BAS Data Sheet No 8
Lithium
Iron Phosphate (LiFePO4) Batteries
Secondary batteries employing Lithium Iron Phosphate (sometimes called
Lithium Ferro Phosphate) chemistry are now available as power supply
sources in gliders. This chemistry gives a marked reduction in cell
weight while avoiding some of the disadvantages of the earlier Lithium Ion
batteries. However their characteristics are dis-similar to the sealed
lead acid cells they replace. This short summary outlines their
advantages and disadvantages and covers some of the precautions to be
taken in their charging and use.
Advantages
(when compared with sealed lead acid (SLA) batteries)
·
Weight. The popular size (151 x 64 x 95mm) 7.5 A/H version weighs 1 kg
against 2.8 kg for its SLA equivalent
·
Greater number of power cycles (up to 2000) while retaining capacity
compared with around 500 for SLA batteries
·
Does not loose capacity when stored unused for long periods. They should
normally be stored uncharged whereas a SLA battery should be stored
charged.
Disadvantages
·
They are not self balancing. A SLA battery is made up of six 2v
individual cells connected in series. If one cell has less charge than
the others, then when the battery is charged, the other 5 cells ‘mark
time’ while the sixth cell catches up and the unbalance is corrected. A
LiFePO battery is made up of four cells with a nominal voltage of 3.2
volts each. If one cell should become unbalanced, then it is not
corrected when the battery is recharged. The good cells shut down
preventing complete charging of the unbalanced cell. The unbalance is
never corrected and the battery looses capacity as a result. The solution
to this problem is that electronic circuitry is built into the battery to
reduce the chance of unbalance occurring. Some batteries have this
circuitry installed while the cheaper versions do not have the
capability. You should ask the supplier to confirm that electronic cell
balancing is installed.
·
They require a dedicated charger that provides a regulated 14.4 volt
supply at less than 2 amps. When charging, the terminal voltage will rise
slowly to 14.4 volts when the charging process should switch off. If a SLA
charger is used, then when this point is reached, it switches to a trickle
charging mode which is death to a LiFePO4 battery.
·
They have a much lower internal resistance than a SLA battery. This means
that if an external short circuit occurs, a large discharge current will
flow with a significant fire risk. It is essential that the battery is
externally fused at the terminals with a 5 or 10 amp fuse.
·
They are on average five times more expensive than the equivalent SLA
battery.
·
They are subject to transportation restrictions. Unlike a SLA
battery, they cannot be transported through the postal system.
However some carriers will accept them with special conditions and for air
freight they require specific packaging and labelling
February 2017
This Data Sheet is Copyright of BAS and is not to be copied or
reproduced without permission
|