Review on 12V DC Male Cigarette Lighter Adapter for Cars with 42cm Cable - High-Performance uXcell Product by Justin Ott

I think you can think of many applications where this would work. I have a portable air compressor that uses a 12v battery. The charger that came with the compressor runs on AC power and doesn't have an indicator light to tell you when the device is fully charged, so charging the battery can shorten its lifespan. The compressor has a 12v socket and this plug allows me to use this socket to connect to a battery tender and keep the unit fully charged without worrying about overcharging.