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Aircraft battery (Gill versus Concorde)

In the remote control world, Lithium batteries such as A123 are so far advanced compared to NiCad or lead or any other chemistry that I hope they make their way into aircraft soon. They’ll never be a ‘drop in’ replacement, unfortunately.

In the remote control world, Lithium batteries such as A123 are so far advanced compared to NiCad or lead or any other chemistry that I hope they make their way into aircraft soon. They’ll never be a ‘drop in’ replacement, unfortunately.

Why not? There are already LiFePO4 drop-in replacement batteries for motorcycles, why not for aircraft? It’s only a question of certification. By the way, unlike other lithium-based chemistries, LiFePO4 batteries are even less hazardous than lead-acid ones.

LKBU (near Prague), Czech Republic

The simple problem is charging.

Lead acid chemistry is charged by presenting a constant voltage to the battery (14V or 28V for 12V or 24V systems, respectively) and the battery draws whatever current it needs to charge up. As its cell voltage rises, the current reduces and at 14/28V you eventually get just a trickle charge. Actually more than a trickle charge strictly speaking – trickle is OK indefinitely by definition – but only about 1A which is OK for the duration of any flight. The charging voltage ought to be battery cell temperature compensated but if you don’t bother nothing much happens. So the charging circuit is very simple: there isn’t one. You just have to make sure the voltage regulator on the alternator is set to the right figure – 14V or 28V – but you need to do that anyway otherwise you might blow up all the avionic.

Lithiums need clever charging methods. They can be charged from a constant voltage but it is crude and overcharges them. So, for best battery life, you end up with a separate “charger” arrangement which involves some electronics. Obviously this is done all over the place but GA has a rather more conservative policy

The use of nickel cadmium batteries (in light jets) is horrible and I cannot understand how it was ever allowed. It is completely wrong because the batteries have a very low impedance so they draw a heavy charge current, their cell voltage is (as with all batteries) temperature dependent to a degree which is on the same order of magnitude as the natural cell voltage variations, so you can easily get overcharging and then thermal runaway. I used to design chargers for NICDs and they need a constant current charge, with one of several charge termination methods which, like lithiums, are nontrivial if you want the best life and performance out of it.

Administrator
Shoreham EGKA, United Kingdom

I don’t think we’ll see lithium chemistry in piston aircraft anytime soon. The simple reason is that lead acid batteries are very good at delivering a very high current for a short time, easily 150 amps when cranking. For a lithium battery to be able to do that, it needs to be rather big and complex. It will hold a lot more energy but we don’t really need that. All we need is to start the engine and hold enough power to serve as a backup for the alternator for some time.

What does make sense (and what I am going to do) is to install a second Li-Po battery in the cabin (below the floor board I am thinking) and connect it to the avionics bus as an additional backup. When my alternator packs on a Greek island, I still want to be able to make that IMC flight back to Germany.

The simple problem is charging.

That I don’t see as a big problem. Part of the aircraft LiPo battery would be an integrated charger module. It takes the normal 28V on its connectors but the battery enclosure has its own voltage regulator to do the charging. That stuff next to nothing today.

A 30 volt lithium battery capable of over 250 amps continuously for 1 minute would weigh 1100g. What would the lead acid equivalent weigh?

https://www.hobbyking.com/hobbyking/store/__19158__Turnigy_nano_tech_5000mah_8S_65_130C_Lipo_Pack.html

I would go for A123 cells myself for the reduced fire risk and slightly higher weight. Harder to get quotes like that though. I agree with Peter that charging is the problem. Also, without being connected up to a Pb battery I think your avionics would see a higher variation in voltage and noise?

I don’t think we’ll see lithium chemistry in piston aircraft anytime soon. The simple reason is that lead acid batteries are very good at delivering a very high current for a short time, easily 150 amps when cranking. For a lithium battery to be able to do that, it needs to be rather big and complex. It will hold a lot more energy but we don’t really need that. All we need is to start the engine and hold enough power to serve as a backup for the alternator for some time.

I don’t think we’ll see them any time soon, but simple lithium polymer batteries are already quite capable of peaking at 150 amps. I have some large radio controlled helicopters and the batteries for them are straightforward. Most of the cheap ones have a peak rating of 120A, some of the better ones much more. Of course you could have the starting system run on 44.4v nominal and then you need far less amps. These batteries are being mass produced now.

The reason why I don’t think we’ll ever see lithium starter batteries on GA piston aircraft is because lead acid is a dirt simple system which works well. No need for complex charging electronics and no need to worry about them catching fire. (Plenty of RC guys have suffered battery fires, usually as the result of a crash, but sometimes during charging due to a fault in the charger of fault in a battery cell). Lithium batteries are also much more expensive per amp hour than lead acid (and knowing how much a typical RC heli battery runs, then multiply that by 10 for a GA approved one, you could be looking at over £1000 for a battery suitable to replace a lead acid!) In a typical GA plane there would be a weight saving but only on the order of three or four kilograms, and if those 3 or 4 kg are really that important, it would be far cheaper for the aircraft owner to eat less and exercise more judging by the shape of the typical GA pilot!

