Lithium ion phosphate cells (LiFePO4) are beginning dominate the energy storage sphere among more discerning designers and customers. These cells have been used in electric vehicles since 2011 and are now appearing in stationary power applications.
The expected LiFePO4 battery life in daily cycling off grid systems is 13 to 15 years, and as much as 20 years in back up installations with occasional cycling. The end of life (EoL) is defined by the cells containing 70% of their beginning of life (BoL) capacity. The capacity deterioration over time is however 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 EoL capacity is acceptable.
The initial higher cost of installing a LiFePO4 system as compared to lead batteries is dramatically compensated 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 30% of the cost of typical lead acid deep cycle batteries.
The main reason for this is that the cells offer up to ten times the number of cycles than your average deep cycle lead battery and as much as five 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, neither of which are suitable for lead batteries.
|Comparison aspect||Lead acid||LiFePO4|
|Cycle life (50% DoD with 70% remaining capacity, 30°C ambient temperature)||500 to 1500 cycles depending on manufacturer and model.||Up to 7000 cycles|
|Calendar life||Average (poor in high temperature or partial/full discharge condition or infrequent cycling).||Excellent – no chemical degradation, partial charge storage is no problem, regular cycling is not required, heat tolerant.|
|Charge/discharge (round trip) efficiency||60 to 70% typical depending on current. Typically rated capacity is based on ten hour discharge (C10) that must be derated in many applications.||96%, consistent throughout current range. Rated capacity is based on 30 minute discharge (2C), a one hour or longer discharge will actually give 10% more than the rated capacity.|
|Temperature resilience||Poor – temperatures above 25°C significantly reduce the calendar life.||Excellent – ambient temperatures up to 40°C will have no appreciable effect on the cell life.|
|Up front cost||Cheaper||20 to 70% more expensive up front than a comparative lead acid pack depending on what lead acid cells are used for comparison and the type of application.|
|Life cycle cost per kWh||R4,20 to R8 depending on battery type and model.||R1,80 to R2,70 depending on size of battery pack (larger packs are cheaper upfront per kWh capacity because the BMS costs are absorbed better in larger batteries)|
|Quick charge time||Typically should not be done in less than five hours.||One hour.|
|Discharge current||Higher discharge than C10|
(ten hours), or 0,1 x C 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.|
|Poor||Weigh three to four times less: reduced transport costs and installation effort. Volumetric density more than two times higher: less than half the space required.|
|Pack capacity||Loss of 30 to 40% in heat (70-60% pack round trip 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 96% round trip efficiency and ability to discharge on regular occasions to 80% DoD with much lower effect on life.|
|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 in the battery for a round trip|
cycle – big savings in charging energy and capital on PV installations etc.
Another benefit to the customer is the far greater round trip efficiency of the LiFePO4 cell, which is typically better than 96%. A “round trip” is a complete charge and discharge cycle. A typical round trip 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. Lead acid cells are very inefficient at high state of charge (SoC). In a grid connected back up scenario this results in significant energy savings when recharging the batteries, and in a photovoltaic (PV) installation it enables a reduction of the size of the PV array by as much as 30% with the same usable energy.
There are many other advantages of LiFePO4 cells over lead cells so a full elaboration is not included in this article. A summary is however provided in Table 1.
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 usually be sized to about 50% of the “nameplate” lead acid ampere hour (Ah) rating.
This factor originates simply from the fact that typically only 65% of the rated (nameplate) energy is available from a lead acid battery in most backup applications owing to energy losses (poor efficiency). This is in contrast to 100% of the rated energy being available from LiFePO4 cells (most of these cells can deliver more than the rated capacity for moderate current applications).
Apart from the impact from efficiency, 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 required LiFePO4 pack energy capacity rating about 50% of that required for a lead battery for the same useful energy output.
|1||4||No. of typical solar deep cycle batteries.||8||LiFePO4 cells.|
|2||260||Ah each.||100||Ah each.|
|3||12||V per battery (six cells).||3,2||Vnom per cell during discharge.|
|4||260||Ah total (one string).||100||Ah total (one string of sixteen cells).|
|5||44||Vnom during discharge total.||51,2||Vnom during discharge total (3,2 V x 8).|
|6||11 440||kWh rated (260 Ah x 44 V).||5120||Wh available and rated for 100% DoD (100 Ah x 51,2V).|
|7||65%||Derating of lead acid battery.||100%||No derating required.|
|8||7436||Wh available for 100% DoD (11 440 kWh x 65%).||5120||Wh available and rated for 100% DoD (200 Ah x 3,2 V).|
|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 23 000||Pack cost excl VAT.||R 42 000||Pack cost excl VAT.|
|12||1,82||Ratio for upfront cost.|
|13||700||Cycle life (50% DoD).||3800||Cycle life (80% DoD).|
|14||2 602||kWh in lifetime.||15 564||kWh in lifetime.|
|15||R8,83||Cost per kWh.||R2,70||Cost per kWh.|
|16||31%||Percent cost per kWh.|
LiFePO4 cells maintain their rated nominal voltage for about 95% of the discharge, whilst a lead cell voltage drops continually. For example the LiFePO4 nominal discharge cycle voltage for a“48 V” battery is actually 52 V versus 46 V or less for a lead acid pack. A higher mean voltage during discharge also equates to higher power delivery for the same Ah output. An example comparing a 100 Ah LiFePO4 pack to a 260 Ah lead acid pack is included in Table 2. 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 comparative LiFePO4 pack rating is 45% (just less than half) of the lead battery rating. The LiFePO4 pack costs 65% more than the lead acid 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 acid batteries used in this example.
The LiFePO4 pack is made up of any number of cells to make up the required voltage and Ah capacity. A typical “48V” pack will have 16 cells in series of a nominal 3,2 V each. The cells 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. A proper BMS should also communicate with the connected equipment (inverters, charge controllers, energy managers etc.) in order to provide valuable battery information to the system and the owner as well as issue commands to this equipment that ensure system integrity and longevity.
The BMS should also perform several self-diagnostic measurements and calculations to determine the health and status of the battery at all times.
This information should ideally be accessible via a computer or through another data connection system.
Contact Antony English, Freedomwon, Tel 087 550-9512, email@example.com