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Old 03-11-2016, 05:15   #1
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Very small AGM for starter?

I currently have a group 27 AGM for my starter battery and 2 group 31 AGMs for my house. I'd like to combine the 27 with the 2 31s to increase my house bank from 210 amp/hrs to 300 amp hours. I have an isolator so the alternator charges two banks independently and a solar regulator that also tops off the engine bank after the house is full. I don't sail in colder climates so my engine starts right away every time (knock on wood).

I'm considering the Deka ETX30LA (400CCA). Switching to a much smaller starter battery seems to have lots of advantages and no disadvantages.
  1. I have just enough room in my battery box to fit a small 4th battery so no need to build another box.
  2. Even this small battery should have no problem starting my 39hp 3JH3E.
  3. I will configure my battery switches to that I can easily start with the house bank if need be.
  4. I get a bigger house bank w/o losing the redundancy of 2 banks.

Any disadvantages I'm not thinking of?
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Old 03-11-2016, 05:32   #2
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Re: very small AGM for starter?

If the space would work out for you, I've read it'd be better to replace your 1xG27 and 2xG31s with 3 new G31s. It's said to be better to have all three batteries within a bank be the same brand/size/type/chemistry/age/etc.


Or... you may even have room to replace the 3x larger batteries with 4x 6V golf cart batteries in that box. About 440 Ah. If that leaves room somewhere (else?) for a small start battery, maybe an option to consider.


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Old 03-11-2016, 05:40   #3
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Re: very small AGM for starter?

Quote:
Originally Posted by ranger42c View Post
If the space would work out for you, I've read it'd be better to replace your 1xG27 and 2xG31s with 3 new G31s. It's said to be better to have all three batteries within a bank be the same brand/size/type/chemistry/age/etc...
Indeed.
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Old 03-11-2016, 05:47   #4
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Re: very small AGM for starter?

Quote:
Originally Posted by ranger42c View Post
If the space would work out for you, I've read it'd be better to replace your 1xG27 and 2xG31s with 3 new G31s. It's said to be better to have all three batteries within a bank be the same brand/size/type/chemistry/age/etc.


Or... you may even have room to replace the 3x larger batteries with 4x 6V golf cart batteries in that box. About 440 Ah. If that leaves room somewhere (else?) for a small start battery, maybe an option to consider.


-Chris
I agree as well. but unfortunately, I don't think I have the space to even go with 3 31s.

I wonder how much of a problem it is to combine 2 31s and a 27 in one bank...
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Old 03-11-2016, 06:03   #5
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Re: very small AGM for starter?

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Originally Posted by MooGroc View Post
I agree as well. but unfortunately, I don't think I have the space to even go with 3 31s.



I wonder how much of a problem it is to combine 2 31s and a 27 in one bank...

Is the 27 much longer down its useful life than the 31s?

You really don't want to add a bad battery with a good one. The bad battery will drag the good down quickly and soon enough you'll have two that need to be replaced.

That said, no one is going to recommend that you combine the batteries, but you won't burn down the boat if you do. Combine away and monitor if that is the current best option to increased capacity, I say. Also remember that your house bank can start the engine so you don't have to rely on that small starter.
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Old 03-11-2016, 08:57   #6
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Re: very small AGM for starter?

I had the same 2+1 house/starter arrangement as you. I hooked up all three into the house bank and installed a Red Flash 750 from DMS Technology as the starter battery. These are tiny - you can hold one in your hand - and were designed originally to start jet fighter engines. It spins my Yanmar 3YM30 over very nicely and has never let me down.
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Old 03-11-2016, 09:28   #7
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Re: very small AGM for starter?

Another option is to simply convert to one battery bank. It sounds like you practice pretty careful battery management so are unlikely to need an isolated starting battery with that small engine.

Then store a backup small AGM starting battery with low self discharge rate (like an Optima Red-top) with jumper cables. It doesn't need to be in your battery box. Give it a top-up charge once a year. You'll probably never need it.
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Old 03-11-2016, 09:47   #8
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Re: very small AGM for starter?

It works for me. I have a 55 small AGM for starting and 350 for house . The engine starts with only a slight touch on the button and if needed I can use the house batteries to start the engine by just turning a knob.
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Old 03-11-2016, 11:46   #9
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Re: very small AGM for starter?

