By Terry Kight - Technical Officer Operations/Radio/Airworthiness.
Batteries and their care provide the occasional headache, so I have been asked to provide some general information that will give a sound knowledge base and encourage informed discussion.
So let's look at the battery picture in a general sense. We will go right back to basics assuming there may be a member or two who has absolutely no interest in or knowledge of, electrical batteries. For the pros and techno wizzes amongst us, please bear with me. First is terminology. An AA size canister containing electricity-producing innards is a CELL. Not a battery. We refer to C size cells and AAA size cells and so on. When we package more than one cell together, the combined unit is called a BATTERY.
Batteries can push a flow of electrons through a conductor that joins the two terminals together, and the pressure provided by the battery that forces that flow through the conductor, is termed the Voltage (V). The flow of electrons is called the current, measured in Amps (A). Obviously a very thin conductor (wire), like a small bore water hose, is unable to allow a very large current (quantity of electrons) to pass. It provides a Resistance to that flow, and Resistance is measured in Ohms (Ω, or Omega). A low resistance will allow a larger current to flow. Think of Voltage being the pressure at the bottom of a pipe connected to a reservoir much higher up. The higher the reservoir the higher the pressure. That pressure will force a current to flow if the tap (switch) is turned on. The diameter of the pipe will restrict the actual current (quantity of water) that can flow. A very large pipe has a very low resistance to the flow and a very large current can result because of that.
The principle is used in a fuse - a very short, small diameter wire that presents a resistance to current. The current causes an increased rate of electron movement (energy) that is dissipated as heat. If too much current flows - perhaps because of a short circuit in your instrument panel- the wire melts. The fuse has blown and the current stops. So do your instruments.
This combination of volts pushing a current through a Conductor is called Power, measured in watts (W). Volts x Amps = Watts. Power is not Energy. Power is Energy divided by Time.
The difference is most important. We want our electrical cells to push power into a circuit, but they must create or store energy to allow that to happen. More energy storage will keep our Nav instruments and radios going for a longer time.
As an aside, Watts can be converted to Horsepower,(hp) and not surprisingly there are mechanical / hydraulic, electrical, and Metric horsepowers. They are all a little different as below. I have no idea why, and it's totally irrelevant to our study of batteries. But interesting. One useful thing from this is that 1 electrical horsepower equals 746 watts. Exactly. My toaster uses 750 watts and that means I use 1hp to cook two bits of toast. The issue is that watts equal power.
|Watts||Mech/Hyd hp||Electrical hp||Metric hp|
|1 W||0.001341 hp||0.001340 hp||0.001360 hp|
Back to Batteries.
Batteries are made of electrochemical cells. Two different metals immersed in a conductive liquid will generate electricity. Much effort has gone into varying the combinations in an (often futile) attempt to achieve few side effects, long life, low weight and low cost.
There are two types.
These cells are "use once and throw away when flat" types. The reduction / oxidation reaction in conjunction with the electrodes provides power until the energy available from the electrons is dissipated.
Perhaps the simplest cell is the Lime or Lemon cell. Take one of these high tech vessels already full of juicy chemicals and conductive acidic electrolytes, stick two dissimilar electrodes into the lemon's inside, quite close together but not touching. One is of zinc, such as a hot dipped galvanised nail. This is the Negative (-) pole of the cell. The other electrode is a stiff bit of copper wire, polished. That is the Positive (+) pole. Although conventional current is deemed to flow from positive to negative, electrons will flow from the negative to the positive, a voltage of about 0.75V will be measured using a digital multimeter, and the citric acid in the lime or lemon (not too ripe) is the electrolyte. Hydrogen will be given off at the copper electrode. The zinc electrode will be consumed. Not particularly efficient - the zinc gets used up and you need 5 - 6 million lemons to equal a car battery's power, but the idea seemed good when Volta thought of it some 215 year ago.
The most common example today is Carbon Zinc. Common examples include carbon zinc torch cells in their many iterations. They use ammonium or zinc chloride as the electrolyte, a carbon positive anode electrolyte and a zinc case as the negative (electron supply) cathode. Manganese dioxide, fine carbon and other tricks provide variants. The voltage is 1.55V.
The original cell of this type was the Leclanché cell using a glass jar with a sintered pot inside that contained the positive terminal's carbon mix. These powered the local sounders of the overland telegraph line from Adelaide to Darwin from 1872.
