So, if you are looking into this, what attributes would be especially useful in a battery specifically intended to power an aircraft? Things that come to mind:
- Fine grain in-battery health monitoring. Perhaps our airplane battery shouldn't be a single big pile of cells wired in series and parallel, but many monitorable (and in-flight shuntable) packages of cells. Got an overheat/anomalous voltage in one sub-battery? Take that sub-battery offline and continue flying. Do diagnostics on the ground, but prevent an inflight fire with monitoring and inflight actions before things turn critical.
- Cooling to improve safe C-rate (charging and discharging). Lots easier with more cells exposed to your chosen coolant working fluid (air or otherwise).
- Sub-battery containers to contain/limit effects of a thermal runaway. Facilitate active suppression of a fire? Intumescent coatings are light and effective within limits, can they slow the spread of fire and buy an extra 30 critical seconds in a bad situation?
- Plug-and-play subbatteries of convenient size and weight? There will be use cases where it is not optimum to charge batteries in the aircraft (where safe charging rates are just too slow, and a leisurely recharging on the ground extends the expected lifetime of the batteries). Being able to swap out twelve 10 lb sub-units is a lot more convenient than a single 120 lb battery or two 60 lb units.
To me, the safety aspects are top priority. A fire in a car or scooter battery is a big deal, but still the options in those cases are likely MUCH better than a runaway inflight fire in an airplane. I'd go so far as to suggest that pilots who aren't concerned about inflight fire (regardless of power source) haven't given the issue much thought (or they fly gliders).
Oh, and we need cheap, light, powerful, and reliable.
You are right - safety should be top priority.
Monitoring, reasonable discharge "C" rate, battery case slowing down fire.
I have been flying with 5 electric types.
Pipistrel Velis and
Pipistrel Alpha Electro - looking similar but definitelly not the same for flying. 2 battery packs (one in the front, one behind pilots).
Phoenix D-14. Two assymetric battery packs (16+34 kg) in the front.
Phoenix U-15E. Three battery packs. 2x 40 kg in the wings, 70 kg in the front.
Sagitta. One seater. Retractable motor. Two battery packs behind pilot (2x 15 kg).
Lets be realistic
Homebuilder will not make battery like proffesionall company. We have to try to make it with maximum safety = simplicity.
I would stay with idea of small battery for one seater like Sagitta.
For easy manipulation lets assume approx. 2x 15 kg (2x 33 lb) = 30 kg total (66 lb).
That can give us 6 kWh = TakeOff power up to 15-18 kW, Power for climbing +- 10 kW.
If we use typical well approved 18650 cells VTC6 (3,7 V, 3 000 mAh) than we will need +- 540 cells.
Case...temprerature monitoring can be done simply by adding multiple temp sensors between cells.
Question is what material should be used for case.
Of course would be ideal some material able to hold fire inside for as long as possible.
It means another question - how long time is long enough...
With plane like Sagitta is expected flight under cloud base and soaring.
In EU is not typical to fly often over 2 000 m (6 500 ft.).
If you notice temp. increasing you need to go down = trying to increase descent ratio to maximum.
Lets assume 5 m/s (+- 1 000 tf/min).
That means really long descent during approx. 7 minutes + time for landing.
So it is critical to give pilot notice as soon as possible that in battery is temp changing over limits.
Metal case will be probably too heavy, so we have to look for special protections.
Available is for example
KERATEM able to survive 1 100 °C.
We used this on fire wall on UL airplanes.
It is a "sticker" - glass fabric covered by Aluminium.
It is very easy to work with.
Also development of fire retardant paints is going well forward...
And here we have two conflicting requirements.
Close cells into case (no ventilation = no way to manage some cooling) or make active ventilation / cooling (but than you must be sure to lead possible fire gases out from cockpit).
From my point of view simplicity = safety.
Close cells into fireproof box, make good monitoring of temp by multiple sensors.
With "conservative" way of takeoff you use max. power (max, discharge rate) for as short as possible time. When you in the air at safe altitude (different on each airfield) decrease TO power to "continuous" climb power.
"Cruise" power should be 1C (=5-6 kW).
Going back to Sagitta I was using following settings:
- takeoff power 15 kW (possibility to go up to 20 kW).
- at 50 m (150 ft) reducing to 8-10 kW.
- cruise at 5-6 kW.
I beleive that that is "achievable" performance for homebuild airplane with acceptable electric propulsion system complexity and acceptable safety.
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