Battery pack selection for self-launching glider

[kubark42]

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?

 

[Grantmac]

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]

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.

 

[Stealth_Chopper]

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

 

[Grantmac]

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]

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.

 

[helno]

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]

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

 

[Solarsail]

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.

May I suggest nano-LiFePO4, like the A123? This will not catch fire, so you have covered that base. You say you don't need much energy but need a lot of capacity. The nano-LiFePO4 is around 50% energy density compared to lithium-ion, but since you are carrying not too many kWh, you probably will not notice the difference. LiFePO4 also has longer cycle life compared to LiPO and Li-ion.

Finally - the sweetness - nano-LiFePO4 can easily deliver 25C and under certain conditions 100C.

You may also be able to do this with standard LiFePO4, which is costwise close to li-ion and energy density wise about 80% of li-ion.

LiPo is least desirable. Simply not safe.

Can you indicate what kind of power you need for climbing, and what size of storage you want?

 

[Solarsail]

A 12 - 15 kg DIY Li-ion pack will have between 2.5 kWh to 3.5 kWh of energy capacity depending what cells are selected and how the packing is done. The same thing with 12V auto li-ion batteries will probably be 1 kWh to 2 kWh. I am surprised that you say DIY will cost $3K to $4K. At the current rate of $200 /kWh, that should cost as little as $700, if you DIY your own li-ion pack. There is really no point of buying off-the-shelf very heavy 12V li-ion batteries. How are you going to transmit cell temperature or cell performance data to your flight computer? You just can't do that easily. How are you going to expand the storage if needed? By putting on 2x more weight than is necessary. And how are you going to drive the cell down to 2.0 V in an emergency when the auto battery will cut you off at 3.0 or 3.2 V?

Not sure why you don't give us the capacity and storage needed for your project. Neither have you specified the current and voltage required.

Another option aside from nano-LiFePO4 is supercaps backed up with li-ion, as you need high capacity for only a short time. And you could be charging the supercap when you are in cruise mode. Supercaps are not much more expensive or heavier than li-ion. So I would have 2 kWh of li-ion (9 kg) or less and 0.75 kWh of supercap (probably 5 kg). You probably don't need to balance the supercaps if you put a 470 ohm resistor in parallel with each supercap so that overnight the caps discharge and do a bottom self-balance.

There are also li-ion cells by Samsung that supply 3C. Just put as many of these in parallel as you need to arrive at the desired amperage.

As I don't know the loads or the performance data, these figures are just a guess. The whole point of trying to minimize your energy capacity is counter-practice, and I don't know what the benefits are when li-ion cells are so cheap and relatively lightweight. Anyone who has had any battery operated vehicle will tell you there is no such thing as sufficient energy storage.

 

[kubark42]

May I suggest nano-LiFePO4, like the A123? This will not catch fire, so you have covered that base. You say you don't need much energy but need a lot of capacity. The nano-LiFePO4 is around 50% energy density compared to lithium-ion, but since you are carrying not too many kWh, you probably will not notice the difference. LiFePO4 also has longer cycle life compared to LiPO and Li-ion.

Finally - the sweetness - nano-LiFePO4 can easily deliver 25C and under certain conditions 100C.

You may also be able to do this with standard LiFePO4, which is costwise close to li-ion and energy density wise about 80% of li-ion.

LiPo is least desirable. Simply not safe.

Can you indicate what kind of power you need for climbing, and what size of storage you want?

The Nano LiFeP04 are interesting, thanks for the tip. The specific energy isn't as good, that's true, but compared to high discharge LiPo --which are about 50% higher in theory--, it's not so bad when taking into account a true 100% DoD. There might also be some weight savings in protection circuits and fireproof boxes, since the batteries naturally will only vent at low temperature.

I'm not particularly concerned about high cycle life. The glider will probably not launch 2000 times in its entire life, so if the batteries last even a quarter of that I think we'll feel satisfied.

The power and size is model dependent. Gliders go from as small as 75kg up to 750kg. The launch profile will stay the same, though. 95% of the value is getting off the ground, climbing for 3-6 minutes, and then shut-down from there. This implies the chemistry and approach are the same, although the absolute quantities will change by an order of magnitude.