A chemical variation was the Meidinger cells. These powered the 3200km telegraph wire itself. They were quite massive but still only gave 1.5 volts. 100 in series gave the line voltage and a hefty shock for the unwary. They have a lead plate in copper sulphate solution, and a zinc cylinder with magnesium sulphate solution.
All variations on a theme. More sophisticated Lithium cells with greater energy capacity are yet another.
Secondary Cells comprise the other type. These are rechargeable. The chemistry is regenerated by pushing energy back into the cell, making it ready to disgorge another load of energy. Nickel Cadmium, (Nicad) Nickel Metal Hydride (NiMH) and Lead Acid (Pb) are the most common types today.
In days long gone when the writer was gainfully employed, large aircraft commonly used NiFe cells (pronounced knife) that used Nickel (Ni) and Iron (Fe) electrodes in a caustic soda electrolyte. Invented in Sweden by Jungner in 1899, (who also invented Nickel Cadmium cells) and developed by Edison in the USA, these have lost favour due to slow charge, size, weight and high cost (and the fact that major lead acid battery maker Exide bought Edison's NiFe manufacturing company and closed it in 1972). However, they will last 50 to 100 years or more with the ability to repeatedly discharge to zero capacity without damage.
All have limitations.
Our gliders usually use one or two nominal 12 volt lead acid batteries as their power source. Each contains 6 nominal 2 volts (2V) cells connected in a series string. The electrolyte is usually in a gel form and the batteries are called sealed lead acid (SLA) types.
We say nominal because if the total measured battery voltage is actually 12 volts the battery is essentially flat. Empty and require recharging. Feed them with enough energy (Watts x Time) in the right dose, and they will store quite a lot of it and become Fully Charged. A fully charged 12V battery (after a short period of use to wipe the surface charge off its innards), will read 12.55 - 12.7V. When being topped up with a trickle of charge called a Float charge - it will read around 13.6V. When being given a full serve of electricity to jam it full, it will be fed with 14.4V. These figures vary a little according to temperature, battery condition and variations in battery chemistry.
Like all such batteries the label capacity is given as x Ah, meaning they contain enough stored energy to deliver a current of x amps for a period of 1 hour. This is calculated at the 20 hour rate, indicating the battery can deliver its full capacity at a current of x/20 amps for 20 hours. Common sizes include 7Ah batteries, that can deliver 7/20 = 0.35A for 20 hours in theory. (Capacity decreases as the rate of discharge increases).
Sounds good but the catch is such batteries are not designed to deliver more than half of their capacity and still provide a reasonable life. In fact, any regular discharge over 25% of rated capacity lessens life significantly.
A terminal voltage of 12.2 volts is the lowest that these batteries should ever be discharged to without risking permanent damage. I would recommend a routinely repeated end point voltage of 12.35V. That only leaves about half the energy in the battery.
That reduces the usable capacity of our (say) 7Ah battery to 3.5Ah of usable stored energy at best. Other factors include the fact that many brands on the market cannot supply their rated capacity and also deteriorate rapidly, and our energy supply is looking grim.
Everything has worked well so far, so what's the problem? Why are people talking about using other types?
The problem is frequent flat batteries with shortened battery life.
The problem only occurs when we load up our circuits with additional power hungry devices. USB powered devices are often quite savage on current draw; smart phones, screen based displays, moving maps, loggers, flight computers and varios, radio transmitters etc all add up to a far greater load than yesterday's basic cockpit demanded.
Long flights can flatten the battery before you land, potentially leaving you without radio communications.
The solutions are obvious:
- Don't use power hungry instruments. This is not going to be a workable solution.
- Install flexible solar charge panels on the external hull. This is an excellent solution when practical but is expensive and requires precise and automatic battery charge management.
- Install additional battery capacity. Additional mass in terms of CoG parameters can be difficult to manage and can require a reweigh.
- Install an additional battery. As for 3 above, but more so if additional battery is in tail area.
- Take a USB recharge battery with you, don't plug extra equipment into the aircraft system, and make sure everything is fully charged before you take off.
- Install a different type of battery. Today, the metal Lithium is being used to make alternative secondary batteries. There are many different types.
We will now consider some Lithium variations, and eliminate those unsuitable for aircraft use.
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 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 in aircraft 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 LiFePO4 types 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 a 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 LiFePO4 batteries 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 LiFePO4 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 without harming the battery, and a substantially longer life.
However, our batteries are frequently fully discharged, not half, 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 but it will be at substantially greater cost.
Alternative? Take a small USB recharge battery (just make sure it is not a Lithium Polymer type!) with you on your flight, 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.