While we have battery jumper for all our cars, for the ones we don’t use too much, you need a Battery Tender. These are top rated on Consumer Reports and are trickle chargers:
Amazon.com: Battery Tender 021-0128 Battery Tender Plus 12V Battery Charger: Automotive. $47 and this is a 1.25 amp charger
There is a junior version that is very small and convenient. It is $26 and trickles at 0.75 amps. So good for many cases.