Battery pack selection for self-launching glider

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Solarsail   10 W

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Re: Battery pack selection for self-launching glider

Post by Solarsail » Apr 04 2021 5:46am

kubark42 wrote:
Apr 04 2021 3:30am
Well, if your concern is spontaneous combustion, then forget about the hybrid approach and you will have to stick with the A123 26650 (M1A). In which case 15 kg will give you roughly 1.56 kWh, and 47 kW. This should check all your boxes. These cells have high cycle life.

BTW, you mention that 25% of power is lost to drag. But the glide ratio L/D is 30. So should you not lose only 1/30 = 3.3% to drag?

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Re: Battery pack selection for self-launching glider

Post by kubark42 » Apr 04 2021 8:25am

Even LiFePo4 can spontaneously combust. They are more resilient, though. The goal is to reasonably minimize the chances of fire, which I think can be done either with the hybrid idea or a pure LiPo approach. I have bought some A123 cells to test the hybrid pack and see if it works.

Power is F*v, so while the force is m*g/(L/D), the drag losses are m*g/(L/D) * v. The v term is going to always be around 25m/s, almost independent of the glider.

Also consider that when a propeller pylon is deployed the drag increases tremendously. On the AC-5M (published 35:1 L/D), the ungainly engine pylon is literally as much drag as the rest of the plane combined, so the L/D drops to 18:1. This means that drag power is doubled.

Fortunately, a motor has much less drag than an engine, so we can expect a bump in L/D. I'm conservatively penciling it in at 22:1.

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Solarsail   10 W

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Re: Battery pack selection for self-launching glider

Post by Solarsail » Apr 04 2021 1:18pm

kubark42 wrote:
Apr 04 2021 8:25am
For a hybrid pack, I would recommend 14s LCO and 16s LFP. 14 * 4.2 = 58.8 V. 16 x 3.7 = 59.2. If you parallel them without the diode, you get 58.8 / 16 = 3.675 for the LFP which is perfect. You can then use one charger, and two BMS. You can still use a bunch of diodes in parallel to isolate the two, but they would need to be heat sunk.

Another option would be 18s LFP and 16s LCO without diodes. Of course this depends on the motor rating.

You probably would want to overrate the inverter by 25% to 50%, for emergency power.

5 m2 of germanium 30% multi-junction photoelectric cells can give you 1.5 kW on a good day. That would be a great way to charge in-flight.

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Re: Battery pack selection for self-launching glider

Post by kubark42 » Apr 05 2021 1:31pm

Solar cells are probably a topic for another day/plane. There's only so much retrofitting that can reasonably be done.

I just did some simulations massaging the cell ratios and it looks like it would depend heavily on the bulk cell discharge capacity. If it can do around 5C, which is about what we'd want for a 10-15 minute total pack time, then it looks like an 18:12 ratio is better than 18:13 or 18:14. As the bulk cell count creeps up, the LiFePo4 takes longer to reach full discharge.


Screen Shot 2021-04-05 at 2.27.32 PM.png
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Screen Shot 2021-04-05 at 2.27.11 PM.png
Screen Shot 2021-04-05 at 2.27.11 PM.png (85.28 KiB) Viewed 72 times
The vertical lines are when the LiFePo pack reaches 5% SoC and the Li-ion pack reaches 20%.

If the LiFePo pack reaches its low discharge shoulder at the same time as the li-ion pack, then it's less ideal because the LiFePo can take far more full-discharge cycles than the Li-ion.

We really only want the high specific power pack in the first couple minutes, when >2.5m/s climb is a safety requirement. Once >300m off the ground, climb rate becomes a secondary consideration. The primary consideration is sustained time aloft while hunting lift.

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Solarsail   10 W

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Re: Battery pack selection for self-launching glider

Post by Solarsail » Apr 07 2021 4:05pm

....
Last edited by Solarsail on Apr 07 2021 4:08pm, edited 1 time in total.

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Re: Battery pack selection for self-launching glider

Post by Solarsail » Apr 07 2021 4:07pm

kubark42 wrote:
Apr 05 2021 1:31pm
Can you also plot the graph for 18s LFP and 16s LCO without diode? Could you also draw pack voltage on the chart?

I wonder if you have included the internal resistance in your simulation. The LFP cells will have 1/6 of the IR of the LCO cells. I think this will assure that at high C, 5/6 of the current comes from LFP. While at low-C, the LCO supplies most of the power.

You may be right having uneven voltages, if you don't care for high-C after a few minutes. With 18s LFP/16s LCO, the LCOs will recharge the LFPs. This may not be desired.

In which case you would need two chargers which complicates things. You could also drop the charge voltage for the LCO using a regulator. But that may defeat the CCCV action of the charger.

I wish BMSs had a CCCV regulator on board.

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Re: Battery pack selection for self-launching glider

Post by kubark42 » Apr 07 2021 6:13pm

Solarsail wrote:
Apr 07 2021 4:07pm
Can you also plot the graph for 18s LFP and 16s LCO without diode?
Which specific battery were you thinking? The way I did the simulation was to extract the data points from battery discharge tests, and then use a converging interpolating function to solve for the voltage drop and current split. I would need a graph like below in order to do a new simulation.
Screen Shot 2021-03-24 at 8.23.09 AM.png
Screen Shot 2021-03-24 at 8.23.09 AM.png (507.04 KiB) Viewed 43 times
P.S. I can heartily recommend Engauge Digitizer for systematically extracting data from published graphs.

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