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Old 25-02-2021, 13:11   #1
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Reason for having multiple house battery banks?

The previous owner upgraded the boat's DC system extensively. It's bloody complicated. We have two separate 12 VDC house banks and a starter bank.

I don't understand the reason for having two separate house banks. Please share your thoughts on this arrangement.

House bank 1: four, 6V, 190 Ah AGM batteries located aft.
House bank 2: six, 6V, 190 Ah AGM batteries located midships.
All ten 6V batteries are identical. Total capacity: 950 Ah at 12 VDC, 700 lb.
There is a switch to take each bank offline.
There is a switch to parallel the banks together.
Each house bank supports different loads.
Each house bank has a separate battery charger.
I don't as yet understand how the solar MPPT charges both banks.

Our starter bank has three, 12V, 92 Ah AGM batteries. One battery is located midships at the engine and two are located forward at the bow thruster. Total capacity: 276 Ah at 12 VDC, 225 lb. This bank powers the starter, generator and bow thruster. This bank is charged via battery charger, engine alternator or generator alternator. I think this bank should also power the windlass but I have to check on that.

Thanks!
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Old 26-02-2021, 00:10   #2
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Re: Reason for having multiple house battery banks?

Since batteries impedance values vary, you don’t want to connect them in parallel, or at least want to minimize the number of paralleled batteries in a bank. Also, you want charging current to be relatively high for a high capacity bank, which again limits reasonable number of batteries in a bank. I can see why the starter and thruster share the same bank - both of those loads are active only once in a while.
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Old 26-02-2021, 01:33   #3
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Re: Reason for having multiple house battery banks?

The only advantage to having 2 house banks is if one physically fails. You can shut it off and power from the other. Al though if each bank is powering its own loads. You do not have this advantage. You’d still lose half the boat.

There are many advantages of a single. Though it becomes harder to do with batteries in 2 spots.
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Old 26-02-2021, 04:35   #4
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Re: Reason for having multiple house battery banks?

As SMAC says there are many advantages to a single bank. Any large draw will deplete a single bank by a lesser percentage than the same draw on half of two banks. Charging of a single bank will also be more efficient.


The battery capacity both house and start seem excessively large for the size of boat. I can only think that the previous owner wished to spend time off grid and that there is insufficient off grid charging ability.


Some catamarans will opt for 2 banks purely to save weight by eliminating multiple wiring runs across the bridgedeck and accept this as a trade off in efficiency.


I would plan what you think is the best arrangement for you and adapt the existing system to suit.
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Old 26-02-2021, 05:47   #5
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Re: Reason for having multiple house battery banks?

A thought supporting a single bank failure as discussed above:

You can always take a battery or set of batteries out of a failed bank while underway. It would reduce your capacity of course, but you’d still have power.
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Old 26-02-2021, 06:46   #6
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Re: Reason for having multiple house battery banks?

Quote:
Originally Posted by vpbarkley View Post
I don't understand the reason for having two separate house banks. Please share your thoughts on this arrangement.

House bank 1: four, 6V, 190 Ah AGM batteries located aft.
House bank 2: six, 6V, 190 Ah AGM batteries located midships.
All ten 6V batteries are identical. Total capacity: 950 Ah at 12 VDC, 700 lb.

Thanks!
Because the batteries are in different locations it would be hard to efficiently parallel them because of cable lengths. Could be part of the reason for two banks, two chargers, etc.

It would still give some redundancy in case of a failure as well.

Most older boats were designed with two banks, as the 1/2/off/Both switch was standard thinking.
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Old 26-02-2021, 06:53   #7
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Re: Reason for having multiple house battery banks?

Seems like a great candidate for Lithium. 600Ah where the 4 6V currently are would replace every other battery on the boat. Even the start batteries if you were so inclined. You would save 600lbs and gain storage space.
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Old 26-02-2021, 07:04   #8
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Re: Reason for having multiple house battery banks?

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Originally Posted by wholybee View Post
Seems like a great candidate for Lithium. 600Ah where the 4 6V currently are would replace every other battery on the boat. Even the start batteries if you were so inclined. You would save 600lbs and gain storage space.
This!!!

We wanted to say it also, but didn’t want to derail things to the inevitable LA vs LI debate.

It’s exactly what we did and we are extremely happy with the results.
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Old 26-02-2021, 07:23   #9
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Reason for having multiple house battery banks?

I would go out of my way to put all my house batteries in one bank. But that only works if they can be in the same location.

Potentially you could have parts of a single bank in separate locations if you fed the positive and negative from opposite ends. Possible but potentially difficult, depends on all the details of the situation.
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Old 26-02-2021, 09:53   #10
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Re: Reason for having multiple house battery banks?