Andreas IOM

Given you appear to know something about LiPo batteries, what kind of battery would you use for a 28V avionics backup system? My plan is to stow a battery somewhere below the floorboard and connect it to the avionics bus through a CB. The most suitable battery packs I have found so far are from pedelecs. Another option are the Milwaukee power tools which operate on 28V and the batteries are well packaged with charging stations, etc.

Also, without being connected up to a Pb battery I think your avionics would see a higher variation in voltage and noise?

Why? Because of the capacitor properties of a lead acid battery? I can’t imagine this to be a real problem. Note that the main use would not be an emergency power source (need hopefully arises seldom) but to power avionics on the ground. I absolutely hate tapping the main battery for avionics on the ground and even more so running the engine for that purpose.

Yes – current is not an issue and a few hundred amps can be delivered by a under-1kg battery easily.

That also means the battery will catch fire very fast if shorted or even damaged (pierced) and as alioth says loads of model planes have burnt up post-crash because the battery got rammed into something.

In the model business there are special fireproof bags – example – into which you can put an ignited battery and it keeps the flames contained. The smoke will still come out… I carry one of these in the plane, in case of e.g. a laptop battery fire. However I would probably try to engage the AP, close the throttle, and open the door and chuck it out just before reaching Vs…

The models sometimes have a piece of foam in front of the battery, so it doesn’t get totally smashed in a crash. They also cost a fortune and are cited by the IC-engine model flyers as the main reason they stick to IC engines. You can spend a whole day at a flying field with a £10 can of fuel, whereas you might need £500 of LIPO batteries to last that long. And they don’t last a long time – my son used to be heavily into this and he must have got through LIPO batteries into 4 figures in value, over the years.

If one was to scale model plane batteries and their prices to the size of our batteries, you would be looking at approaching £10k, so something would need to change. However it is clear that model plane stuff is mostly grossly overpriced.

Yes the LIPO voltage is not stable, versus charge condition. This is OK for any normal application where you will have (a) an intelligent charger and (b) a switching power supply to power whatever it is powering, so if the battery varies from say 16 to 12V during its normal charge state (from fully charged to just about flat) it doesn’t matter, but clearly for us one would need some serious electronics. This technology is now well developed for cars etc (and solar panels going back into the grid, etc etc) but you have to ask whether we really need this in the plane, given the rather sorry record of GA electronics design competence…

If I was to use a LIPO battery as an avionics backup, I would use some commercial LIPO battery (not a “model plane market” battery as, having seen those products for some years and how long they last for, most are probably the lowest grade of chinese crap) charge it with a dedicated charger, and when needed have a simple mechanical switch which switches it over to a +28V regulator which feeds your +28V bus via a diode. The (schottky) diode will drop some 0.5V so won’t conduct if you switched it in during normal aircraft system operation. So one is never charging it while trying to use it, which keeps the electronics simple and avoids your existing power system (alternator+regulator) having to know anything about it. The B&C backup alternators work the same way – they are set 0.5V below the normal bus voltage. Good luck with the paperwork

You might want to mount the LIPO battery in a box outside the airframe, releasable with an explosive bolt

Last Edited by Peter at 24 Jan 10:28
Administrator
Shoreham EGKA, United Kingdom

I would use A123 cells which stand up to 1000s of discharge cycles and are much less of a fire risk than standard lithium polymer, at a slightly increased weight. They have a different construction and chemistry. I have some 6-7 year old packs for my Logo 600 helicopter that have lost about 10% of their capacity. If they were strictly a backup battery you could charge them with a constant voltage trickle charger, then use a diode or relay so that they kicked in when the main battery was down/disconnected.

Looking up a battery for a C152, it only has about twice the capacity of the $157 LiPo battery I posted earlier, so at least 9,500 of his 10k cost would be certification.

Living in a flat, and having seen LiPo batteries deflagrate, I used to charge my LiPo batteries on the stove (heat turned off) in a saucepan with a collander over the top. And I still used to worry. There are lots of nice youtube videos.

kwlf – I would like to see that $157 5Ah 29V model battery compare against lead acid for performance in the real world. I agree the spec looks right but I have never seen these things last more than a few months before they are for practical purposes shagged.

With the A123 cells, how would you build up the required capacity? They can’t be simply parallelled (due to current sharing issues) so one would need to put them in series, which needs individual charging (for which there are plenty of chargers) and if you want to build say 20Ah equivalent for 28V you would need approx 80 of them (2.3Ah each, 3.3V). There are no commercial chargers for that configuration. I hope I got the maths right… I don’t know how this is done in electric cars… I think they use higher voltages e.g. 100V (keeps the currents lower so the electronics are a lot more efficient) so they must be putting loads of cells in series, but I assume they don’t put any cells in parallel (they use cells that are big enough to deliver the final Ah figure).

Last Edited by Peter at 24 Jan 11:16
Administrator
Shoreham EGKA, United Kingdom
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