A 12 - 15 kg DIY Li-ion pack will have between 2.5 kWh to 3.5 kWh of energy capacity depending what cells are selected and how the packing is done. The same thing with 12V auto li-ion batteries will probably be 1 kWh to 2 kWh. I am surprised that you say DIY will cost $3K to $4K. At the current rate of $200 /kWh, that should cost as little as $700, if you DIY your own li-ion pack.

Another option aside from nano-LiFePO4 is supercaps backed up with li-ion, as you need high capacity for only a short time. And you could be charging the supercap when you are in cruise mode. Supercaps are not much more expensive or heavier than li-ion. So I would have 2 kWh of li-ion (9 kg) or less and 0.75 kWh of supercap (probably 5 kg). You probably don't need to balance the supercaps if you put a 470 ohm resistor in parallel with each supercap so that overnight the caps discharge and do a bottom self-balance.

As I don't know the loads or the performance data, these figures are just a guess. The whole point of trying to minimize your energy capacity is counter-practice, and I don't know what the benefits are when li-ion cells are so cheap and relatively lightweight. Anyone who has had any battery operated vehicle will tell you there is no such thing as sufficient energy storage.

The price we see for suitable cells is significantly higher, by about 2x. Then realistically round up to 4-5kWhr, because at this price you want to be able to still launch when the batteries are at 80% capacity. Then you have to factor in tooling costs as well as incidentals such as the box and internal circuitry. Maybe $4k is on the high side, but none of us have come out for any less than $2500, so $700 seems very distant. Maybe that could be closer if we sacrificed on quality, but I'm not aware of anyone ready to take risk on this very critical part of the plane. In a perfect world, we could just resurrect a Tesla/Volvo/Nissan pack and use those cells, but we can't know for sure the cells are still good and thus aren't comfortable with anything other than new cells from a trustworthy source.

Supercaps have terrible specific density, AFAIAA, 10-100x less than batteries. They're better for instantaneous surges, such as peak acceleration, where the batteries would be damaged under the peak. We're more like a rocket, needing steady-state power up until the engines shut off and we go from there.

I wouldn't really say we're expressly trying to minimize energy. Instead, we need to minimize weight, weight, weight, risk, and cost. And weight. The easiest way to do all of that is to get the right pack, so we're not carting around a huge brick. Recall that a heavy battery is one people are more likely to drop, more likely to get lazy with and charge in an inappropriate spot, and, in the event of a crash, is more likely to hurdle through the plane crushing the pilot.

 

[kubark42]

May I suggest nano-LiFePO4, like the A123? This will not catch fire, so you have covered that base. You say you don't need much energy but need a lot of capacity. The nano-LiFePO4 is around 50% energy density compared to lithium-ion, but since you are carrying not too many kWh, you probably will not notice the difference. LiFePO4 also has longer cycle life compared to LiPO and Li-ion.

Finally - the sweetness - nano-LiFePO4 can easily deliver 25C and under certain conditions 100C.

You may also be able to do this with standard LiFePO4, which is costwise close to li-ion and energy density wise about 80% of li-ion.

LiPo is least desirable. Simply not safe.

Can you indicate what kind of power you need for climbing, and what size of storage you want?

The Nano LiFeP04 are interesting, thanks for the tip. The specific energy isn't as good, that's true, but compared to high discharge LiPo --which are about 50% higher in theory--, it's not so bad when taking into account a true 100% DoD. There might also be some weight savings in protection circuits and fireproof boxes, since the batteries naturally will only vent at low temperature.

I'm not particularly concerned about high cycle life. A glider will probably not launch 2000 times in its entire life, so if the batteries last even a quarter of that I think we'll feel satisfied.

The power and battery size is model dependent. Although gliders go from as small as 75kg up to 750kg, the launch profile will stay the same. 95% of the value is getting off the ground, climbing for 3-6 minutes, and then shut-down from there. This implies the chemistry and approach are the same, although the absolute quantities will change by an order of magnitude.

