battery warranty of 10 years with no fine print or diminishing value 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 15 years, and more than 20 years in grid connected back up installations with occasional cycling. The end of life is defined by when the cells contain 60% 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 even longer periods if a lower end of life capacity is acceptable.
The initial cost of installing a LiFePO4 system as compared to Lead batteries is only marginally higher since the 2018 price decreases in the Freedom Won range. For smaller batteries up to 10kWh this premium is about 40% but for batteries larger than 100kWh this premium is only about 10%.
These premiums in initial cost are 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. 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 flooded lead acid types.
Another top benefit to the customer is the far greater efficiency of the LiFePO4 technology, which is typically better than 98%. A typical efficiency for lead batteries is 65%, although this can be 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 connected back up scenario this high efficiency 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.
Table: Summary of Benefits of Using LiFePO4 Cells in Stationary Power Applications
|Comparison Aspect||Lead Acid||LiFePO4|
|Cycle Life (50% DoD with 70% remaining capacity, 30 deg C ambient temperature)||500 to 1300 cycles depending on manufacturer and model||More than 7000 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 (round trip) 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||10 to 50% more expensive up front than Lead Acid depending on what lead acid cells are used for comparison and the size of the battery – the Lithium premium on a larger Freedom Lite battery is lower.|
|Life Cycle Cost per kWh||R3.00 (USD0,21) to R6 (USD0,41) depending on battery type and model||R1,20 (USD0,08) (approx.)|
|Quick Charge Time||Typically should not be done in less than 5 hours||2 hour standard, 45 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
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 50% of the “rated” capacity of a lead acid pack because of 98% efficiency and ability to discharge on regular occasion to 90% DoD with negligible 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 2% 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 over time means that a LiFePO4 can be sized to about 50% of the lead battery in terms of Ampere hours. This factor originates simply from the fact that only 65% of the rated total energy is available from a lead acid battery in most high power backup applications, whilst 100% of the rated energy is available from 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 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 (kWh) rating 50% of the lead acid rating.
LiFePO4 cells maintain their rated nominal voltage for about 95% of the discharge, whilst a lead cell voltage drops continuously. When working out the Wh of a lead 12V battery one must use about 11.4V for the average voltage under load (1,9V per cell). The nominal voltage for LiFePO4 is 3.2V per cell or 52V for a typical “48V” system.
An example comparing a 200Ah LiFePO4 pack to a 400Ah lead pack is included in the below table. The theoretical energy capacity for the lead battery is reduced to 85% of the rated capacity to replicate a real world discharge scenario vs the manufacturer’s 10 hour constant current test regime used for the nameplate Ah rating. The usable capacities are adjusted to be in line with the typical DoD expected in the design of 50% and 70% for the lead and lithium examples respectively. The LiFePO4 pack costs only 40% more than the Lead battery, however, after taking into account the cycle life and the kWh produced in the lifetime of the packs it is clear that LiFePO4 costs only 22% of the Lead batteries used in this example.
Table: Comparison Example of Lead Battery vs. LiFePO4
|8||200Ah Lead Acid/Gel||16||units of Sinopoly SP-LFP200AHA (Freedom Lite 20/14)|
|4||Batteries (units) in Series||16||cells in each string|
|200||Ah Nameplate Capacity||200||Ah Cell Nameplate Capacity|
|200||Ah each @ 10hr rate||220||Ah at 3 hour rate (one cell per unit)|
|12||V per battery nameplate||3,4||V per cell nameplate|
|11,4||V per battery during discharge||3,2||V per cell during discharge|
|400||Ah total pack||220||Ah total pack|
|48||V rated total||54||V rated total|
|45,6||V nom of pack during discharge||51,2||V nom of pack during discharge|
|18240||Wh @ 10 hr discharge (260AhX44V)||11264||Wh available and rated for 100% DOD|
|85%||Derating of 10hr Capacity||100%||No derating required|
|15504||Wh available for 100% DOD||11264||Wh available and rated for 100% DOD|
|7752||Wh available for 50% DOD||7885||Wh available for 70% DOD|
|50%||Percent of LiFePO4 pack rated capacity required to match Lead Acid|
|R 6 000||Unit Retail Cost Excl VAT (one 12V battery)||n/a|
|R 48 000||Pack Upfront Cost||R 67 000||Retail Cost Excl VAT (Complete Freedom Lite 20/14 battery)|
|1,40||ratio for upfront cost|
|950||Cycle life at 60% of BoL Capacity||6000||Cycle life at 60% of BoL Capacity|
|2,6||Life in years – 365 cycles per year||16,4||Life in years – 365 cycles per year|
|7364,4||Kwh in lifetime||47308,8||Kwh in lifetime|
|R 6,52||cost per kWh||R 1,42||cost per kWh|
|22%||Cost of LiFePO4 life cycle compared to Lead Acid|
The batteries supplied by Freedom Won are available in various sizes. The Lite 15/11 is the most popular because it is ideal for a typical 3 bedroom house. They can be connected in parallel if future upgrades in capacity are required. The LiFePO4 pack in the Freedom Lite is 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 Lite with the BMS configured for each application.
For further information, assistance and sales of LiFePO4 cells please contact sales or enquiries at Freedom Won Pty Ltd via the website www.freedomwon.co.za.