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Old 03-01-2015, 11:53   #1
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Need dedicated starting battery?

Hi everyone,
My T40 which is new to me as of last year has three 105ah Agm batteries. 2 are combined as a house bank, and the 3rd is the starting battery. They all live together. I would like to increase my house capacity and don't have enough space to easily add a 4th battery.
My question is can I just combine the 3 batteries into a bank for everything including engine? I have a heart battery monitor which I keep tabs on so doubt I'll drain the bank. As a backup I have a 5.5kw genset with separate starting battery which can power a large battery charger which could also help start the main engine if needed.
Am I missing something here?


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Old 03-01-2015, 12:32   #2
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Re: Need dedicated starting battery?

Suppose you have started to drag as a storm cell comes through, winds at 55 knots, and you have just begun to charge your batteries.....

FWIW, we've always had a separate start battery, so may just be extremely conservative.

But we did have such a dragging event, just the other day, but eventually held, and did not have to start the engine. Just as well, the hail was thundering down, and lightening and thunder, and I didn't want to go above decks anyway! At any rate, the cell that's part of a frontal passage may have significantly more wind in it than predicted.

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Old 03-01-2015, 12:32   #3
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Re: Need dedicated starting battery?

I'm a "land" electrician so this may not be the same answer as a marine electrician would have:

As long as you can start your genset, you can charge your batteries. Do you have the wiring schematic so you know how the three batteries are wired and to which loads they go? If done properly I don't see that it should be a problem as long as you always have the ability to start your genset.

What I would ask is what loads are you putting on the batteries that drain the two house batteries to the point you need a third? And if you need a third battery because of the loads you're applying, would having a third solve your problem? Do you foresee the third being drained too? And then there's the issue of burning up the batteries.

I'd want to know the loads and how everything is wired before I'd do what you're suggesting.
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Old 03-01-2015, 12:33   #4
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Re: Need dedicated starting battery?

No, you aren't missing anything. There is nothing wrong at all about using a house bank for starting the engine and forgoing a dedicated start battery. The only things that would give pause to this are 1) the wiring runs don't make sense, 2) the engine is large enough to require an actual dedicated start-type battery.

You will be much better served with that extra 105Ah AGM put to use in your house bank.

On the other hand, is your generator start battery far removed from your engine? You could always either use this for both, or provide a parallel switch for it to the engine if needed.

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Old 03-01-2015, 12:41   #5
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Re: Need dedicated starting battery?

Quote:
Originally Posted by colemj View Post
You could always either use this for both, or provide a parallel switch for it to the engine if needed.

Mark
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Old 03-01-2015, 12:49   #6
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Re: Need dedicated starting battery?

I have cruised on a couple of boats that did not have separate starting batteries. My opinion and my experience, no matter how careful you are it will just be a matter of time before you run the batteries down too low to crank the engine.

Also, during several years cruising there were at least a couple of times that I needed to start the engine immediately!! If these two situations happen to coincide it could be a bad day. Recharging the batteries from the gennie is fine except, if your battery bank is really discharged it may take some time to get enough juice back into the system to crank the engine.

If I had nowhere to mount a separate starting battery I would dual purpose the generator battery. Just make sure it is large enough to crank the main engine and, if it is very far from the main engine make sure you use really large cables.
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Old 03-01-2015, 13:03   #7
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Re: Need dedicated starting battery?

Quote:
Originally Posted by skipmac View Post
I have cruised on a couple of boats that did not have separate starting batteries. My opinion and my experience, no matter how careful you are it will just be a matter of time before you run the batteries down too low to crank the engine.

Also, during several years cruising there were at least a couple of times that I needed to start the engine immediately!! If these two situations happen to coincide it could be a bad day. Recharging the batteries from the gennie is fine except, if your battery bank is really discharged it may take some time to get enough juice back into the system to crank the engine.

If I had nowhere to mount a separate starting battery I would dual purpose the generator battery. Just make sure it is large enough to crank the main engine and, if it is very far from the main engine make sure you use really large cables.
What skipmac said. ^^^ He put it much better than I did.
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Old 03-01-2015, 13:15   #8
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Re: Need dedicated starting battery?

On the other hand, I have owned boats continually since the mid-80's, and have been living aboard 24/7 for 6+ years and have never managed to run down the house batteries below the ability to start an engine.

On our current boat, we start the genset off the house bank. The wire run is 30' (both ways - huge voltage drop) and it never fails to start.

Pretty much all smaller boats use the house bank for starting because there is simply no room for dedicated batteries.

In the OP's case, he has alternate solutions already in place. Given the huge immediate value and advantages of adding that AGM back into the house bank versus a very low probability "what if", this decision seems rather cut and dry to me.

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Old 03-01-2015, 13:27   #9
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Re: Need dedicated starting battery?

If you have a gen set can you start you main engie off it immedietly?
If you can the I think you are fine.
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Old 03-01-2015, 13:35   #10
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Re: Need dedicated starting battery?

No experience here, but getting your engine to start seems like a pretty darn important thing. If space is an issue, you might consider a lithium start battery. The A123 M1 26650 cells are very powerful, sold a lot of my stash to people building start batteries, built one for my little car.
Not cheap, but priceless when the house bat is dead/damaged and you need to start the motor asap.
Many people sell them these days...at pretty big markups. $300 for $80 in cells put in a box.

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Old 03-01-2015, 14:03   #11
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Re: Need dedicated starting battery?

Even a nearly completely dead battery will start an engine if you open decompression valves, get the fly wheel going, and close them up. Needs two people if start control not near the engine, but works great. We lived with our nearly defunct LA batteries for the last half of this season using this method.


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Old 03-01-2015, 14:07   #12
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Re: Need dedicated starting battery?

Quote:
Originally Posted by MarkJ View Post
If you have a gen set can you start you main engie off it immedietly?
If you can the I think you are fine.
At 5.5 kw, you'd think it would be enough.
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Old 03-01-2015, 14:15   #13
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Re: Need dedicated starting battery?

Quote:
Originally Posted by malbert73 View Post
I would like to increase my house capacity and don't have enough space to easily add a 4th battery.
My question is can I just combine the 3 batteries into a bank for everything including engine?
We have been cruising for two years with that setup.

Ralph
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Old 03-01-2015, 14:15   #14
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Re: Need dedicated starting battery?

Quote:
Originally Posted by 44'cruisingcat View Post
At 5.5 kw, you'd think it would be enough.
Actually, it would depend more on the size of the charging source than the genset.

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Old 03-01-2015, 14:25   #15
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Re: Need dedicated starting battery?

how big is the gen battery and how far away? I would not get rid of a starting battery. however the gen one could maybe do both



how big is your battery charger? I doubt it's "large" with a 300ah bank. if you had a 100a charger it probably wouldn't take too long running the gen to get some charge back in. but depends how soon you need to start the engine when something goes wrong.

the other consideration is your 3 batteries will now be in different shape. as 2 have been used / drained as house bank and one hasn't. depending on age may not be best to combine them. may require 3 new batteries for best results.
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