On my RadCity 5 Plus, it was a 10 amp red mini-blade fuse. I had a “variety pack” in my car that I’d gotten at a local auto part store, but looked it up any way, just in case. This may be different on other models/batteries. Ironically, it did require a warranty voiding opening on the battery casing. :-)
Almost got scammed by Rad Bikes this way. Battery mysteriously failed 10 months into their 1 year warranty. Rad Bikes accused me of sabotaging the battery and refused to honor their warranty, but “generously” offered me free shipping on a $300+ replacement battery. Turns out the off-the-rack fuse they use blew; identical down to the manufacturer to the ones used in cars. Replacement fuse was <20 cents and fixed the problem instantly.
“American Automakers Refuse to Participate in Race to Make Accessible, Fuel-Efficient Vehicles”
Fixed that title for you, Verge.