If you're just connecting a battery tenderizer then a cable like the one shown in the previous post works fine. I've got those little things dangling out of all sorts of things around here (golf cart, lawn mower, infrequently used pickup truck, airplane). If you want to do a higher rate of charge or even to something that is going to allow you to "jump start" the plane, then you're going to need to go to a larger connector such as the traditional aircraft APU plugs.

Note the latter usually are connected after the battery contactor in planes so they aren't particularly useful for "charging" purposes (and if your battery is totally flat, jumpstarting it won't close the contactor and allow you to charge the battery with the alternator).

Note, that it's technically against most electrical codes to charge aircraft batteries in situ inside hangers.