Check the amp draw on that. The 12v outlet has a low fuse rating. A cig lighter adapter on a battery tender connection(SAE connector) will remedy that.suppose that as the title of the post says, "just to heat a cup of soup / coffee", why not use a simple 12v heater ?? .. if we were to cook, or we did not have a source of energy, yes, such Maybe alcohol or compressed gas would be the best option, but having 12v at hand, a 12v heater seems like the smallest option.
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If California wasn't always just a hot fart away from being on fire I would buy one of these. Way simpler than carrying fuel and it folds flat for easy storage. Sadly we are always in "fire danger" around here.Previous linky no work. This is what I use. Either wood or slip a Trangia copy into it. Folds flat and easy to use