Generator Starting Battery

C

Crossroads

Member
Joined
Jan 1, 2000
RO Number
2208
Messages
308
I have an older Onan 7.5KW diesel generator. My manual (it's old too) does not list a starting battery amperage requirement, it only says 105AH. Currently I'm using a 4D that I need to replace. I don't want to put another 4D in there due to the human carrying weight (I'm also old). I'd like to parallel two 12 volt group 24 cranking batteries. Anyone done this? I'm not even sure I need two batteries in parallel since most of the group 24 cranking batteries have 1000 MCA.
 
I'm thinking a single AGM from Sears or Optima. Light weight (well lighter than a 4D) and maintenance free. If it is completely independent from the battery charger, it won't matter what the rest of your battery bank is.
 
A single group 27 starting or dual purpose deepcycle/starting should be very adequate.
 
I have an 8kw Northern Lights and use a group 31 with no problems.
 
I just switched my gen battery from a full size to a garden tractor battery. to make more room for another deep cycle for the house group. I have started the gen a few times just to check it out and it worked fine. I won't be able to tell you till the end of the season how it held up. Before I switched I tried starting the gen on a small battery jump pack that only has a 4AH gell cell and it started it 5 times in a row with no problem thats how I decided I could move to a smaller batt. I figured the Gen HP is the same as my lawn tractor so why should it need a bigger batt. I always keep a jumper pack onboard and some jumper cables so if I get in a bind I can pull from another batt.
 
I have a MDJE on my last boat and had a single group 31 cranking battery. Never an issue. A 4D is certainly overkill - not that overkill is bad.
 
If your genny does not start in a half dozen tries, it is not going to start. Save the battery space. I use a garden tractor U-1 on my 5E. Never had a problem. It is twice the minimum size speced for that engine.
 
My 5hp generator does not have a separate battery. It is hooked to the port engine group 24 starting battery. Never been an issue. Of course since I recently bought the boat it has new batteries all around so they are fully serviceable.

I just reread your post and noticed that you are dealing with a diesel generator. It would have higher starting current draw, but I still think a group 24 would be no problem.

G
 
My circa 1991 Onan 20kw starts instantly off a dedicated Group 31 Napa Commercial. Check what the manual specifies in cranking amps; amp hours is a weird spec for a starting battery.

George
 
Mine was on the stbd group (house & start) I moved it to port (start only) and then decided I would rather have it on it
s own as a just in case ( last ditch ) if both bank somehow got depleted ( ie the emergency start didn't disengage) and while I was on the hook both bank got drained to a no start condition. I could always start the genny and let it charge both banks up for an hour or so then start the engines.
 
Well I put my 2 year old Orbital Blue Top on the boat yesterday. I've been using that Orbital as a trolling motor battery on a small flats fishing boat. The Orbiatl spins the diesel genny just fine. I'm thinking about leaving it in there but I'm concerned about the Prosport 12 Charger running a lead acid 8D on one leg and the Orbital AGM on another.
 
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