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post #1 of 10 Old 10-25-2007 Thread Starter
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Battery Banks 1 or 2

I have seen it mentioned a number of times, that one big bank of batteries is better than two separate banks. I have been unable to find the reasoning for this so could someone explain it in simple terms, as I am a twelve volt moron.(as no doubt are a lot of others)

Why should I not run Auto pilot and radar from one bank and the fridge and other items from the second, would this not give me some sort of redundancy.

A separate starting Battery is a given.

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post #2 of 10 Old 10-25-2007
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Nigel Calder does it best, here's a try (this can be like a religion, so it might get fractuous).
I use 1 bank, 315 ah, with a starter battery that is exactly the same (AGM deep) on a ACR, battery switch always on 'both', 80w solar the only charger I've need all year (and the occasional iron jenny adding in). I also have a solar booster and battery monitor keeping it all toasty.

Charging works fastest/best with stages, and most chargers use 3. Bulk rate is the fastest rate - and is usually based on percentage of charge (which is calculated by the voltage level, but that's too deep) -

If you have two batts that hold 100 ah each as 2 banks, and they are 50% full then when charging them at 20ah (average charger) they will bulk rate charge to the 80% level in 3 hours (10ah per battery, half the output of the charger) then go into the next stage of charging. At this point you have two banks, each with 80 ah in them, meaning a usable 30 ah each before you cycle the battery - and you have to watch them to make sure you don't cycle the battery.

Meanwhile, as one bank 200 ah, drained to 100ah, dumping the same 20ah via a charger; keeping in mind that 80% of 200 is still 160 you reach 80% in the same 3 hours - but now you have the use all the a total of 80 ah without having to worry about switching banks, cycling the battery.

Using an altenator, with larger ah output the single bank gets even better - you can really pour in ah much faster if the bucket is bigger to begin with.

The rate of fill stays pretty much the same for the next two stages of charge, it still works on percentage of overall all charge (read by voltage of the battery) - the bigger the bank, the more ah you can pour in before going to the next, slower charge stage.
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post #3 of 10 Old 10-25-2007
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The big bank theory is based on a couple of things:

First, the deeper you discharge the batteries, the shorter their life. By making a big bank for a given amp load and time, the degree of discharge is reduced.

Second, the last 10% or so of topping charge takes a loooooong time. By making a big bank, the capacity is larger and re-charging is more efficient. This is more important for charging off engine, wind or solar and less so if you mostly recharge from shore power. (edit) I didn't see Chuck's post until after I posted this. His explanation is more detailed but along the same lines.

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Last edited by CapnHand; 10-25-2007 at 09:38 AM.
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post #4 of 10 Old 10-25-2007
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Simon-

The reason one big bank of batteries is generally better than two separate house banks is that the battery life is related to the level of discharge. A larger bank can go longer before being discharged to an unsafe level. It is also easier to charge and maintain the charge level on a single bank, rather than two separate banks. Finally, the larger the battery bank is compared to the loads on it, the larger the effective capacity of the battery bank is—this is because of Peukert's Law. The easiest way to see this is check the specifications of a battery... it will have different Amp-hour ratings at 5A and 20A discharge levels.

Of course if you don't have a separate starting battery bank this is moot.

As for running radar and autopilot from one bank, and the reefer and other items from the other—yes, separating them would make sense...but it does complicate your wiring and setup quite a bit....since you now have three banks, and the two banks for the house side still need to both be fairly large, since radar and autopilots probably draw at least as much as the refrigerator. It would provide a bit of redundancy, but IMHO, the additional complication it adds to the entire electrical system isn't worth the hassles.

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Last edited by sailingdog; 10-25-2007 at 09:46 AM.
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post #5 of 10 Old 10-25-2007
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Note - I don't subscribe to this theory about why one large bank is problematic but the reasoning is sound:
If one cell in one battery of a large bank goes bad and shorts, the entire bank is at risk of damage or worse making a one battery problem a disaster.

For that reason, there are many who split their larger banks into multiple parts. whether you choose to follow the above logic or opt to have one large bank (as I have done) is a personal preference.

Either option has it's pros and cons.
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post #6 of 10 Old 10-25-2007
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You can always pull the bad battery...and jumper around it. However, most batteries failing are due to poor maintenance and treatment.... which is far more likely in a two-bank setup than it is in a single larger bank. The batteries are less likely to be run down significantly, more likely to be evenly charged, easier to monitor, etc...

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Originally Posted by k1vsk View Post
Note - I don't subscribe to this theory about why one large bank is problematic but the reasoning is sound:
If one cell in one battery of a large bank goes bad and shorts, the entire bank is at risk of damage or worse making a one battery problem a disaster.

For that reason, there are many who split their larger banks into multiple parts. whether you choose to follow the above logic or opt to have one large bank (as I have done) is a personal preference.

Either option has it's pros and cons.

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post #7 of 10 Old 10-25-2007
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Does this simplify

Boat 1 Boat2

Alternator: 400 W 400W
Consumption: 35 Ah 35 Ah
Bat. Capacity: 70 Ah 350Ah
Run engine per day: 5 hours 1 hour

I am trying to understand DC stuff and this helped me. I hoped it helps others.
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post #8 of 10 Old 10-25-2007
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I run two banks

I run two banks one house bank 375ah and one emergency/start/backup battery of 125ah. My house bank, 375ah, is position selector #1 and is used 100% of the time for everything including starting. My emergency/start/backup battery is on position #2 and is a group 31 at 125ah. It is never used except for my stereo memory wire to keep some activity & draw on it. My batteries are tied together via a relay and charged by solar and the alternator. When the voltage hits 13.2 volts the relay automatically combines the batteries for charging & keeps both banks fully charged at all times.

So one big bank for the house loads and one back up battery in case you kill the house bank by accident..

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post #9 of 10 Old 10-25-2007
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We had a single 1100 A/H AGM bank and thus could withdraw 500+ A/H's and put back those amp hours as quickly as desired without any limits other than my charger (130A). To my mind, the simplicity and quicker charging of one large house bank outweighs any risk of a bad cell.
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post #10 of 10 Old 10-25-2007 Thread Starter
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Thank you all for the imput, it seems one large bank and a starter battery is the way to go.

Simon
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