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Old 26-11-2014, 07:32   #1
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Adding a battery bank

Boat came with a house battery bank of 2 group 31 lead acid batteries, which just fit in the space under the quarterberth. I replaced them with new ones a month ago. Needing more capacity, only option was to add another bank (2 more, same exact batteries) about 4ft away (~7ft of cable distance). I know you are supposed to take the load off opposite ends of the whole bank, so my question is - is there any way around running 3 cables over to the other bank? Does the advantage of wiring it that way outweigh the extra length of 1/0 cable?
I'll try attaching a diagram.
Thanks,
David
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Old 26-11-2014, 08:43   #2
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Re: Adding a battery bank

Diagram:

perhaps "bank 1" and "bank 2" are misleading, as it's all one bank.
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Old 26-11-2014, 09:15   #3
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Re: Adding a battery bank

While symmetrical wiring can balance heavy starting loads it is unnecessary with house loads. The differential voltages introduced by house loads on an unsymmetrical circuit are smaller than the traditional variations from battery to battery due to manufacturing so the wiring is eliminating voltage differences that are small compared to battery differences over which you have no control.

Even if you have momentary heavy loads, battery to battery current will equalize the batteries within a second or two after the load is removed.
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Old 26-11-2014, 09:40   #4
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Re: Adding a battery bank

Thanks so much, that makes perfect sense.
David
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Old 26-11-2014, 10:47   #5
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Re: Adding a battery bank

If you have the room you would get much better results by installing a pair of 6 volt deep cycle golf cart batteries. The GC2 size is very close to the size of a group 31.

Depending on the maker's specs 2 X 12V Group 31 batteries in parallel would give you maybe 150-160 amp hours where 2 X 6V GC2 deep cycles in series give 220-230 amp hours.

Dimensions L x W X H
G31 12 7/8 x 6 1/2 x 9 3/8
GC2 10 1/4 x 7/18 x 11 1/4
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Old 26-11-2014, 10:56   #6
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Re: Adding a battery bank

Thanks, I did consider that, but they certainly wouldn't fit in the stock location (due to the height), and I didn't want to mix battery types on the same bank, nor put them on a 2nd separate, switched, bank.

In retrospect, perhaps the 2nd switched bank wouldn't be too big of a deal. Maybe next time around.
David
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Old 26-11-2014, 10:59   #7
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Re: Adding a battery bank

I would disagree with the statement below. You are not considering how the electrons flow when charging, and the voltages the batteries reach during charging.

Wiring as the OP's diagram indicates is indeed the way to do this.

Chris

Quote:
Originally Posted by Andina Marie View Post
While symmetrical wiring can balance heavy starting loads it is unnecessary with house loads. The differential voltages introduced by house loads on an unsymmetrical circuit are smaller than the traditional variations from battery to battery due to manufacturing so the wiring is eliminating voltage differences that are small compared to battery differences over which you have no control.

Even if you have momentary heavy loads, battery to battery current will equalize the batteries within a second or two after the load is removed.
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Old 26-11-2014, 11:16   #8
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Re: Adding a battery bank

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Originally Posted by planetminder View Post
Thanks, I did consider that, but they certainly wouldn't fit in the stock location (due to the height), and I didn't want to mix battery types on the same bank, nor put them on a 2nd separate, switched, bank.

In retrospect, perhaps the 2nd switched bank wouldn't be too big of a deal. Maybe next time around.
David
Hi David,

First, setting up the house batteries in two banks is generally not the best option for several reasons. If you are interested I can go into details about why.

Second, mixing battery types or older batteries with new (even if the same type) is not the ideal but if it fills a need then it's not going to destroy the system or blow up your boat or kill all the batteries in six months. Mixing totally different battery chemistries is a little more problematic but different size lead acid batteries together you should be fine.
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Old 26-11-2014, 12:21   #9
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Re: Adding a battery bank

If you bought batteries a month ago, you can certainly add more of the same, in parallel, and still have only one bank.

