Starting Batteries for Generator

I have two dedicated Group 31 batteries for starting either my 12.5kw generator or the other 7.7kw unit. I am questioning the need for two batteries. Would not one suffice? I am planning to relocate the batteries.

IMO yes. My 13KW unit seemed very happy with one GP 31. More important IMO is some additional way to charge or jump the battery. e.g. battery charger at dock or jumper cables. I always wanted redundant means of starting genny and main engines.
Gennys use pretty small engines so not much battery is needed.

Yes, I would think one battery would be OK with a 1,2,All, Off switch.

Some of us take the approach of a single large bank that handles the house and engine/generator starting requirements with a single isolated battery somewhere (genset/helm radio etc) as a backup. Following this approach individual starting batteries for two gensets is not necessary.

One batt will be fine for both gennies. They are not started or run at the same time. But as others have posted, have some other source as a backup, like a crosstie switch to an engine or house bank. You don't want one dead gennie batt meaning no gennie.

I have no gennie batt, just tied it in to engine start bank. That bank has a crosstie to house bank. Works fine.

If you are appropriately wired for redundancy why change? Batteries are cheap and a completely independent genset battery duo gives great peace of mind for those that are far from tow boat or assistance.

Heck, I've got an 8D to start the genset. Love it as it provides weight well below the CG and with finished FRP top is a great seat in the ER! The DeFever ERs are large enough to easily keep the battery ballast in place. IMHO a pair of dedicated genset batteries is great planning.

I agree with sunchaser. Redundancy is good, why not leave well enough alone? A pair of jumper cables does come in handy. I had a genset start battery die prematurely once and I could easily jump start it from one in the thruster bank. Used it that way for a couple weeks while away from civilization. Came in handy a few other times, helping stranded dinghys and the like.


On the other hand, if the space is really that valuable, then get a good AGM (I liked the Sears Platinum Marine 31) as the solo unit. It cranked my 20kw 4 cylinder Onan easy.

The noisemaker batt should be robust enough to be removed and serve as a starting batt for the main engine.

"Stuff happens" ,

FF said:

The noisemaker batt should be robust enough to be removed and serve as a starting batt for the main engine.

"Stuff happens" ,

Click to expand...

Don't see why since the genny can charge the main batts via the charger.

Soon somebody will suggest a second boat for extra redundancy. []

bayview said:

Don't see why since the genny can charge the main batts via the charger.

Soon somebody will suggest a second boat for extra redundancy. []

Click to expand...

True, but a battery can fail completely. My genset battery simply died a sudden death of old age right after the survey. The engine start battery was then able to start the genny. If the reverse has been true, my genset battery could start the engine.

Consider the cost benefit, plus redundancy does have advantages. Balance the cost of one batt against the cost and bother of modifying cabling.
Of course if you have plans for the space one occupies, or this is part of a general battery relocation as per your post, other considerations apply.

A single M24 starting battery should be enough. Your manual should state the minimum MCA needed.

"Don't see why since the genny can charge the main batts via the charger."

TIME can be a factor , a charger can not refill a batt instantly.

To move a batt the purchase, in advance, of a strap that grabs on to the batt posts , a few dollars, is a huge help.

https://www.amazon.com/Pico-0867PT-Battery-Carrier-Package/dp/B00030COZG

Thanks guys for all of your advice. I have decided, at least initially, to use just one battery. Two would require more wiring, a second box, and a 1-2-Both-Off switch although I may install a switch anyway for future use. I have attached several pictures in case anyone is interested. You will see that the existing box is way too large and occupies most of the space between the stringers. The battery will be relocated to an existing shelf outboard of one of the Lehman 120s. The house banks have already been relocated to a central location in two boxes very close to the Magnum 2812 inverter-charger.