A 12 - 15 kg DIY Li-ion pack will have between 2.5 kWh to 3.5 kWh of energy capacity depending what cells are selected and how the packing is done. The same thing with 12V auto li-ion batteries will probably be 1 kWh to 2 kWh. I am surprised that you say DIY will cost $3K to $4K. At the current rate of $200 /kWh, that should cost as little as $700, if you DIY your own li-ion pack.

Another option aside from nano-LiFePO4 is supercaps backed up with li-ion, as you need high capacity for only a short time. And you could be charging the supercap when you are in cruise mode. Supercaps are not much more expensive or heavier than li-ion. So I would have 2 kWh of li-ion (9 kg) or less and 0.75 kWh of supercap (probably 5 kg). You probably don't need to balance the supercaps if you put a 470 ohm resistor in parallel with each supercap so that overnight the caps discharge and do a bottom self-balance.

As I don't know the loads or the performance data, these figures are just a guess. The whole point of trying to minimize your energy capacity is counter-practice, and I don't know what the benefits are when li-ion cells are so cheap and relatively lightweight. Anyone who has had any battery operated vehicle will tell you there is no such thing as sufficient energy storage.

The price we see for suitable cells is significantly higher, by about 2x. Then realistically round up to 4-5kWhr, because at this price you want to be able to still launch when the batteries are at 80% capacity. Then you have to factor in tooling costs as well as incidentals such as the box and internal circuitry. Maybe $4k is on the high side, but none of us have come out for any less than $2500, so $700 seems very distant. Maybe that could be closer if we sacrificed on quality, but I'm not aware of anyone ready to take risk on this very critical part of the plane. In a perfect world, we could just resurrect a Tesla/Volvo/Nissan pack and use those cells, but we can't know for sure the cells are still good and thus aren't comfortable with anything other than new cells from a trustworthy source.

Supercaps have terrible specific density, AFAIAA, 10-100x less than batteries. They're better for instantaneous surges, such as peak acceleration, where the batteries would be damaged under the peak. We're more like a rocket, needing steady-state power up until the engines shut off and we go from there.

I wouldn't really say we're expressly trying to minimize energy. Instead, we need to minimize weight, weight, weight, risk, and cost. And weight. The easiest way to do all of that is to get the right pack, so we're not carting around a huge brick. Recall that a heavy battery is one people are more likely to drop, more likely to get lazy with and charge in an inappropriate spot, and, in the event of a crash, is more likely to hurdle through the plane crushing the pilot.

 

[JackFlorey]

Over the last decade, the R/C world has really gotten a good handle on 15C-20C batteries.

Yep. But why not go with a 10C 18650 li-ion? It's unlikely that you will need less than 6 minutes of power start to finish. And li-ions are going to be somewhat more reliable and less likely to burn in the first place.

 

[kubark42]

Over the last decade, the R/C world has really gotten a good handle on 15C-20C batteries.

Yep. But why not go with a 10C 18650 li-ion? It's unlikely that you will need less than 6 minutes of power start to finish. And li-ions are going to be somewhat more reliable and less likely to burn in the first place.

If your mission profile is to constantly run near-- and even exceed for several minutes at a time-- the battery's peak continuous rating I don't think that battery would survive very long, and you'd probably be dissatisfied with the battery voltage performance at those high loads.

For that reason, people I know who are building packs are using 10-20C li-ion cells, and they get downrated to 4-5C actual draw.

 

[JackFlorey]

If your mission profile is to constantly run near-- and even exceed for several minutes at a time-- the battery's peak continuous rating I don't think that battery would survive very long, and you'd probably be dissatisfied with the battery voltage performance at those high loads.

I am not suggesting that anyone run beyond the battery's rating. Size for 10C max discharge. Use a good cell like the US18650VTC6. We've had excellent results discharging them at 15-20 amps. You will get a lifetime in the few hundreds of cycles - but since you will start with somewhat more capacity than you need (since you are downrating to 10C) that's not a big deal.

