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LiFePO4 - 10 Key factors

RE Solutions > Energy Storage


1) Cycle life


The Cycle life is the amount of times a cell or a battery can be charged and discharged to a specific point. It has nothing to do with the calendar life (how old the battery is).

The Cycle life is heavily dependent on how deep the battery is discharged. If you discharge too deep, then you can kill a battery in three months. Some batteries are specified for 100 cycles from 100% full to 50% DOD (typically a car battery). Car batteries are full most of their lifetime, so 100 Cycles are enough for a one or a two year period.

LiFePO4 cells can be cycled, 2000 to 3000 cycles at 70-80% DOD. These tests were done under 0.5CV. This means that the battery is discharged within 2 hours. Where batteries are used for smaller loads like 10-20%, even more cycles are possible.

2) Depth of Discharge (DOD)

One of the key parameters of any Battery installation is DOD. This property will determine the capacity to be installed. If one needs 100Ah capacity, you will have to install 200Ah, if you can only discharge to 50% DOD. Technically, double the cost of the installation.

Similarly, if you install a 100Ah battery, you will only have 50Ah available, otherwise the battery life will be shortened to less than a year. A lack of proper battery management results in a shorter Cycle life, and it is the most common reason for failure in batteries.

3) Charging time

LiFePO4 can be fully charged in a very short time. Charging time under normal conditions is 3 hours, but 1 hour, or even 30 minutes are possible. Although the Cycle life will decrease at very fast charging, it might be useful for critical backup systems, where fast charging is not a regular occurrence, or where the Cycle life is not that critical.

But even a 3 hour charging period has many advantages, where generators need to recharge the batteries. Solar PV installations have a limited time frame to charge a battery as it is does not have the entire day to charge, and perhaps on some cloudy days just 3-4 hours.

4) Constant Capacity under different loads

If a 100Ah battery is discharged at 1C or 100A (100% Capacity) of the cell, it will still deliver about 95% of the capacity, and it does no suffer from Peukert's law.

In 1897, German Scientist W. Peukert first documented the capacity of a lead acid battery in terms of the rate at which it was discharged. As the rate increased, the battery's available capacity decreased. This is a common issue with lead acid technologies.

This holds serious issues for the efficiency of the battery. If you pay R150 to charge a 100kWh battery and you can only get 50% back, you end up paying R300 or R3.0/kWh for the 50kW you could extract from the battery.

5) Temperature stability

LiFePO4 can charge and discharge at much higher temperatures. Higher temperatures do not decrease the calendar life of the chemical composition. Some lead technologies are rated for a lifetime of 15 years, but at a stable 25C. If that battery is not kept at 25, the 15 year lifetime of even the most expensive lead technologies, can decrease to 2 years. This means that the battery must be kept under controlled conditions, and the cost of keeping the air-conditioner on for 15 years adds a massive cost for some of these technologies.

The Working temperature range for LiFePO4 is from -20C to +60C (Extremely cold or hot weather will not affect its performance). Charging however needs to be done at a temperature higher than 0C, but it can discharge at -20C.

6) Energy Density

High Energy density reduces the weight and size of the cells.

7) Low Self Discharge rate

Low self-discharge rate is less than 3% a month. That means that a battery does not need to be trickle charged on an ongoing basis. At most, one will probably need less than a 20 minute charging period every three months to keep the batteries in a fully charged mode.

8) Environmental friendly

LiFePO4 does not contain any toxic or heavy metals like Lead and Cadmium. It does not contain any acid or alkaline and is regarded as one of the most environmentally safe batteries available. It also has excellent recycle potential.

9) Safety

LiFePO4 has an extremely stable chemical composition and is safe to use.
The cells are intrinsically safe. It will not explode or catch fire under collision when over charged or short circuited. The cells have a high thermal stability of up to 500C. (500% of capacity, a virtual short circuit)

10) Total cost of ownership (TCO)

When all these factors are taken into account in a financial simulation, it will be evident that the TCO over a 10 or 20 year period is better than any other battery technology in most applications. It is becoming clear as to why LiFePO4 batteries are busy gaining ground on a massive scale in Europe.

As part of his M-Degree in Renewable energies, the founder, Dewald le Grange, has developed such a financial model to compare certain scenarios over a 10 to 20 year period, simulating ROI and IRR of a Solar PV installation with storage. This model is available on request.


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