Smart battery maintainers (also called trickle chargers or float chargers) keep a parked vehicle's battery fully charged using household electricity. Plug it in, leave it, the battery stays at 100% indefinitely.
The key word is 'smart.' Old trickle chargers ran constant current and could overcharge a battery, damaging it. Modern smart maintainers monitor voltage and switch between charging modes to maintain without damaging.
Best use cases: vehicles parked for weeks (RVs, motorcycles, classic cars, ski-trip cars, vacation homes), daily commuters with very short trips (where the alternator never fully recharges the battery), and EV/hybrid 12V auxiliary batteries (which are small and easily depleted).
Cost: $30–$80 for a quality unit. Lifespan: indefinite. Battery life extension: 30–50% for chronic short-trip vehicles, much more for seasonal vehicles.
Brands worth considering: NOCO Genius, CTEK, Battery Tender. Avoid no-name imports — battery damage from a bad maintainer costs more than the device.