[KennyBlanken]

Internal resistance of the battery would be a significant problem, you'd need a fairly sizeable battery, and most lithium ion cells have a maximum discharge rate of around 1-5C unless they're high-discharge-rate cells which are usually more expensive and lower capacity.

Using a vacuum flask would dramatically lower heat loss.

[mardifoufs]

Some cheap lipo cells can get you a sweet 60-120C discharge rate with a pretty okay capacity. They are also widely available on the used market, usually sourced from EV batteries or energy storage packs. But yes, for the typical 18650/cylindrical li-ion battery, this wouldn't be especially feasible at all

[dahfizz]

The kettle used was vacuum insulated.