 

[kubark42]

I am not suggesting that anyone run beyond the battery's rating. Size for 10C max discharge. Use a good cell like the US18650VTC6. We've had excellent results discharging them at 15-20 amps. You will get a lifetime in the few hundreds of cycles - but since you will start with somewhat more capacity than you need (since you are downrating to 10C) that's not a big deal.

That cell is 3A, so wouldn't that be 5-7C, instead of the 15-20C which would fit the ideal launch profile?

 

[JackFlorey]

That cell is 3A, so wouldn't that be 5-7C, instead of the 15-20C which would fit the ideal launch profile?

Per data sheet that cell can discharge at 30 amps as long as temperature is limited to 80C or under.

 

[Solarsail]

The Sony VTC6-2 seems to be a good candidate.

A123 nano-LiFePO4 ANR32113UB: 4.5 Ah, 34C (30s peak 60C)
ANR26650M1A: 2.5 Ah, 30C (30s peak 60C)

Also the Sony VTC4, 2.1 Ah, at 14C, and VTC5A, 2.6 Ah, at 9.5C

LG has the INR21700 M50T - 5Ah, 6C
The HE4 18650 2.5 Ah at 12C
HB 1.5 Ah at 20C

Samsung INR:
48G 21700, 4.8Ah, 7.3C
40T 21700, 4 Ah, 7.5C
30T 21700, 3 Ah, 12C
25R 18650, 2.5 Ah, 8C
20R 18650, 2 Ah, 11C

So let's say you need 150 A max and 60 V max (51 V mean), and you choose the A123 Then you get:

32113UB: 16S1P, 3.3 kg, 0.23 kwh, 14 kg/kWh
26650M1A: 16S2P, 2.3 kg, 0.26 kWh, 8.9 kg/kWh

48G: 14S4P (only 140 A), 3.9 kg, 0.98 kWh, 4.0 kg/kWh
30T: 14S4P, 3.9 kg, 0.61 kWh, 6.4 kg/kWh
20R: 14S7P, 4.9 kg, 0.72 kWh, 6.8 kg/kWh
Sony VTC4: 14S5P, 3.5 kg, 0.54 kWh, 6.5 kg/kWh
VTC6 (6C, 3 Ah): 14S12P, 8.4 kg, 1.29 kWh, 6.5 kg/kWh
LG M50T: 14S5P, 4.9 kg, 1.28 kWh, 3.8 kg/kWh
LG HB: 14S5P, 3.5 kg, 0.23 kWh, 9.1 kg/kWh

If you need 150 A and 180 V max (153V mean), then
LG M50T: 42S5P, 14.7 kg, 3.83 kWh
M1A: 48S2P, 6.9 kg, 0.78 kWh
30T: 42S4P 11.8 kg, 1.85 kWh
VTC4: 42S5P 10.5 kg, 1.61 kWh

If you need to go to 240 V or 300 V, your only choice is the M1A to keep the weight below 12 kg. This cell will give you 210 V at 8 kg.

Of course if you need 150 A, you should design for 180 A.

Looks like your best bet for minimum weight is the ANR26650M1A. This cell has a long history and showed up in power tools about 20 years ago. My DeWalt is still working like new! In fact I am going to run my cell spot welder to assemble my 50E pack with these ANR26650M1A (300A required 2S3P).

If I were to choose, I would go with the Samsung 48G or LG M50T (or the VTC4), and put in at least 8 kg of that, which will give me 2.1 kWh. But this 8 kg limit will work only up to 120 V.

If your sailplane allows a pilot up to 100 kg, then disallow anyone over 90 kg, and install 18 kg of cells. Or remove half the packs for fat people (sorry, not PC). Carrying packs for recharge is silly and cumbersome. Anywhere that you may park the plane, just install a 230 V outlet. You can charge a 10 kg M1A in 9 minutes with a 30A circuit. Probably takes more time to remove the pack, carry it to and from, and re install pack, and check for proper installation. What if the thing is dropped, or gets wet? What if the pack is 120 V, and the pilot gets a shock? Why bother with these extra complications and inconveniences. Any connector carrying 150A has a chance of full or partial failure. A non-removable pack does not need a connector.

 

[Solarsail]

The price we see for suitable cells is significantly higher, by about 2x.

