Although lithium-ion batteries have been around for a while, 2020 has seen an increased uptake of the technology, and it quickly became one of the most talked about developments for the UPS industry this year. The batteries offer several advantages over the traditional VRLA predecessor but with so many types of lithium-ion cell on the market, which one is the most suitable for UPS applications?

The two names are often used interchangeably, which from a technical point of view is correct, both batteries are of lithium-ion composite relying on the movement of lithium ions between a positive and negative electrode. However, the chemistry of the two batteries is very different, giving them different properties, features and uses.

What is a Lithium-ion (LiPo) battery?

A lithium ion polymer is a rechargeable battery using an embedded lithium compound as the cathode, the standard compound being lithium cobalt oxide, but it can also be manganese oxide. Due to its high energy density, these types of batteries are commonly found in mobile phones, laptops, and wearable devices but the as the technology becomes more affordable, it is becoming an increasingly popular choice for UPS manufacturers too.

What is a Lithium iron phosphate (LiFePO4 / LFP) battery?

Unlike their polymer counterpart, Lithium iron phosphate batteries use phosphate as the cathode and are considered the safest lithium battery type currently available on the market. They benefit from low resistance properties, which enhance their thermal stability and safety.

The benefits and limitations of both lithium-ion battery technologies

Energy density
A high energy density is one of the major advantages of LiPo batteries when comparing it with other lithium-ion compounds. Providing 15Wh per 200kg, a LiPo battery delivers a 40% higher power to weight ratio than lithium iron phosphate, which is why it tends to be the battery of choice for power intensive devices. The higher energy density also means a reduced footprint and lighter weight battery is needed to provide large amounts of power and lower run times. A huge advantage for use alongside UPS in industries where space is at a premium.

Cycle life
The higher energy density of a liPo battery makes it marginally less tolerant to high operating temperatures when compared with other lithium-ion batteries. It has a cycle life of 500-1,000 which is still significantly more than a VRLA battery which has a 200-650 cycle life and is the typical choice of battery in today’s UPS systems.

For lithium iron phosphate, the cycle life is 1,000-10,000, the phosphate cathode is intrinsically safer that other cathode materials. With stronger chemical bonds, the cathode can withstand higher temperatures meaning they are harder to ignite in the event of mishandling and do not decompose at high temperatures. An LFP battery can run at an operational temperature range up to 70 degrees Celsius, but every manufacturer will have their own temperature tolerance recommendation.

Depth of discharge (DOD)
Depth of discharge may also be referred to as usable energy. A lithium ion battery reaches between of 80-100% DOD, depending on the compound used, which is ideal for energy storage as it means there is no need to worry about over discharging; an important feature for a UPS which requires deep-cycle batteries. This is a significant improvement on traditional VRLA batteries, they have a negative correlation between the DOD of the battery and the number of charge and discharge cycles it will perform, with an average DOD of 50%.

Thermal runaway
Unlike with other lithium-ion batteries, LFP batteries can get to much higher temperatures before they begin to react and combust. For this to start happening the battery would have to reach 350 degrees Celsius, which is rare because the rate of heat transferring out of the cell increases with cell temperature.

Which battery is most suitable for a UPS system?

The type of lithium ion battery you go for greatly depends on the application and situation it is being used in. Although they do have their similarities, the lithium iron phosphate battery is based on a lithium-ion polymer, by using phosphate as the cathode material, the battery becomes more tolerant to heat, improves the cycle life and increases the overall battery performance. However, LPF batteries are not as energy dense and to obtain the same amount of power, the battery must be larger and heavier than the polymer.

To find out about Power Control’s UPS systems that incorporate Lithium iron phosphate (LFP) battery technology, please contact power@powercontrol.co.uk or call the office direct on 01246 431431. For additional product and service information please visit www.powercontrol.co.uk