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Dedicated Starting Battery For 2 Generators

Discussion in 'Generators' started by DOCKMASTER, Feb 9, 2023.

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  1. DOCKMASTER, Feb 9, 2023
    #1
    DOCKMASTER

    DOCKMASTER Senior Member

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    I have a 20kw gen and a 9kw gen. Both are currently started from the house battery bank and neither of the generator 12v alternators are currently used to charge the house bank. I am getting ready to install a dedicated starting battery for the gens. I would like to use a single group 24, maybe 31 and have this battery start either gen. What do I need to use to isolate the individual gens so there is no back feed to the other alternator as I plan to utilize the available alternators? I typically only run one generator at any given time but sometimes I have both running for a short period if I'm doing a warm-up so I can transfer over with minimal interruption time when switching over. I also have a two bank shore power fed battery charger that currently only charges one bank so I plan to use the available bank connection for the new dedicated gen starting battery. So the new battery will have three potential charging sources; the shore fed charger or the alternator from each gen. I want to ensure proper isolation for each source.

    I will also install an emergency manual parallel to the house bank just in case this new dedicated battery has any issues.

    Appreciate any input. Thanks!
  2. Capt Ralph, Feb 9, 2023
    #2
    Capt Ralph

    Capt Ralph Senior Member

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    I have a 30 and a 9. Both start & run off a single multi use / closed cell 4D.
    Gen-set alternators are usually small and provide little current. As designed, enough to re-replenish the draw from starting.
    There are times I run both gen-sets for that hot summer day when Josie wants to cook for an army.
    My gen-set battery is the last worry on our boat.
    When installed, I use a different (same model & age) 4D for our electronics. A back up battery switch was installed. NEVER used...
    I probably could of used a good G31 for both. WM had the EastPenn 4Ds on sale 4 years ago.
    Amazing what the price is now (my cost has doubled, list price tripled) https://www.westmarine.com/west-mar...arine-battery-1280-mca-group-4d-15020209.html
    It is a good battery. When they do fail, I may go then to a G31.
    Last edited: Feb 9, 2023
  3. Pascal, Feb 9, 2023
    #3
    Pascal

    Pascal Senior Member

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    A group 24 maybe a bit small for the 20kw, a group 31 is probably big enough

    Personally, I like to keep things independent and prefer one battery per gen or main.
  4. DOCKMASTER, Feb 9, 2023
    #4
    DOCKMASTER

    DOCKMASTER Senior Member

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    I’ll probably go with a 31 as that’s what the house batteries are. Hate to add 2 more batteries for this for something used so infrequent. I rarely use the 20kw. I don’t think it has 50 hrs on it.

    Do I need any isolation if I just use one battery?
  5. Capt Ralph, Feb 9, 2023
    #5
    Capt Ralph

    Capt Ralph Senior Member

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    I use the typical battery switch but backwards. 1 & 2 to each gen-set and common (usually the engine) to the battery.
    This allows me to be able to isolate a gen-set if it needs service and not dis-able the other gen-set. OR all off if needed.
    That switch has not been touched in near 4 years either. Always in both (1&2) enabled.
    Your not the kind (we know well), but for others reading this, don't cheap out on the battery.
    And allow for a backup switch from something just-in-case.
  6. Capt Ralph, Feb 9, 2023
    #6
    Capt Ralph

    Capt Ralph Senior Member

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    Were anchored out on the north end of Lake George again tonight. The inverter and lil genny are cycling fine.
    I can send pictures or diagrams if needed.
  7. Capt J, Feb 9, 2023
    #7
    Capt J

    Capt J Senior Member

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    You're going to need a 31 at a minimum for the 20 kw. You should run it more often as sitting does it NO favors.
  8. Capt Ralph, Feb 9, 2023
    #8
    Capt Ralph

    Capt Ralph Senior Member

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    A good thought. Alternators (generator ends) luv to collect moisture. Get her warmed up and loaded from time to time.
    Blow da gunk out also!!
  9. mapism, Feb 10, 2023
    #9
    mapism

    mapism Senior Member

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    I see that nobody else asked so far, so maybe I'm missing something.
    But aren't your gensets 12V and house bank 24V?
  10. ranger58sb, Feb 10, 2023
    #10
    ranger58sb

    ranger58sb Senior member

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    Were it me, I'd prefer separate battery for each generator. Odyssey Extreme PC2150 for the large genset, PC1500 for the smaller. (These are older names; they've renamed their batteries but you can find 'em...)

    I took our genset battery off of the 3-bank charger... relied on the generator's alternator only. The AGMs don't self-discharge all that much... and we carry jumper cables on board too...

