Help me design a LiFePO4 battery for my homebuilt airplane

[cringely]

I have a homebuilt airplane, a Thorp T-18C, which carries two people and has a Lycoming IO-320-B1A engine rated at 160 hp. I'll shortly be getting it back in the air following a six-year rebuild (the plane was originally built in 1999). It needs a new 12V battery (for starting, radios, etc., just like a car) and I am thinking of building my own.

Here is the plane:



Here is the battery I'm replacing: http://concordebattery.com/flyerprint.php?id=35

It's 12V, 22 amp-hours, 225 CCA, and 22.7 lbs (ouch!). Oh, and because it's for airplanes it costs $260.

There are now lithium batteries available for this application. They crank better and weigh a LOT less (7 lbs compared to 22), but are even more expensive at $609: https://www.aircraftspruce.com/catalog/elpages/earthxbatteries_11-16211.php

Since this is an experimental amateur-built aircraft there really is no rule against my building my own battery, so I am starting (if this is wrong, please tell me) with four of these cells from Battery Hookup: HEADWAY 38120 HP 3.2V 8AH LIFEPO4 BATTERY. They are now sold out but my four cells arrived today.



There is a BMS available, which I didn't buy but can still order one.

The whole package is about three lbs.

Now I have to decide how to build the actual battery. It will be mounted on the firewall right by the engine, replacing the former battery, so the cable lengths are all short. I doubt that I will EVER charge this battery at below-freezing temps. I wouldn't start the engine below freezing for one thing (pre-heat -- airplane engines are pampered), and it won't charge without the engine running. I can build it 2X2 or as a 1X4 flatpack, which might mount better on the firewall. Oh, and Will, the plane is in LAS VEGAS.

 

[snoobler]

Good looking plane.

Trying to wrap my head around something thinking WAY back to my flying days in the Mouseketeer (oddly enough, last flight was in 1999). Battery was in the tail... threw me for a loop when you said it was on the firewall.

Knowing your old battery was 225CCA means that it could deliver 225A and hold above 7.5V.

Your LFP battery is about 1/3 the capacity of your AGM, but they have a 25C rating, or 200A. This is on the line assuming you actually need 225A to crank the engine. Since you will likely have less voltage drop, you may find the engine spins faster during start, and you may need less than 225A to start as well.

What is your alternator's peak voltage and current? You need the alternator to be below 14.4V, and the cells can readily handle 80A charge. Likely not an issue because a start isn't going to drain the battery that much, and you'll hit bulk voltage pretty quick. Also good to know that your alternator will float at 13.6V max.

A BMS is mandatory - one that can sustain 200A+ is needed.

 

[evad]

I would think of a alternate source of power for the radio and any critical items incase the bms shuts things down. Just a small battery.

 

[svetz]

Lead acid batteries love being stored at 100% charge and degrade when not stored at 100% charge.

Lithium are the other way around, see SoC degradation of LiFePO4 for more. It's not a big deal to store them with 100%, you just loose a few percent capacity per year.

How frequently do you fly or would it be a problem to recharge the battery before flying?

The reason I ask is there is another alternative. A supercapacitor battery drop-in replacement (e.g., https://www.amazon.com/Maxwell-Durablue-500Farad-Capacitor-Battery/dp/B07V5VN9QB) for about the same price as the expensive heavy airplane battery, but they also weigh a lot less, last forever, and are fairly insensitive to vibration.

They're not ready for prime-time replacement of lead acid batteries as they self-drain quickly (who wants a dead battery after returning from the airport after a two week vacation)? But for vehicles used a lot, or where you don't mind a 2 minute charge up, they can be very good and cost effective.

Batteries hold voltage steady, for example a lead acid is still around 10V even though it's exhausted. Capacitors are near linear, they don't hold voltage steady by themselves (although it's my understanding the Maxwell batteries have some internal tricks (boost converter?) to get around that).

PROS	CONs
Seconds to minutes to charge Weight on par with Lithium Millions of Cycles Don't lose power in extreme cold (-40F) Can be recharged in extreme cold (-40F) Have huge C-Rates	Probably lose 50% charge per week, so need recharging if not used Don't have the power density of LA or LFP

You might even be able to use a solar tender to keep them charged if the plane isn't in a hangar.

As a final caveat, I don't have any experience with these... so not speaking from experience, just what I've read.

 

[BiduleOhm]

Trying to wrap my head around something thinking WAY back to my flying days in the Mouseketeer (oddly enough, last flight was in 1999). Battery was in the tail... threw me for a loop when you said it was on the firewall.

Well weight and balance are really important on a plane and the battery is a heavy and easy thing to move to adjust the W&B during the design stage

I would think of a alternate source of power for the radio and any critical items incase the bms shuts things down. Just a small battery.

Very good point. But instead of having another battery a simpler and lighter solution would be to have a relay by-passing the BMS for the critical loads

A supercapacitor battery drop-in replacement (e.g., https://www.amazon.com/Maxwell-Durablue-500Farad-Capacitor-Battery/dp/B07V5VN9QB) for about the same price as the expensive heavy airplane battery, but they also weigh a lot less, last forever, and are fairly insensitive to vibration.

That's a very nice idea but in a plane you really need to have a decent battery capacity to power the critical loads in case the alternator fails and sadly I'm pretty sure supercaps aren't good enough for that.