As far as the golf cart batteries, I like them. Lot of juice for a reasonable space and weight, and they are cheap. I paid $85 each for my 8 GC2 batts at Sams Club, and they are my propulsion bank. I do have a pair of group 31s left from back when I had an infernal combustion engine, but I only use them for lighting, cell, and laptop. 31s are light and easy to handle but they don't pack much juice.

If you have a diesel or gas engine with electric start, it is IMHO a good idea to have 2 separate banks. That way if a short drains your house bank while you sleep on the hook, if you remembered to ensure you were only running on one bank, you still have engine start capability and use of radio or navigation electronics.
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Old 26-11-2014, 13:36   #10
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Re: Adding a battery bank

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Originally Posted by GrowleyMonster View Post
If you have a diesel or gas engine with electric start, it is IMHO a good idea to have 2 separate banks. That way if a short drains your house bank while you sleep on the hook, if you remembered to ensure you were only running on one bank, you still have engine start capability and use of radio or navigation electronics.
Yes you should have a separate bank for starting. One bank for all the house batteries, one for engine starting and the two systems isolated from each other.

Recommend this from personal experience.
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Old 26-11-2014, 16:40   #11
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Re: Adding a battery bank

Yes, I do have a separate starting battery in a separate switched bank. I'm thinking of installing an automatic charging relay so I don't have to worry about switching back and forth between banks (and running the alt. direct to the house bank) - just leave it on the house bank. But I'm not sure using the house bank to start the engine (and the start battery as backup) is the best idea - especially with all the distance among the batteries in my house bank. And my old Westerbeke takes a bit of cranking when cold.

David
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Old 26-11-2014, 19:45   #12
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Re: Adding a battery bank

Quote:
Originally Posted by planetminder View Post
Yes, I do have a separate starting battery in a separate switched bank. I'm thinking of installing an automatic charging relay so I don't have to worry about switching back and forth between banks (and running the alt. direct to the house bank) - just leave it on the house bank. But I'm not sure using the house bank to start the engine (and the start battery as backup) is the best idea - especially with all the distance among the batteries in my house bank. And my old Westerbeke takes a bit of cranking when cold.

David
Don't worry ur starter will croak before ur batts.
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Old 27-11-2014, 10:20   #13
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Re: Adding a battery bank

here`s my hmmm again...1st, grp 31`s are mainly starting batts, have thin plates, let off amps quickly to start your engine. 2nd, deep cycle batts have thick plates, to let amps out slowly, for lights, etc...the thicker plates allow them to take lots of charges/discharges over the yrs of use. i use what is called 'sweepers' they are 6v, and 260 amps ea...in series, they are 12v, and 260 amps. rule of thumb is...never let them get below 12.2v,[half discharged] before re-charging...ie, longer life! the battery people that sold you the grp 31`s, should have informed you of that information! get the most amps for their foot print you can get! i would also not use any cable less than 2/0! larger cable, better current
flow, plus all cables in the system have to be the same size too! not 1/0 here, and 2/0 there, etc...it has to do with the flow of the current!...any ?`s just ask me...oh, and i would`nt have any swithes between banks! with each sw, there are losses...and last, always charge them as one large bank!...clyde
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Old 27-11-2014, 11:58   #14
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Re: Adding a battery bank

Well these are Trojan 30HXS, marketed as a deep cycle floor machine battery. Not sure of the plate thickness - it'd be nice if manufacturers offered that on the spec sheet. I would have used 6v golf cart batteries in a second if I could find any that were less that 10'' tall. All the research I did warned against mixing battery types, even of different manufactures. But now I've seen some other opinions to the contrary, leading me to believe I should have added a couple 6V golf cart batteries in series to the bank instead. Hard to really know what to do for sure, with this Internet thing...
David
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Old 27-11-2014, 13:40   #15
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Re: Adding a battery bank

Hi there,
I am looking at buying a catamaran here in Australia and I am really I terested in your electric propulsion. Can you give me more details as to how you went about the conversion and how you keep the batteries charged enough to provide you with enough " engine" power when needed?
Thanks
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