[roadster]

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.