I did almost exactly what you describe on my Alerion. I combined the existing batteries into a 3 x Group 27 bank (AGM) and added a Gr 24 starting battery. I have never had the slightest problem starting, and I have the house battery in reserve. Works perfectly, along with the Blue Sea System ACR and their Dual Circuit battery switch.

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Old 03-11-2016, 12:05   #10
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Re: very small AGM for starter?

A 39hp engine will take a lot of amps to crank. Be sure that the battery contemplated will work with the starter. Know more than one starter that was burned up by not having enough cranking amps one of which ended a cruise prematurely. If you had a sub 30hp diesel, a group 24 would work just fine but may not for your larger engine.

If you have room above the battery box, might look at large 6volt batteries. They have the same footprint as Golf Cart batteries but are taller giving 300 plus amp/hours in the same footprint as a 200 amp/hour bank. Wire them in series to get 12v. Downside is you need to add them in pairs but two should work for the amp/hours that you are after.
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Old 03-11-2016, 14:20   #11
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Re: very small AGM for starter?

Quote:
Originally Posted by MooGroc View Post
I currently have a group 27 AGM for my starter battery and 2 group 31 AGMs for my house. I'd like to combine the 27 with the 2 31s to increase my house bank from 210 amp/hrs to 300 amp hours. I have an isolator so the alternator charges two banks independently and a solar regulator that also tops off the engine bank after the house is full. I don't sail in colder climates so my engine starts right away every time (knock on wood).

I'm considering the Deka ETX30LA (400CCA). Switching to a much smaller starter battery seems to have lots of advantages and no disadvantages.
  1. I have just enough room in my battery box to fit a small 4th battery so no need to build another box.
  2. Even this small battery should have no problem starting my 39hp 3JH3E.
  3. I will configure my battery switches to that I can easily start with the house bank if need be.
  4. I get a bigger house bank w/o losing the redundancy of 2 banks.

Any disadvantages I'm not thinking of?
I had a one Deka and an one AGM battery in the same battery bank and the AGM was toast in less than one year. Both were new at the time of the installation. Not impressed with AGM batteries... my opinion...
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Old 06-11-2016, 04:15   #12
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Re: very small AGM for starter?

Quote:
Originally Posted by MooGroc View Post
I agree as well. but unfortunately, I don't think I have the space to even go with 3 31s.

I wonder how much of a problem it is to combine 2 31s and a 27 in one bank...
Three G27s?


Quote:
Originally Posted by roverhi View Post
A 39hp engine will take a lot of amps to crank. Be sure that the battery contemplated will work with the starter.
OP's starter may have a sticker that tells minimum CCA/MCA requirements, and the engine specs will also include that info. In the grand scheme of things, 39hp is relatively small.


Quote:
Originally Posted by seabreez View Post
I had a one Deka and an one AGM battery in the same battery bank and the AGM was toast in less than one year. Both were new at the time of the installation. Not impressed with AGM batteries... my opinion...
You had a flooded lead acid battery and an AGM battery in the same bank? Hard to imagine the "AGM-hood-ness" of the one battery being the cause of early failure.

-Chris
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Old 06-11-2016, 04:31   #13
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Re: very small AGM for starter?

You had a flooded lead acid battery and an AGM battery in the same bank? Hard to imagine the "AGM-hood-ness" of the one battery being the cause of early failure.

-Chris[/QUOTE]

I did this installation with the advice that it would not be a problem to combine the different types of batteries. I then returned the battery to the dealer with my original invoice and he was sure I had the wrong battery or the invoice. I left the battery with no reply and no warrantee. The battery was 19 months old... go figure.
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Old 06-11-2016, 05:06   #14
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Re: very small AGM for starter?

Quote:
Originally Posted by seabreez View Post
You had a flooded lead acid battery and an AGM battery in the same bank? Hard to imagine the "AGM-hood-ness" of the one battery being the cause of early failure.

-Chris
I did this installation with the advice that it would not be a problem to combine the different types of batteries. I then returned the battery to the dealer with my original invoice and he was sure I had the wrong battery or the invoice. [/QUOTE]


Yeah, I suspect your dealer's reaction on return probably indicts the accuracy of that original "advice."

-Chris
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Old 06-11-2016, 05:20   #15
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Re: Very small AGM for starter?

I'm about to do the same thing on my boat. Only, instead of using an AGM for my new starting battery, I will use a NOCO GB40. It is cheaper, smaller, and more lightweight than a lead acid battery.
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