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It's not at all COTS, but the Rutan Voyager [1] spent 9 days in the air on it's round-the-world flight.

[1] https://en.wikipedia.org/wiki/Rutan_Voyager

Similarly not-COTS, but a series of UAVs, a Zephyr "7" made a record 336 hour flight - 14 days, with an unofficial runs of three minutes (!!) short of 26 days aloft for a recent Zephyr S.

https://en.wikipedia.org/wiki/Airbus_Zephyr

The Zephyr is using Lithium-Sulfur batteries, which have a better energy-to-weight ratio ("Currently the best Li–S batteries offer specific energies on the order of 500 W·h/kg" according to Wikipedia [1]).

These aren't commercially available though. Twice the performance of currently available automotive batteries is a major advantage for aircraft applications.

Pretty cool.

[1] https://en.wikipedia.org/wiki/Lithium%E2%80%93sulfur_battery

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