'add some charge to the generator start battery while cruising along for several hours. '
1. Is the genset, start & house battery same chemistry, say flooded lead acid [fla] with similar charging voltage requirements ?
If yes then a very simple solution I have used with success is a voltage sensitive relay [vsr]
The ones I have used a "BEP VSR"
Voltage Sensing Relays | bep
sealed and designed for marine ~140amp rated. they have a led that indicates when charging.
They turn on at around 13.2v and off around 12.7 so only once the source battery starts to charge [propulsion engine] it closes. Once source battery off charge [ engine stopped ] they open. so doesn't flatten your source battery.
vsr has two studs & one thin black wire
one stud gets hooked up to source [start] battery positive and the other gets hooked up to your load [genset] battery positive with a thin wire to your negative bus.
I always put a 150A fuse in series too.
If not similar battery chemistry and or different charging requirements between the start, house battery & genset batteries then still use a vsr and a dc to dc adjustable converter
then the battery charging only gets turned on when your propulsion engine is above charging speed, and your system is less likely to flatten the start battery.
if you keep the wire under say 8 gauge max[~4mm]] then the source alternator wont work as hard and it'll reduce the initial charge current a little and so last longer.
Genset alternator overheating
is genset permanent magnet [often two wire with no fan, like a lot of small tractors] or variable field alternator [like a normal car type alternator] ?
if its permanent magnet you cant overdrive it [they actually work flat out the whole time], if its variable field yes you can over drive it.
some dc to dc devices do have a current limit option [victron energy dc to dc do and I've used them]
sing out with questions.
happy new year all and may your fishing rods have a good bend in them
Kind Regards Donald