Battery pack selection for self-launching glider

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kubark42   1 mW

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Battery pack selection for self-launching glider

Post by kubark42 » Feb 22 2021 11:13am

I'm converting an AC-5M to electric, and I've been struggling for a while now to figure out how I want to do my battery pack. I keep coming back to LiPo as the right solution. When I look at the existing eGlider solutions, I've come to realize that they have unnecessarily large packs because they need the amps. So the packs wind up costing more, and worse, weighing significantly more. Weight is the enemy in aviation, it makes you come down faster, sooner, and harder.

The mission profile for a self-launching glider is very different from an ePlane. In the latter case, you need hours of motor time, but in an eGlider you need 3-5 minutes to climb high enough to connect with a nearby thermal. 3-5 minutes pack life sounds much more like an R/C hotliner than it does an EV.

Over the last decade, the R/C world has really gotten a good handle on 15C-20C batteries. There's extensive test data and suppliers are easy to come by. Companies have been around long enough to establish reputations and the products are mature. Fires aren't a thing of the past-- there will always be mistakes and foolish behavior-- but buying good quality packs and chargers it seems to overwhelmingly mitigate the fire risk.

The price and weight for a LiPo pack which can launch the AC-5M to 300m (~1000') is $500 and 3kg. The same pack in LiPo is going to be $3-4k and 12-15kg. The Li-ion pack will be good for another 50 minutes of cruise flight, but that's not the standard mission profile and so that extra battery time is typically unused dead weight.

Safety is a concern, but the 3kg lithium fire is a lot easier to manage than the 15kg one. Furthermore, from an operational standpoint a 3kg pack is one people won't hesitate to remove from the airplane for charging, whereas a 15kg one might lead people to be lazy and charge the battery in situ.

So there are some strong reasons to use LiPo over Li-ion. However, using LiPo is bucking the industry trend, and I hate to reinvent wheels if it's not required.

Am I missing something important here?
Last edited by kubark42 on Feb 25 2021 3:03pm, edited 2 times in total.

Grantmac   10 kW

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Re: Battery pack selection for self-launching glider

Post by Grantmac » Feb 22 2021 1:04pm

Finding a way to eject the pack in event of a fire is much easier with the smaller pack.

Keep in mind that using Lipo at high drain will reduce pack life way down. But then again how many flights per year are you going to get?

kubark42   1 mW

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Re: Battery pack selection for self-launching glider

Post by kubark42 » Feb 22 2021 1:43pm

Grantmac wrote:
Feb 22 2021 1:04pm
Finding a way to eject the pack in event of a fire is much easier with the smaller pack.

Keep in mind that using Lipo at high drain will reduce pack life way down. But then again how many flights per year are you going to get?
As much as I would love to eject a burning pack, that's probably not in the cards. There are logistical and practical hurdles (and even a few regulatory ones) which make it that it's a lot easier to manage the fire in the plane. One of the nice things about a small pack is that there less energy to dissipate in the event of a thermal runaway. An insulated metal box properly vented to the outside of the plane is probably sufficient to keep a small fire from spreading to the airframe.

You raise a good point about reduced pack life. This will occur with either choice, and at the same relative rate (assuming that packs are spec'ed for the minimum size), so the 8x less expensive pack costs less 8x per launch. It also costs 8x less per year, because lithium batteries have a maximum shelf life no matter if they are used frequently or not. I fully expect the airplane to outlast several battery pack lifecycles, so this is highly relevant across the lifetime of the airplane.

Of course, saving $1000 and burning your plane in exchange is a false economy, which is why I want to be careful to have thought it all through.

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Stealth_Chopper   10 W

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Re: Battery pack selection for self-launching glider

Post by Stealth_Chopper » Feb 22 2021 2:34pm

. . . "Am I missing something important here?"
Yes, fun. :D
Why not 'just' fly a sailplane twice today and you'll be completely "immersed" piloting the aircraft.
Good Luck
Mike

Grantmac   10 kW

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Re: Battery pack selection for self-launching glider

Post by Grantmac » Feb 22 2021 4:47pm

A very marginal battery could end up not working at all after losing just 20% capacity. So cutting it really close could result in a very short usable life span.

Then again from what I understand once you've taken off a sail plane can climb very efficiently. Throttling down once you've reached a safe abort altitude could make a small pack function just fine.

kubark42   1 mW

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Re: Battery pack selection for self-launching glider

Post by kubark42 » Feb 23 2021 10:31pm

Grantmac wrote:
Feb 22 2021 4:47pm
A very marginal battery could end up not working at all after losing just 20% capacity. So cutting it really close could result in a very short usable life span.

Then again from what I understand once you've taken off a sail plane can climb very efficiently. Throttling down once you've reached a safe abort altitude could make a small pack function just fine.
I think you've got it exactly right. The 20C discharge rate is required to quickly get to 300', the minimum safe altitude for a power failure. After that, the climb rate can be much more leisurely, dropping to 5-10C. Once the plane gets to 1000', there's not much point in climbing anymore at all and it can simply go off upwind in search of a thermal.

In order to survive 1-2 minutes at the smaller LiPo pack's 20C current, the li-ion pack needs to be 5x larger (4C rate). This is critically important when taking into account your point about marginality.

It would be nice to have a double pack, one optimized for 1 minute at 60C and one optimized for 5 minutes at 5C. I'm not aware of such a thing, though, and am not particularly keen on taking on the research project to manage the switchover point.
Last edited by kubark42 on Feb 25 2021 6:57pm, edited 1 time in total.

helno   10 W

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Re: Battery pack selection for self-launching glider

Post by helno » Feb 24 2021 1:59pm

Since the extra capacity is not going to kill your weight budget why not go for a pack that doesn't get worked quite so hard.

Sure the cells can handle 20C but doing that often will greatly reduce the lifespan.

Make the 15 kg pack into two parts and it would be easy enough for someone to carry one module in each hand. You could also parallel it to charge and get away with a less expensive charger.

thepronghorn   1 kW

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Re: Battery pack selection for self-launching glider

Post by thepronghorn » Feb 25 2021 6:00pm

For 15-20C applications, pouch cells are the right format. 18650s can't support such high discharge rates.

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