    -Chris
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  11. rtrafford, Feb 10, 2023
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    rtrafford

    rtrafford Senior Member

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    Yes, I use a 31 for each generator, same make and model as my setups for mains, house, and thruster. In a pinch the batteries are interchangeable to solve a crisis, and I can rotate them as they age with maximum flexibility.
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  12. rtrafford, Feb 10, 2023
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    rtrafford

    rtrafford Senior Member

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    Yes, but the 24 is simply 2x12 31's in series. My generators are 12. Mains and house and thruster are all 24. I use the same make and model (31 Odyssey) for each system. Then I can shuffle batteries around as and if needed, replace one as needed.
  13. mapism, Feb 10, 2023
    #13
    mapism

    mapism Senior Member

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    LOL, yeah, I'm well aware that a 24V bank is made of 12V batteries in pairs - or 6V batteries in banks of 4, which is what I've got. :)

    But the OP said he's using the domestic bank for starting the gensets, and if the former is wired for 24V and the genset starter is 12V (which is just a guess, mind - hence my question), it would be unusual to take 12V out of a 24V bank.
    Possible, of course, but not good practice (unbalanced load).
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  14. rtrafford, Feb 10, 2023
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    rtrafford

    rtrafford Senior Member

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    I agree, although I'm more concerned with the generator alternator interfering with the house charger...so I'd prefer to keep a dedicated battery for the generators...but, yes, you could take off 12v from that series without issue, but I agree that I don't care for the setup. If space and weight is an issue, go with one dedicated battery for both generators, and keep that system apart from the house.
  15. mapism, Feb 10, 2023
    #15
    mapism

    mapism Senior Member

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    +1 to all that from my part, FWIW.
  16. Capt J, Feb 10, 2023
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    Capt J

    Capt J Senior Member

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    I've seen that done for electronics banks. I wouldn't do it with a generator. The large draw of the starter will really unbalance the bank, and then the other battery will try to equalize very quickly creating a lot of heat and really shortening the lives of both batteries.
  17. Fishtigua, Feb 10, 2023
    #17
    Fishtigua

    Fishtigua Senior Member

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    Talking of gennies being run with little or no load on them, we had such a problem on one yacht. The brand of yacht, no names, had designed the stabilisers to work only with the genny running.

    This was fine when guests were onboard, a decent hotel load on the system. Sadly, on a long passage, there was little or no load but for the fins, a light load, going through the system. The genny bores glazed-up really quickly, causing a rebuild in a short time. The only way to stop that happening again was to turn the oven and stove onto full power. It got jolly hot in the wheelhouse doing that.

    Solved by fitting a really small genny that could run the fins and one A/C compressor. That involved making a new mount platform, fuel lines, rewire, switch panel and making new cable runs.

    Bloody designers not thinking, yet again.
  18. mapism, Feb 10, 2023
    #18
    mapism

    mapism Senior Member

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    My point exactly. Then again, we are now just speculating.
    Maybe Dockmaster will now tell us that he has a 12V domestic bank, or that he has 24V gensets.
    To my knowledge, both alternatives would be very unusual on a 54 footer, but what do I know...?
  19. DOCKMASTER, Feb 10, 2023
    #19
    DOCKMASTER

    DOCKMASTER Senior Member

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    My house bank is 12v and main engine starting banks are 24v. This is how I’ve been using the house bank to start my gens which are 12v starters. I don’t think a 12v house bank is that unusual on a 54 that I’m aware. My current house bank is 6x Full River group 27 AGM. We use the 27 as they fit perfect in their spot.
    I did more research and talked to several folks I trust and looks like there is no concern with a single battery fed by both 12v alternators on the gens should I ever run them concurrently. Worst case is one alternator sees voltage from the other and reduces output. This is not a concern for me. I can’t put both gens on the a/c buss at the same time so no reason to run both except as mentioned previously for a warm-up and switch over.
    As for low hours on my 20kw that’s from limited use of boat in total. I think the 9kw only has 250 hrs. These gens were just installed in 2020 and with COVID issues and others last year was the first season I got real use of the boat. And our season is very short regardless. I think my new CAT’s just turned 150 hrs at most.
  20. mapism, Feb 10, 2023
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    mapism

    mapism Senior Member

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    Well, good thing that I said what do I know...?
    In Europe, I can't recall to have ever seen any boat above 50 feet or so whose domestic bank wasn't 24V, but every day is a school day! :)

    Anyhow, back to your original point, I tend to agree that it's OK to have a single 12V battery starting both gensets, fed by their alternators in parallel.
    Having said that, since the cost of one additional 12V starting battery is neither here nor there, personally I'd rather have a dedicated battery for each genset, wired with a selector allowing to start either one or the other genset with either one or the other battery.
    But of course, the fact that you can always count also on the 12V domestic bank for firing the gensets makes such redundancy a bit rich...
    Last edited: Feb 10, 2023
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