Bicester Aviation Services

Lithium Iron Phosphate (L_iF_ePO₄) 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