I have a Noco Genius 1 which is very versatile and includes settings for 6 volt batteries and even Lithium ones. I wouldn't advise leaving any charger permanently connected though. Not only do they consume power at about 5 watts when not actually charging but they are also not necessarily the best thing for overall battery life. My own Grom has stood for a couple of months and the battery still shows good voltage so I reckon one charge per month is plenty.
The GENIUS2X4 is a 8-amp, 4-bank, battery charger, battery maintainer, and battery desulfator for 6-volt and 12-volt batteries.
It charges and maintains everything I got... grom, monkey, ktm, yamaha, vespa, waverunner, RZR, FJ.. and effectively + automatically senses and sets the charge profile for flooded, AGM, or lithium. Leave them connected 24x7 when aren't being run. As roadster mentioned, they also make it with less banks... highly recommended!