By Terry Kight - Technical Officer Operations/Radio/Airworthiness.

This article was included in the October 2015 Alpine Flyer

Use of Charge Station to recharge / jump start vehicle batteries. Can we do this?

Often asked.

Answer: We could do this but we don't and won't. It is not permitted. Reason: our main storage battery will give longest service if it's not discharged too deeply - by drawing more than 25% of its capacity. Using a big (220Ah) battery to recharge a smaller (80Ah) one overnight will excessively discharge the big battery. So for big car and winch batteries we use a mains powered charger instead.

Jump starting

Our AGM (Absorbed Glass Mat) type deep cycle storage battery is not constructed to supply high motor vehicle cranking currents. These types will do it but it's not good for their health.

Use of Green battery disconnect switches fitted to Tercel, Charade and Ford.

Some members use these as a removable connector instead of as a rotary switch. The units must NOT be dismantled. It's tricky to reinstall the dovetail insulating block without cross-threading the screw or worse - losing parts.

One full turn anticlockwise please - switch disconnects battery. Clockwise one turn until tight -switches battery back into circuit. Support the switch assembly with your free hand to take the rotational strain away from the battery post. Please spread the word.

Aircraft Batteries

Since the topic of the month is batteries and it's fair to say there is a fair bit of misinformation going around, I have written a lengthy primer on the subject and included it in our Web page. It discusses in detail why our lead acid battery capacity is often below par, battery history and battery management.

It leads to a conclusion concerning lithium iron (NOT Ion) batteries and that has been extracted and added below:

"We will now consider some Lithium variations, and eliminate those unsuitable for aircraft use.

Lithium Ion cells

In 1979 chemist John Goodenough at Oxford presented his rechargeable 3.6V cell that used Lithium Cobalt Oxide and Lithium Manganese Dioxide. It is known as Lithium Ion (Li-Ion). Thinner plastic laminated versions, called Lithium-ion polymer - Li-Po - are now common.

If abused, and sometimes even when not abused, these can spontaneously burn with great ferocity. The fire can only be extinguished by smothering. In 2013, Boeing grounded its fleet of  787 Dreamliners at a cost of $50 million a day because the aircraft's two Yuasa 32V lithium cobalt oxide standby battery packs tended to spontaneously ignite.

Not ideal for a glider.

Our Oxford whiz thought it wasn't Goodenough, and in 1996 went on to develop a stable and cheaper Lithium Iron Phosphate (LiFePO4) cell. These are sometimes also named LFE or LiFe. They do not easily ignite.

At this stage of their continuing development these cells are the best option for replacement of Pb cells if greater performance becomes necessary. Changes are still being made - Yttrium doping improves performance and the result is called an LiFeYPO4 (LFYP) cell.

We know that we should only use half of our SLA battery's rated capacity, we know the battery voltage steadily falls during use, and we know that our radios are usually rated for a supply voltage of 13.6V - 13.8V. We know that the energy capacity of our SLA batteries is often too low for our application.

Cell voltage comparison: Pb cells give 2V, NiCad and NiMH give 1.2V, Li-Ion starts at 4.2V and falls progressively to about 3V

The LiFePOtypes provide an output of 3.2V that remains steady until the last 5% of capacity. To make a nominal 12V battery, we use 4 cells (12.8V).

They have half the volume and a third of the mass of an equivalent capacity PB SLA battery.

This means that we can double the energy stored by using a lithium iron battery of the same size, and it will still be a little lighter than a SLA of half the capacity. Further, we can quadruple the usable energy because the Lithium Iron battery is not damaged by deep discharge (to a minimum of 2.5V per cell) that limits the usable energy in our SLA type. Even better is that by the time an equivalent capacity 12V SLA battery is dead at 12V, our LiFe battery is still happily at 12.6V and yet has delivered only half of its capacity.

Note that LiFePO4 capacity is lower than that of similar sized LiPolymer types, but that is not a problem for us. In any event, the use of LiPolymer batteries is not allowed in our aircraft due to safety issues.

The list of advantages claimed for LiFePObatteries is extensive and includes excellent subzero temperature characteristics but completely discharging to less than 2.5V will ruin the cell.

Constant current recharging must be done in an exacting manner, and our club's solar charge panel is programmed to do this when properly set for LiFePO batteries. It can also balance individual cells within a battery.

Cost? A 7.5Ah battery will cost upwards of $160 plus freight and a specialised charger if you don't use the Club one. Costs are coming down, and although lifetime costs are very low, the initial cost is around 600% more than an equivalent SLA. But then one is getting twice the energy delivery, and a substantially longer life. However, our batteries are frequently fully discharged, not half as they should be and if we want an advantage from Lithium Iron beyond long life we need to use a larger capacity battery. It will still be the same size but lighter than the SLA it would replace.

Alternative? Take a small USB recharge battery with you, (just make sure it is not a Lithium Polymer type!) make sure everything is fully charged before the flight, and don't plug extra equipment into the aircraft system."

By Terry Kight - Technical Officer Operations/Radio/Airworthiness.