In quantity you should be able to find stuff on Alibaba at below $300 / kWh.

Then realistically round up to 4-5kWhr, because at this price you want to be able to still launch when the batteries are at 80% capacity.

But if weight is at a premium, why launch with 80% capacity? And 5 kWh will be about 25 kg, and my understanding is that you want to keep it under 10 kg and about 2 kWh.

As you can't provide the energy storage capacity, current or voltage, how can we even discuss costs? You say, the MTOW range is a factor of 10 or so. So the pack price will be anywhere.

If you want minimal weight, assuming 16s8p or 32s4p or 64s2p for the M1A (128 cells), that gives you 1.04 kWh, a max of 31 kW, and which gives you 6 minutes of climb at 10.4 kW. That will be 9.2 kg. With the 48G and 60 cells you get the same energy (1.05 kWh) and a max of 7.7 kW and that will be 4.2 kg.

Supercaps have terrible specific density

Yes, I went off the wrong data. Supercaps are not an option in any form.

I wouldn't really say we're expressly trying to minimize energy. Instead, we need to minimize weight, weight, weight, risk, and cost. And weight.

To minimize weight and cost assuming 60V controller and 8 kW climbing, you can do it at 4.2 kg with the 48G: 1.05 kWh and 7.7 kW max power. With M50T: 4.9 kg, 1.25 kWh, 7.7 kW max power. To minimize risk you need the M1A: 9.2 kg, 1.04 kWh and 31 kW max power.

I really doubt that you want to have only 1.04 kWh on board, or barely enough to reach 1,000'. Makes no sense to me. The first thing you will hear from your customers is that they want emergency power for outlanding or getting out of terrain, or simply to go somewhere that the thermals are better. I mean some e-bicycles have more storage than that.

You should settle for the 48G with at least 2.5 kWh (10 kg). For those who can pull a power cord to their sailplane, cut costs and risks by making the pack unremovable. Also offer solar charging -- including inflight solar. Also offer the option to jettison the pack with a chord attached where the chord can be released in a suitable location. I think you will discover that the 48E is safe enough, if it gets adequate cooling. The Sony cells are probably the least risky non-LFP -- but weigh 60% more than the 48G at low kWh.

 

[Solarsail]

To raise 400 kg takeoff weight to 1000', the energy needed is U(kWh) = 400*9.81*1000/(3.28 * 3600000) = 0.332 kWh. Assuming a glide ratio of 30 (will be less while the motor is out) and a prop efficiency of 70%, motor of 90%, inverter of 90%, battery of 95%, and everything else at 95%, you get energy needed:

0.332 / ((1-1/30)*.7*.9*.9*.95*.95) = .332/.494 = 0.67 kWh. At the rate of 1000' per 6 mins, power required would be 6.7 kW.

You have mentioned 4 to 5 kWh. That would take you to 6,700' (in theory and neglecting the altitude density).

It appears you may want 2 kWh, which can take you to 3,000'. With the option to add 2 kWh modules up to 10 kWh in parallel. Each 2 kWh module will be about 9 kg and 5 liters.

Check this out:

https://www.ennoid.me/battery/pack

 

[spinningmagnets]

When it comes to battery volume, there is often a trade-off between current and capacity. For instance the Samsung 20R 18650-format cell is well regarded for high current, but it only has 2000-mAh. The MJ1 in the same format has 3400-mAh, but is typically limited to 10A per cell.

I recommend you look at hybrid EV pouch cells. The cells in a pure-EV Tesla are optimized for long range (300 miles is tupical), and they only achieve high amps through using thousands of them in a pack.

A plug-in hybrid like the Chevy Volt or Honda Clarity must provide full vehicle performance from a much smaller pack. For instance, the Clarity is rated at 40 miles electric-only range, and the volt is known to provide 80 miles.

It may benefit your pack weight to use the lowest volts that will work, such as 48V, 24V or even 12V

 

[kubark42]

I really doubt that you want to have only 1.04 kWh on board, or barely enough to reach 1,000'. Makes no sense to me. The first thing you will hear from your customers is that they want emergency power for outlanding or getting out of terrain, or simply to go somewhere that the thermals are better. I mean some e-bicycles have more storage than that.

