EarthX has a range of LiFePO4 batteries that are plug and play for experimental aircrafts. These contains all the needed electronics for charging the battery:
http://earthxbatteries.com/wp-content/uploads/2016/06/Aircraft.Brochure.2016.cropped.pdf
You can just purchase an ETX680C and directly replace an Oddisey PC680, very common battery in small aircrafts.
I would suspect for a battery like this the charging circuit would be built into the battery itself so that you could connect it to a normal 14V/28V bus and the in-built charging circuit would take care of it.
That would be some interesting electronics, because it needs to control the charge current (using obviously a switch-mode regulator otherwise it would get way too hot) and also pass the several hundred amps of discharge current when starting the engine.
All doable of course but it is a fair amount of stuff to chuck away when the battery is scrapped after some years.
The efficiency would not be better than say 90% even with expensive magnetics and when you are looking at charging currents in the tens of amps, that is a fair bit of heat which needs to come out somewhere, and I don’t see any cooling vents.
So I don’t believe these lithium batteries have a charge control circuit in them.
Laptop batteries, for instance, have an inbuilt charging circuit.
Those I have seen (removable ones) have a current integrator which keeps track of what energy goes in and what energy comes out, and this is accessible over some sort of serial link (I2C?) but there wasn’t a charging regulator in there.
I almost bought this EarthX battery before looking closer into this. These batteries have lots of energy for their weight. However, what you want is lots of power, and in particular, when you really need that power is when it’s cold. A lead acid battery is better in subzero conditions and you want to start the engine. For all other use cases the difference is negligible, because high power at low temperature is the dimensioning factor.
I have decided simply to use standard gel start batteries from “Biltema”. As a backup for the avionics, a small lithium battery would be nice of course.
I am growing more and more skeptical over the years. What is the mass of a typical light aircraft battery? 15kg? Add to this that quite a lot or aircraft use the mass of the battery for balancing purposes. Even if that Lithium battery would reduce the mass by 2/3, those 10kg of extra payload are hardly worth the risk and complication involved, unless there are products available which are proven since many years.
I remember well when the CJ4 was introduced as the first aircraft with a Lithium battery. The first aircraft delivered were almost instantly grounded because of battery problems and had to be refitted with NiCd batteries (the standard in this category of aircraft) until the problems were solved. A few years later, exactly the same happened to the Dreamliner, but on a much larger scale. So if the big players have such problems with this technology, how could small companies like the one featured in that article develop a safe product?
I have been using a Skyrich battery (HJTX30-FP) in my homebuilt for nearly two years and it spins the Lycoming engine and composite propeller very briskly. They may need warming up in cold climates, but this can be done by drawing current. No problem with alternator charging, and you can buy dedicated chargers for 220/240V. I use an old 15 V laptop power supply with a series diode to drop the voltage down to 14.4.
Simon
Peter wrote:
That would be some interesting electronics, because it needs to control the charge current (using obviously a switch-mode regulator otherwise it would get way too hot) and also pass the several hundred amps of discharge current when starting the engine.
There’s no reason that the starting current has to go through the charging circuit. It would be possible to sense when a large discharge current is required and bypass the charging circuits altogether. The charging circuits (switch mode) would be a minuscule fraction of the cost of the actual Li-Ion battery they are attached to. Microcontrollers are incredibly cheap and you can add a lot of intelligence to the charging circuit for very little cost (e.g. Atmel’s ATtiny64 costs about 50p even if you’re only buying one of them from Farnell).
alioth wrote:
I would suspect for a battery like this the charging circuit would be built into the battery itself so that you could connect it to a normal 14V/28V bus and the in-built charging circuit would take care of it.
That’s exactly what I expect in a drop-in replacement for lead-acid batteries. After all, what else – either the required electronics are outside, or inside as those cells are not compatible. Question is how good a job they did. The battery might not explode, it might work just fine initially but it could very well die rather soon (e.g. handling of float charging).
There’s no reason that the starting current has to go through the charging circuit. It would be possible to sense when a large discharge current is required and bypass the charging circuits altogether.
Yes, of course, you put in a big diode (preferably a schottky diode) to bypass everything in the outgoing current direction. In fact a big (BIG) MOSFET might do that job just right. I used to do this for a living. But that doesn’t change what I am getting at. I just don’t believe these batteries contain all the electronics you would need for intelligent charging from a 14V or 28V bus. You are looking at some €100 in components, quite a bit of space, and some heat dissipation.
Of course one could settle that with some research 
but when I asked some vendors in the past they didn’t know.
I was initially worried about high current charging from the alternator. The ammeter whacks over when the engine starts, but soon settles back. I think the LiFePO4 technology is fairly mature.
Simon
Peter wrote:
You are looking at some €100 in components, quite a bit of space, and some heat dissipation.
Nowhere near 100eur in components. The microcontroller to look after it costs pennies and has pretty much all you need actually integrated inside it, the purpose built charging ICs cost pennies. Even the power MOSFETs cost little these days and you can use them in parallel (in other words, use five cheap ones instead of a single mega expensive one, you certainly wouldn’t choose a diode if you could possibly avoid it). Heat dissipation is probably not quite as bad as you imagine. The ESC for my T-Rex 700 RC helicopter (which has a motor as powerful as an aircraft starter motor, and runs for ten minutes at a time, not the few seconds an aircraft starter motor needs to run) gets barely warm. Space is not a problem either, the aforementioned ESC can fit in a matchbox.
I know someone in the US who put a Li-Ion battery in his RV – it had no external charging circuits. They are all built in.
Personally I use a gel lead-acid battery. For me going to lithium would hurt my weight and balance, the battery is required as ballast!
