I understand that a much weaker version of beer, called 'small beer', was historically drunk as a safe way to drink liquid. This was at a time when drinking plain water might well have made you sick, as it would not have been boiled. Do we have any idea what the ABV of 'small beer' drunk around 1800 would have been?
Historically small beer is believed to be between 2 and 3.5% ABV, based on notes from Belgian monasteries which produced small beer from the 3rd runnings of the mash and original French Saison recipes. Those numbers are probably accurate for brewing between the middle ages and about the 1500s.
Small beer got a little stronger a few hundred years later when it came to America. George Washington's famous 1757 Small Beer has been estimated as a little stronger, around 4-4.5% ABV by recreations. Another famous historic small beer recipe from 1820 has been estimated around 3.8% and included lemon peel, clove, ginger, and cream of tartar for acidity in addition to hops.
Basically it runs the same gamut as session beer does today, maybe staying a bit towards the lower end of what we'd consider session.