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Old 10-26-2017, 12:11 PM   #1
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Generator battery?

Do I really need a separate isolated battery for the generator? Could the generator be tied in to one of the starting banks instead? Running twin 6BT's and a 8kW Onan. Can the generator's small alternator charge pairs of group 31 batteries?

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Old 10-26-2017, 12:18 PM   #2
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You could tie the gen. to another battery. Technically there is no problem, the gen, will not care.

It is generally not recommended because if you have a battery problem, dead batteries, you may also not be able to start the generator. Then the boat is really dead.

Many people count on the gen. as the bail out in case that happens.
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Old 10-26-2017, 04:50 PM   #3
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Yup agree, depends on your battery setup more than anything. But I lean toward a separate generator battery that isn't tied to any other part of the boats 12 volt system. You don't need much of a battery to start a 8k Onan. Mine is 7.5k and the starting battery is a group 24 or 27, 500 cca I believe.
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Old 10-26-2017, 04:54 PM   #4
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Thanks, I'll keep the extra battery. Having a hard time finding a shop manual and parts list for an 8MDKB.
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Old 10-26-2017, 04:59 PM   #5
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Your genset is your back up. It should have a dedicated start battery.
And you should look after it. The genset battery is often only charged by the genset, and of course only when the genset runs.That`s a weak point, I gave mine a small dedicated 25W solar panel.
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Old 10-26-2017, 05:00 PM   #6
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My gennie and main engine share a starting bank, the backup is the house bank. Or worst case go get the thruster batt. No problem with this arrangement in 9yrs. Start bank has no loads but to start things.
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Old 10-26-2017, 08:09 PM   #7
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Your genset is your back up. It should have a dedicated start battery.
And you should look after it. The genset battery is often only charged by the genset, and of course only when the genset runs.That`s a weak point, I gave mine a small dedicated 25W solar panel.
Mainship built my '08 34HT w a separate gen batty. What I didn't like was MS tied the 3 bank shore charger to the gen and another batty bank via diode isolator in order to charge 4 battys with a 3 bank charger.
I reworked both the alternator and shore charging and decided to isolate the charging of the gen batty to the gen alternator.
Another TFer suggested another mod I dud implement. I changed the gen batty on/off sw to a 1-2-all-off sw with the #2 position tied via jumper to the other start/house sel sw. This allows emergency cross connection to start the gen using other battys if the gen batty fails.
So far in 3 seasons it hasn't been needed... stand slone gen batty works just fine if you run the gen periodically.
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Old 10-26-2017, 08:34 PM   #8
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I use a separate group 27 battery for my 7.5 kw Onan MDJE. This has been a reliable setup so far (going on 2 years).

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Old 10-26-2017, 10:08 PM   #9
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I am a member of separate gen battery club. However the only time my start batteries failed was due to a curious child who flipped the house switch to all. Just happened to be the same day the gen battery decided to call it quits. Of course we were at a remote anchorage. Luckily I had jumper cables and a new dingy starting battery.

I learned my lesson, gen battery gets load tested every spring, if it doesn’t meet spec CCAs then it gets replaced.
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Old 10-27-2017, 01:18 AM   #10
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Originally Posted by dvarholy View Post
Thanks, I'll keep the extra battery. Having a hard time finding a shop manual and parts list for an 8MDKB.


Look on Ebay, may take a while, and if not already done then Cummins as they own Onan.

THere are places that offer manuals for sale. Just some suggestions and it might be worthwhile asking if they don't list.

Boat Motor Manuals - Outboard Repair Manuals - Boat Engine Service Manuals

Online SHOP MANUALS - Cruisers & Sailing Forums
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Old 10-27-2017, 01:36 AM   #11
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Onan genset forum
Bound to be manuals there somewhere.

https://www.smokstak.com/forum/forumdisplay.php?f=1
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Old 10-27-2017, 10:02 AM   #12
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I was thinking about an alternative to a dedicated genset battery as an emergency backup: How about one of those jumper battery packs? Takes up about as much room, but it's portable so it could be used for other things, like jumping a neighbor's boat or as a dedicated 12V power supply for testing equipment.

I may go that route the day I have to finally replace my dedicated genset bank.
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Old 10-27-2017, 07:10 PM   #13
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Jumper boxes loose charge over time so they will need to be plugged in when not in use or recharged on a 3 month schedule.
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Old 10-28-2017, 09:14 AM   #14
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Jumper boxes loose charge over time so they will need to be plugged in when not in use or recharged on a 3 month schedule.
Good point. I was thinking I'd keep it plugged in all the time, and it would be charging whenever the generator was running or shore power was available.
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Old 10-28-2017, 09:36 AM   #15
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"Your genset is your back up. It should have a dedicated start battery."

Always nice, but there are mechanical devices that can start the noisemaker , or even the main engine if your pockets are deep enough.
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Old 10-30-2017, 08:24 AM   #16
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I've seen people discuss the possibility of some sort of non-electric method of starting their engines, in case of a worst-case, all batteries dead sort of scenario. Always thought some way to manually start the generator made a lot more sense. It's a much smaller engine, to start, and once it is going you should be able to charge batteries to start the mains.

So, anyone given thought to manual methods to start the generator? (Talking, of course, of a diesel generator -- not something like a little Honda gas model.)
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Old 10-30-2017, 08:47 AM   #17
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One of the new wave jumper packs like those from NOCO are a great thing to have with you and don't take up any space. No excuse to have one dead, but IMO no excuse to have all batteries on the boat dead either. https://no.co/

I strongly believe the generator should have its own battery and charger system, with some sort of backup available. In my case I could start it of either the thruster bank or the inverter bank with a set of jumper cables I kept on board, but that was just due to the way everything was located on my particular boat.
I had kept the jump pack around as a back up for the dinghy, and it helped a few fellow boaters get going again as well. Not to mention one of our cars.
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