Generator battery size?

[SeaEric]

I have an 8kw Genset on my boat, powered by a 3 cyl Yanmar diesel. The current set up has it starting off of one of the 2 8D batteries that start the mains and run the house. I want to add a dedicated battery that is generator specific, used only to start the gen, nothing else. What size battery do you think will be appropriate and dependable? I was considering using a 4D but that seems like overkill. Oh, one more thing- The Yanmar is a direct injection engine, so no glow plug preheat to worry about. She cranks and starts easily. Would a group 27 or 29 be good enough?

[Banshee36]

Yes.I use a group 27 for a 5kw NL genset. You will be allright

[Jaxfishgyd]

I used a group 24 for 3 yrs on my 9kw genset and never had a problem.

I now have a group 27 as I was given a new one by a man on the dock who bought it by mistake.... (seems he didn't know the difference between an 4D and a group 27 when he bought it)

[Traveler 45C]

A Group 24 starts my 18kw genny with no problems at all.

The best answer for you is to check the Owner's Manual for your genny. It should state in there the recommended Cold Cranking Amps (CCA) required for starting. As long as the battery that you install can deliver the necessary CCA your go to go no matter what size physically the battery is.

I would get the smallest sized battery that will fit within your space limitations with the necessary CCA that will do the job.

[Fanfare]

And install another battery switch/isolator so you can fall back to the main batteries and/or boost the mains with the new battery if needed. I have noticed that if I am prepared, bad things do not happen. Conversely,...

[Traveler 45C]

And install another battery switch/isolator so you can fall back to the main batteries and/or boost the mains with the new battery if needed. I have noticed that if I am prepared, bad things do not happen. Conversely,...

Agreed!!!!

[SeaEric]

Funny that you say that! Thats the situation. I want to make sure that I have one good battery available at all times. I figure the Gen is the best place for it. If I can start the Gen, I can charge the rest. I'm also thinking about keeping a booster pack jump starter on board. I could use one anyway for the other stuff I have going on and it could come in handy on the boat. Thanks everyone for the input!

[Traveler 45C]

Yup, good plan. Thats how I'm set up and I keep a booster on board as well.

[Genesis]

Make that generator battery an AGM. DO NOT attempt to parallel it to the mains; instead, once the generator is running the AC charger should charge the main startup batteries.

If you want to "interconnect" things then use a COMBINER between the generator start battery and one of the other banks. This way when you're charging it keeps topped off, yet its isolated when NOT charging.

Combiners also have a "force override" inlet typically (Yandina's do) which permits a forced combine if you need to parallel INTO that bank.

A 3-cylinder Yanmar doesn't require much of a battery to roll over; a Group 24 AGM (e.g. the one West Marine sells) is WAY more than sufficient.