How far apart are the two house banks? That’s a lot of batts.
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Old 26-02-2021, 09:54   #11
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Re: Reason for having multiple house battery banks?

I had two separate banks on my KP46, the house bank aft, and fwd, under the bunk another bank dedicated to the windlass and thruster. A small charger on the fwd bank and a Xantex 3000 aft. I had a 4/0 cable running between the two banks to parallel (which I did when heaving the anchor as the engine would then charge the fwd bank). The system worked fine and gave me a backup for the engine or gen start should the house and engine batteries be run down.

The system worked fine for me.

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Old 26-02-2021, 10:04   #12
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Re: Reason for having multiple house battery banks?

Have been using two location house bank for 15 years (4X 6V, 2X 6V). They all stay in parallel, no starting battery, and the fwd set are tapped directly to power windlass and bow thruster. If there are any problems with this set up I have yet to detect them. Each pair is individually switched to allow a single pair start test couple of times a year. If one set hesitates to start the 3GM Yanmar they all come out (typically every 5 or 6 years).


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Old 26-02-2021, 10:18   #13
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Re: Reason for having multiple house battery banks?

Space.....when you add additional batteries you have to out them were they can go
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Old 26-02-2021, 10:20   #14
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Re: Reason for having multiple house battery banks?

It doesn't matter where the house batteries are located, just make sure the wires between them are oversized, and don't waste any time "worrying" about "balancing". It's almost an urban myth.




IS IT BETTER TO HAVE ONE OR TWO BATTERY BANKS FOR HOUSE USE?
(By Nigel Calder - I DIDN’T write this!!!)

This appears in Calder’s excellent Boatowners Manual. I bought the 2nd edition in 1998.

The [formerly – pre-1990s] popular arrangement of having two house banks alternated in use needs scrutiny before I go any further.

LIFE CYCLES: As we have seen, the life expectancy of a battery in cycling service is directly related to the depth to which it is discharged at each cycle - the greater the depth of discharge, the shorter the battery’s life.

This relationship between depth of discharge and battery life is NOT linear. As the depth of discharge increases, a battery’s life expectancy is disproportionately shortened. A given battery may cycle through 10% of its capacity 2,000 times, 50% of its capacity 300 times and 100% of its capacity around 100 times.

Let’s say, for arguments sake, that a boat has two 200-ah battery banks, alternated from day to day, with a daily load of 80 Ah. Each bank will be discharged by 40% (80 Ah of one of the two 200 Ah banks) of its capacity before being recharged. The batteries will fail after 380 cycles, which is 760 days (since each is used every other day). If the two banks had been wired in parallel, to make a single 400 Ah battery bank, this bank would have been discharged by 20% of capacity every day, with a life expectancy of 800 days, a 5% increase in life expectancy using exactly the same batteries!

But now let’s double the capacity of the batteries, so that the boat has either two 400 Ah banks, or a single 800 Ah bank, but with the same 80 Ah daily load. The two separate banks will be cycling through 20% of capacity every other day, resulting in a total life expectancy of 1,600 days. Doubling the size of the battery banks in relation to the load has produced a 210% increase in life expectancy. The single 800 Ah bank will be cycling through 10% of capacity every day, resulting in a life expectancy of 2,000 days - a 25% increase in life expectancy over the two (400 Ah) banks, and a 250% increase in life expectancy over the single 400 Ah battery bank!

There are two immediate conclusions to be drawn from these figures:

1. For a given total battery capacity, wiring the (house) batteries into a single high capacity bank, rather than having them divided into two alternating banks, will result in a longer overall life expectancy for the batteries.

2. All other things being equal, any increase in the overall capacity of a battery bank will produce a disproportionate increase in its life expectancy (through reducing the depth of discharge at each cycle).

FOR BATTERY LONGEVITY, A SINGLE LARGE (HOUSE) BANK, THE LARGER THE BETTER, IS PREFERABLE TO DIVIDED (HOUSE) BANKS.
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Old 26-02-2021, 12:56   #15
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Re: Reason for having multiple house battery banks?

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Originally Posted by vpbarkley View Post

I don't understand the reason for having two separate house banks. Please share your thoughts on this arrangement.
It does not matter the size of your battery banks. One for engine (starting) and one for everything else.

If you use 100 amp hours of power and have a 400 amp hour battery bank, you deplete to about 75% capacity.
If you have two 200 hour battery banks and you use 100 amp hours of power, that bank is at the critical 50% depleted that starts to do permanent damage to the batteries.
Why do that?
Unless you have huge battery banks compared to your power needs, it is better for your batteries to have a single, large house bank.
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