Personally I wouldn't worry too much about the start battery, I don't.
The main engine has a dedicated alternator to charge the start battery...so every time you run the engine the start battery is charged...just like your car...do you have a monitor for that ?....I'm guessing the answer is no...
Same with the genset...there's a dedicated alternator that charges the start battery (same battery as the engine charges)
So in total...don't worry about it...just keep it topped up with distilled water.....or better, do what I'm gong to do and replace it with a no-maintenance AGM battery
SV Elyse SM437