I think something has been lost in translation, but I'm pretty sure you will not shut down from battery heat - my batteries barely change temperature regardless of use, and neither controller nor motor have temperature problems even running through most of the capacity at full power.
What does happen is the voltage quickly drops as used capacity goes up, such that while I can get ~20kW (5kt climb) of power at full charge, at 30% charge remaining I can only get 8kW (1-2kt climb). I haven't discharged below that yet but I understand you reach the point where the voltage doesn't sustain level flight at approximately the same time you reach 0% charge.

The FES is most efficient at low power so it's much better to climb to lowest safe altitude, then go sideways at level flight power. I need about 3.5kW for level flight at 350kg (Diana 2-FES), which with the 5.3kWh batteries seems to get me about 150km. I haven't measured the full climb height at high power, but extrapolating my shorter climbs I think I should get 5000ft before I can only go sideways.
Range/altitude seems to scale linearly with weight if you're at 450kg or 500kg like some of those 18m ships with a heavy pilot, you can calculate the fractions accordingly.

I think you can plan on smaller margins with the FES since the engine-not-running drag is much lower with the FES than a stuck-out pylon. You'll have to make your own decisions of course.

On Monday, September 14, 2020 at 8:58:08 PM UTC+2, Mana wrote:
I just had a chat with a fellow pilot whose friend sold his LAK 17b FES because he was disappointed by the ability of the FES to gain altitude. The problem being that when applying sufficient power to gain altitude, the battery would get warm to the point where the controller shuts the motor down. I read recently an accident report tied to the same issue, in that case pilot too low and not in reach of a landing spot (pilot mistake) who counted on FES to regain altitude, but as the battery was too hot it wouldn’t power the motor and the pilot crashed. On the other hand FES works reliably to maintain level flight at a lower power ratio.

Does any LAK 17 or Shark 304 FES (or other glider of similar weight) pilot have any feedback on this? In real life, how much altitude are you able to gain, with fresh and semi full batteries? Is it a matter of being patient and accepting a low rate of climb?

It would be disappointing if when you fly “by the rules” and remain above a landing / outlanding spot until you turn power on, that the FES doesn’t allow to regain altitude, but only to maintain level flight. It changes the flight planning strategy altogether.

I find this surprising and I’ll try to get in touch with the pilot who sold his LAK, but thought I would ask the question in the forum in the meantime.