I'd like to add an outlet to a 1st floor hall closet where I store tools. The water meter for the house is in this closet, but nothing else (e.g. no water heater, furnace, etc.). The closet is roughly 8'x 4'. The outlet will be used to charge batteries for drills, vacuums, etc. -- so very low amperage. I'd like to wire the outlet into a dedicated 20A circuit I have for a sump pump. I don't have a sump pump and don't anticipate getting one, so the outlet is going unused. The sump pump outlet is close to the closet, so tapping into it would be very convenient. I plan on making the new outlet 20A and running 12/2 cable to be consistent. Any concerns with this approach?
Sump pumps have usually been put in on dedicated circuits, possibly to get around the GFCI requirement, but you say you don't have a sump or expect to ever. Based on what I can find in the National Electric Code, I would say it is okay to use it, but you should check local ordinances. Your wiring plan is fine, properly sized wire for the circuit breaker and probably oversized for the charging equipment you expect. NEC 210.8(A)(4) includes crawl spaces in the GFCI requirement, but if your receptacle is going to be in a closet above grade (not in an unfinished portion of a basement) you wouldn't need GFCI there.
If you decide to also leave the sump receptacle in place, in case you need to plug in a light or something in the crawl space, it should be converted to GFCI, but to avoid confusion I wouldn't protect the closet one from there. In other words, use the terminals marked on the back of the GFCI "Line," and do not use the terminals marked "Load." That way if it trips it won't turn the closet receptacle off as well.