For a good number of years the RC community have been treated to the electric power qualities of Lithium batteries. Fast recharging and gradually increasing capacities with falling prices as they become more popular and demand is high. Most RC users moved away from heavy NiCAd and Nimh cells. Although this required a more dedicated charger as each cell needed to be monitored and balanced. Lithium Ion was great for laptops, phones with good capacity and safe in use. Then came along the Lithium Polymer (LiPo) with high power output and fast charging with bigger and bigger capacities and discharge rates now exceeding 60C or power output exceeding 60 times the packs' capacity. The downside though was safety. These packs if mistreated or abused could instanteously overheat and catch fire. Main image Hobbyking
But now there is new chemistry making progress in Lithium batteries.
LiFePO 4 Power Battery, Faster charging and safer performance
Although small capacity Li-ion (polymer) Battery containing lithium cobalt oxide (LiCoO 2) offers a the best mass energy density and volume energy density available, lithium cobalt oxide (LiCoO2) is very expensive and unsafe for large scale Li-ion Batteries.
Recently lithium iron phosphate (LiFePO4) has been becoming the "best-choice" of materials in commercial Li-ion (and polymer) batteries for large capacity and high power applications, such as laptops, power tools, wheel chairs, e-bikes, e-cars and e-buses.
The LiFePO4 battery has hybrid characters: it is as safe as the lead-acid battery and as powerful as the lithium ion battery. The advantages of large format Li-ion (and polymer) batteries containing lithium iron phosphate (LiFePO4) are listed as below:
- Nominal voltage 3.2 Volts
- Peak voltage 3.65 Volts Note our new data on capacity versus charge voltage
- Absolute Minimum discharge voltage 2.0 Volts
- CV charge voltage 3.65 Volts 100% charge
- CV charge voltage 3.5 Volts 95% charge
- Charge Temperature 0°-40°C
- Discharge Temperature -10°-60°C
Large overcharge tolerance and safer performance
A LiCoO2 battery has a very narrow overcharge tolerance, about 0.1V over the 4.2V per cell charging voltage plateau, which also the upper limit of the charge voltage. Continuous charging over 4.3V would either damage the battery performance, such as cycle life, or result in fire or explosion.
A LiFePO4 battery has a much wider overcharge tolerance of about 0.7V from its charging voltage plateau of 3.5V per cell. When measured with a differential scanning calorimeter (DSC) the exothermic heat of the chemical reaction with electrolyte after overcharge is only 90 Joules/gram for LiFePO4 versus 1600 J/g for LiCoO2 . The greater the exothermic heat, the more vigorous the fire or explosion that can happen when the battery is abused.
A LiFePO4 battery can be safely overcharged to 4.2 volts per cell, but higher voltages will start to break down the organic electrolytes. Nevertheless, it is common to charge a 12 volt a 4-cell series pack with a lead acid battery charger. The maximum voltage of these chargers, whether AC powered, or using a car's alternator, is 14.4 volts. This works fine, but lead acid chargers will lower their voltage to 13.8 volts for the float charge, and so will usually terminate before the LiFe pack is at 100%. For this reason a special LiFe charger is required to reliably get to 100% capacity.
Due to the added safety factor, these packs are preferred for large capacity and high power applications. From the viewpoint of large overcharge tolerance and safety performance, a LiFePO4 battery is similar to a lead-acid battery.
Unlike the lead-acid battery, a number of LiFePO4 cells in a battery pack in series connection cannot balance each other during charging process. This is because the charge current stops flowing when the cell is full. This is why the LiFEPO4 packs need management boards.
Simplified battery management system and battery charger
Large overcharge tolerance and self-balance characteristic of LiFePO4 battery can simplify the battery protection and balance circuit boards, lowering their cost. The one step charging process allows the use of a simpler conventional power supplier to charge LiFePO4 battery instead of using an expensive professional Li-ion battery charger.
Longer cycle life
In comparison with LiCoO2 battery which has a cycle life of 400 cycles, LiFePO4 battery extends its cycle life up to 2000 cycles.
High temperature performance
It is detrimental to have a LiCoO2 battery working at elevated temperature, such as 60°C. However, a LiFePO4 battery runs better at elevated temperature, offering 10% more capacity, due to higher lithium ionic conductivity.
At present for us RC modellers the PO4 packs are relatively small being used in transmitters, receivers and small models but the large packs will no doubt be with us soon at a much lower price.
Extract by Powerstream