Aluminum thermostats seem to be pretty bad according to the legend here.
Also, when I cut apart the ET I concluded that there's a magnet in bottom of the ET bobber...or metal, but the car might not bother to check for the level until it's warmed up or 'something' like that. I've heard and seen that the level sensors are sensitive. But, it's the same for windshield reservoir, so an easy swap if you want to test its function.
I doubt you 'need' to disconnect the battery first, but I generally do for most things beyond bulbs because I know I'm not an electronics genius!