FEATURES AND BENEFITS WHEN COMPARED TO SLA
This information will cover the features and benefits of Lithium Iron Phosphate (LiFePO4) batteries when compared to traditional Sealed Lead Acid (SLA) battery technology. Since the discussion is around LiFePO4 and SLA, the article concentrates on 12VDC and 24VDC applications.
DIFFERENT LITHIUM TECHNOLOGIES
It is important to note that there are many types of “Lithium Ion” batteries. The point to note in this definition refers to a “family of batteries”.
There are several different “Lithium Ion” batteries within this family which utilize different materials for their cathode and anode. As a result, they exhibit very different characteristics and therefore are suitable for different applications.
LITHIUM IRON PHOSPHATE (LIFEP04)
Lithium Iron Phosphate (LiFePO4) is a well-known lithium technology in Australia due to its wide use and suitability to a wide range of applications.
Characteristics of low price, high safety and good specific energy, make this a strong option for many applications.
LiFePO4 cell voltage of 3.2V/cell also makes it the lithium technology of choice for sealed lead acid replacement in a number of key applications.
Of all the lithium options available, there are several reasons why LiFePO4 has been selected as the ideal lithium technology for replacement of SLA. The main reasons come down to its favourable characteristics when looking at the main applications where SLA currently exist. These include:
Similar voltage to SLA (3.2V per cell x 4 = 12.8V) making them ideal for SLA replacement.
Safest form of the lithium technologies.
Environmentally friendly –phosphate is not hazardous and so is friendly both to the environment and not a health risk.
Wide temperature range
Weight benefits of LiFePO4 when compared to SLA
Weight has a large bearing on many applications, especially where towing or speed in involved, such and caravan and boating. Other applications including portable lighting and camera applications where the batteries need to be carried.
- LifePO4 is less than half the weight of SLA
- AGM Deep cycle – 27.5Kg
- LiFePO4 – 12.2Kg
- Increases fuel efficiency
- In caravan and boat applications, towing weight is reduced.
- Increases speed
- In boat applications water speed can be increased
- Reduction in overall weight
- Longer runtime
Greater Cycle Life
The greater cycle life means that the extra upfront cost of a LiFePO4 battery is more than made up for over the life use of the battery. If being used daily, an AGM will need to be replaced approx. 6 times before the LiFePO4 needs replacing.
- Up to 6 time the cycle life
- AGM Deep cycle – 300 cycles @ 100% DoD
- LiFePO4 – 2000 cycles @ 100% DoD
- Lower total cost of ownership (cost per kWh much lower over life of battery for LiFePO4)
- Reduction in replacement costs – replace the AGM up to 6 times before the LiFePO4 needs replacing
Flat Discharge Curve
This feature is not common knowledge but is a strong advantage and it gives multiple benefits. With the flat discharge curve of LiFePO4, the terminal voltage holds above 12V for up to 85-90% capacity usage. Because of this, less amps are required in order to supply the same amount of power (P=VxA) and therefore the more efficient use of the capacity leads to longer run time. The user will also not notice the slowing down of the device (golf cart for example) earlier.
Along with this the effect of Peukert’s law is much less significant with lithium than that of AGM. This results in having available a large percentage of the capacity of the battery no matter what the discharge rate. At 1C (or 100A discharge for 100AH battery) the LiFePO4 option will still give you 100AH vs only 50AH for AGM.
- AGM recommended DoD = 50%
- LiFePO4 recommended DoD = 80%
- AGM Deep cycle – 100AH x 50% = 50Ah usable
- LiFePO4 – 100Ah x 80% = 80Ah
- Difference = 30Ah or 60% more capacity usage
- Increased run time or smaller capacity battery for replacement
Increased use of capacity
The increased use of the available capacity means the user can either obtain up to 60% more runtime from the same capacity option in LiFePO4, or alternatively opt for a smaller capacity LiFePO4 battery while still achieving the same runtime as the larger capacity AGM.
- At 0.2C (20A) discharge
- AGM – drops below 12V after
- 1.5 hrs of runtime
- LiFePO4 – drops below 12V after approximately 4 hrs of runtime
- More efficient use of battery capacity
- Power = Volts x Amps
- Once voltage starts to drop off, battery will need to supply higher amps to provide same amount of power.
- Higher voltage is better for electronics
- Longer run time for equipment
- Full use of capacity even at high discharge rate
- AGM @ 1C discharge = 50% Capacity
- LiFePO4 @ 1C discharge = 100% capacity
Greater Charge Efficiency
Another strong benefit in many applications. Due to the lower internal resistance among other factors, LiFePO4 can accept charge at a much great rate than AGM. This allows them to be charged and ready to use much faster, leading to many benefits.
- AGM – Full charge takes approx. 8 hours
- LiFePO4 – Full charge can be as low as 2 hrs
- Battery charged and ready to be used again more quickly
Low Self Discharge Rate
This feature can be very useful for recreational vehicle users who may only use the vehicle for a couple of months a year before going into storage for the rest of the year, products such as caravans, boats, motorcycles and Jet Skis etc. LiFePO4 do not calcify and so even after being left for extended periods of time, the battery is less likely to be permanently damaged. A LiFePO4 battery is not harmed by not being left in storage in a fully charged state. If your applications warrant any of the above features, then you will be sure to get your monies worth for the extra spend on a LiFePO4 battery.
- AGM – Discharge to 80% SOC after 4 months
- LiFePO4 – Discharge to 80% after 8 months
- Can be left in storage for a longer period
Introduced in the early 1990s, the modern prismatic cell satisfies the demand for thinner sizes. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. Other designs are wound and flattened into a pseudo-prismatic jelly roll. These cells are predominantly found in mobile phones, tablets and low-profile laptops ranging from 800mAh to 4,000mAh. No universal format exists and each manufacturer designs its own.
Prismatic cells are also available in large formats. Packaged in welded aluminum housings, the cells deliver capacities of 20 – 50 – 75 – 100 – 200Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure shows the cross section of a prismatic cell.
The prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture.