The internal battery is a litium ion battery, with very low self-discharge rates. No need to "keep it charged" if you aren't turning on the EFIS and running it off the battery. It will stay charged for a long time (years) if it is full to begin with.
The unit charges the battery under only two conditions: when it has master power and is turned on, or when it has keep alive power. So the easiest way to keep the unit charged is to hook up keep alive power to your battery. If you don't have a battery yet, hooking it up so some sort of 1A, 10-30V power supply will also work. It can take a full day to charge a completely dead battery off of the KA power.