 

[Pierre]

Well weight and balance are really important on a plane and the battery is a heavy and easy thing to move to adjust the W&B during the design stage



Very good point. But instead of having another battery a simpler and lighter solution would be to have a relay by-passing the BMS for the critical loads



That's a very nice idea but in a plane you really need to have a decent battery capacity to power the critical loads in case the alternator fails and sadly I'm pretty sure supercaps aren't good enough for that.

Great idea with the BMS bypass - have 2 x busbars of which the one is for critical loads.

 

[Pierre]

Good looking plane.

Trying to wrap my head around something thinking WAY back to my flying days in the Mouseketeer (oddly enough, last flight was in 1999). Battery was in the tail... threw me for a loop when you said it was on the firewall.

Knowing your old battery was 225CCA means that it could deliver 225A and hold above 7.5V.

Your LFP battery is about 1/3 the capacity of your AGM, but they have a 25C rating, or 200A. This is on the line assuming you actually need 225A to crank the engine. Since you will likely have less voltage drop, you may find the engine spins faster during start, and you may need less than 225A to start as well.

What is your alternator's peak voltage and current? You need the alternator to be below 14.4V, and the cells can readily handle 80A charge. Likely not an issue because a start isn't going to drain the battery that much, and you'll hit bulk voltage pretty quick. Also good to know that your alternator will float at 13.6V max.

A BMS is mandatory - one that can sustain 200A+ is needed.

 

[BiduleOhm]

NB: be careful about inadvertently powering the non-critical loads when in backup mode.

The simplest and most reliable solution to avoid that is to use the BMS to power the non-critical loads and the relay (via a diode, ideally two in series) of the critical loads instead of powering the critical loads directly.

You can then add a switch who also power the relay (no diode needed for the switch, and one less thing to fail) for the backup mode.

Also, if you don't use an avionic grade relay because of the price I would recommend two relays in parallel (if possible of different brand and model) just in case one would fail.

Tell me if it's not clear enough as using words to describe a schematic isn't the best thing usually...

 

[Pierre]

Post deleted.

 

[Pierre]

NB: be careful about inadvertently powering the non-critical loads when in backup mode. The simplest and most reliable solution to avoid that is to use the BMS to power the non-critical loads and the relay (via a diode, ideally two in series) of the critical loads instead of powering the critical loads directly. You can then add a switch who also power the relay (no diode needed for the switch, and one less thing to fail) for the backup mode.

Also, if you don't use an avionic grade relay because of the price I would recommend two relays in parallel (if possible of different brand and model) just in case one would fail.

Tell me if it's not clear enough as using words to describe a schematic isn't the best thing usually...

Agree. Aviation is all about 'built in' redundancy -- you cannot just pull up to the side of the road Recently watched "Cessna 310 Pilot" video and on this twin both vacuum pumps failed at the same time. Luckily he had lots of very fancy, modern electronic systems as back up.

 

[BiduleOhm]

Yep, and the good thing about old school motors like the one you have is that they don't require the electric system to be working to run so that's good

And actually a better solution (no diode needed) would be to use a double throw switch to toggle the relay from between the BMS output and the battery negative (BMS usually are on the negative side of the battery so the switch can't be on the positive side), even less things to fail, no semiconductors involved, less weight and complexity, ...

You can even use a double pole double throw switch so it is redundant in itself too

 

[John Frum]

A supercapacitor battery drop-in replacement (e.g., https://www.amazon.com/Maxwell-Durablue-500Farad-Capacitor-Battery/dp/B07V5VN9QB) for about the same price as the expensive heavy airplane battery, but they also weigh a lot less, last forever, and are fairly insensitive to vibration.

Brand: SHUNBIN

That does not inspire confidence.

 

[BiduleOhm]

Brand: SHUNBIN

That does not inspire confidence.

Yep, for a plane I would not use anything else than Maxwell supercaps sourced from a reputable seller.

 

[Pierre]

At least with experimental and non-type certified aircraft one has a bit of leeway when it comes to parts and systems. In my flight school all our aircraft were type certified resulting in heavy maintenance and running costs. Anything that floats or flies will cost you lots money.

 

[BiduleOhm]

Yep, it makes sense to use the aeronautic grade for certain parts but for others it's just wasted time and money.

 

[fat_old_sun]

I have a homebuilt airplane, a Thorp T-18C, which carries two people and has a Lycoming IO-320-B1A engine rated at 160 hp. I'll shortly be getting it back in the air following a six-year rebuild (the plane was originally built in 1999). It needs a new 12V battery (for starting, radios, etc., just like a car) and I am thinking of building my own.

How about high quality supercapacitors installed in the aircraft for runtime 'battery' and a portable lithiium jumpstarter for starting the engine?

 

[BiduleOhm]

It has been suggested already and I've answered why it's probably not feasible at the end of this post. It's sad because it would be a very nice solution.

 

[snoobler]

Yep, it makes sense to use the aeronautic grade for certain parts but for others it's just wasted time and money.

I heard tale of a renegade soul who owned a type certificated aircraft, who refused to pay the many $100s needed to replace the brushes in their generator after learning that a 196X something or other car generator brush was identical, and it cost $1.74. $50 for an armature turning at a local turbo shop, and they were back in business.

 

[BiduleOhm]

Nice story ?

As long as you know exactly what you're doing it's fine to do that kind of thing

Edit: ah, it was on a certified plane... well even if it's ok technical-wise if there's any problem involving your mod then you'll have lots of legal problems, so don't do that on certified stuff... on experimental stuff it's ok tho.

 

[John Frum]

If your airplane falls out of the sky that is bad.
If it lands on my head that is worse.