Of course they will. But will they want to pay for it, and wait for the development, and haul it out to recharge it (this is absolutely non-negotiable until battery tech is rock solid with a decade of experience in this application), and have it compromise every aspect of the flight they're actually going to do? My money is on not. The nations with advanced glider cultures (France, Germany, Czech, etc...) are pros at getting up and staying up, starting with 1000-1500' of altitude at launch.

That being said, I ran some numbers on the LiFePO4 packs and realized that a hybrid approach, pairing a high specific-power LiFePo with a high specific-energy Li-ion battery, is kind of interesting. At a ratio of about 3:2 for LiFePo series cells vs Li-ion series cells (e.g. 18s LiFePo, 12s Li-ion), the LiFePo is almost completely discharged (3% SoC) when the Li-ion is at 20% SoC.

(Of course there needs to be a diode to keep energy from flowing into the Li-ion pack, but that isn't so bad and with good component choice it's reasonable to have only a 1% power loss across it.)

The takeaway is that a high power pack can boost the glider off the ground and yet be tailored to be practically empty right at the 3 minute mark. After that, the high energy density pack provides sufficient power to either sustain or even to climb gently (50fpm).

I have run several simulations and it seems that somewhere around a 1:1 ratio of LiFePo capacity to Li-ion capacity is a good spot. Furthermore, it looks when you take into account the average glider, the 20C rate works out to about 3Whr of LiFePO4/1kg of glider MTOW. The upshot is that the 300kg MTOW has a > 900Whr LiFePo4 battery and a > 900Whr Li-ion = 1.8kWhr.

Nicely, since the LiFePo can go all the way to 3% SoC in this setup, the effective battery capacity is 900*.97+900*.8 = 1.59kWhr, whereas a standard Li-ion with a 20% SoC floor would give 1.8*.8 = 1.44kWhr.

Weight isn't awesome, since the A123Systems 26650 versions of the LiFePO4 are only around 108Whr/kg, and that's before counting all the stuff needed for a pack build. So figure 9kg for the LiFePO4 and another 4kg for the Li-ion. 13kg pack total weight. That's completely doable, though, and is 10+kg less than the alternative pure li-ion pack.

I'm not sure this makes sense from a holistic viewpoint, since now there are two chargers, two BMSes, two connectors, etc... But if I could source the pouches instead of the 26650 cans that would save another 1-2kg, likely.

 

[kubark42]

A plug-in hybrid like the Chevy Volt or Honda Clarity must provide full vehicle performance from a much smaller pack. For instance, the Clarity is rated at 40 miles electric-only range, and the volt is known to provide 80 miles.

That was an intriguing idea, but ultimately the batteries in a car aren't designed to reach full discharge in 2-4 minutes. I'm not sure of the Honda Clarity cells, but it looks like they have a discharge rate in the 4-5C territory.

 

[Solarsail]

Packs should of course be removable, but not necessarily portable. Carrying them to charge makes little sense, even if they are 15 kg. Easier to pull a cable, or install an EVSE where the glider is parked.

In a post above I calculated the power needed for 170 fpm climb. Let's say MTOW is 300 kg and 250 fpm is required.

The energy required is 0.25 kWh. After correcting for the losses, you get 0.5 kWh gross energy. Power required for 250 fpm will be 0.5 * 60 * 250 / 1000 = 7.5 kW.

It appears you are seeking about 2 kWh of storage. The Samsung 21700 48G unit weight is 4 kg/kWh. With a 20% pack overhead, the 2kWh pack will weigh 10 kg. At 7.3C,the pack can supply 14.5 kW power. But you only need 7.5 kW of power. Even at 10 kW, you are well below the C limit of 14.5 kW.

So I fail to understand why you insist on 20C at 0.9 kWh = 18 kW, when you don't need 18 kW, and when the 48G can supply 14.5 kW.

Now, since you are prepared to go to 15 kg, that will be 3 kWh using 48G, which is 22 kW capacity and is greater than the hybrid design of 18 kW -- with energy capacity being 3 kWh vs the hybrid 1.8 kWh.

For the same weight (of 15 kg) a 48G design will give you 22% more power, about 3x the power that you need, and 67% more energy storage compared to a hybrid. (Even 5C can satisfy your power requirement.) 20C will just set you back by increasing weight or decreasing energy storage, and would require pretty fat cabling. Not to mention the complications of two different chemistries, such as the BMS and charger.

You will also save about another 5 kg with a 10 kW motor and inverter, vs. a 20 kW motor and inverter. Three phase inverters can weigh more than the motor itself.

Note that 3 kWh goes a long way to mitigate range anxiety in a glider. I would probably want 6 to 8 kWh in my glider. Increasing MTOW from 300 to 320 kg I believe is not a big deal.

Nicely, since the LiFePo can go all the way to 3% SoC in this setup, the effective battery capacity is 900*.97+900*.8 = 1.59kWhr, whereas a standard Li-ion with a 20% SoC floor would give 1.8*.8 = 1.44kWhr.

Note that the 20% SoC floor will count towards the reserve.

 

[kubark42]

I appreciate the analysis. Unfortunately, the reality of takeoff forces us to have a minimum climb rate which is significantly higher than 0.9m/s. The absolute minimum safe takeoff climb is generally understood to be around 1.5m/s, and this is only acceptable on long (>3500') paved runways on days when there is not likely to be strong sink. This is contrary to most airports where gliders fly, which are grass and on the order of 2000', and the likely atmospheric conditions on days we want to fly (strong updrafts mean there must also be strong downdrafts).

The target climb rate is 2.5m/s, with 2m/s being marginally acceptable in real-world conditions and 3m/s being desirable.

When we're looking at the specific numbers of an AC-5M, with its existing 18.3kW powerplant and 46" prop its climb rate is just barely on the positive side of marginal. Any less climb rate and the plane would not be safe to fly as a self-launcher. So I'm confident that 16kW (with a 55" prop) is around the minimum acceptable electrical system.

As you've noticed in your calculations, climb rate is the real driver of power consumption. The airframe drag is a pretty constant no matter if climbing or sustaining. Everything else goes to climb.

In general, because the airframe drag is ~25% of the overall power budget, reducing weight by 10% increases climb by 15-20%.

Packs should of course be removable, but not necessarily portable. Carrying them to charge makes little sense, even if they are 15 kg. Easier to pull a cable, or install an EVSE where the glider is parked.

We're going to have to agree to disagree here. Best practices are to remove the pack from the plane and charge it separately. The glider is either in a hangar-- which means a fire could spread to millions of dollars in other people's planes-- or in a trailer-- which means a fire could spread to adjacent gliders. Either way, the neighbors will appreciate knowing there are not potentially dangerous levels of batteries being charged in proximity to their planes.

Note that 3 kWh goes a long way to mitigate range anxiety in a glider. I would probably want 6 to 8 kWh in my glider. Increasing MTOW from 300 to 320 kg I believe is not a big deal.

Increasing MTOW is not possible. First off, it's not legal, only the plane's builder can increase MTOW. Second off, even for the builders it is extremely ill-advised to increase the paper MTOW without enhancing the structure. Richard VanGrunsven, the creator of the RV line of homebuilts, has strong words on this topic: https://www.facebook.com/notes/vans-aircraft-inc/what-price-a-masterpiece-by-dick-vangrunsven/237594966250883

Increased weight touches upon so many domains, going far beyond flight forces. For instance, it's unlikely that the landing gear, tire, suspension, brakes, etc... can all absorb a routine landing weight 10% over the maximum the manufacturer designed. Safety factors in aviation are frequently 1.2-1.5x.

Nicely, since the LiFePo can go all the way to 3% SoC in this setup, the effective battery capacity is 900*.97+900*.8 = 1.59kWhr, whereas a standard Li-ion with a 20% SoC floor would give 1.8*.8 = 1.44kWhr.

Note that the 20% SoC floor will count towards the reserve.

Excellent point. The 20% is still there if you really need it.