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Old 06-06-2018, 11:14   #1
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Two house battery banks - necessary?

I have about 960AH capacity on my boat, split between 2 house banks. They each have separate switches so they *can* be isolated, but is there really a need for this? The only reason I can think of is in case one bank has a battery that shorts, I could isolate it and continue on. But is that common enough to merit the trouble?

I also have a separate starting battery, separated from the house bank with an automatic charge relay.
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Old 06-06-2018, 16:38   #2
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Re: Two house battery banks - necessary?

I have the exact same set up on my boat - even the same amount of amp hours - with the exception that our starting battery can be isolated by a third battery switch. It was a while ago but I was having a detailed conversation with a technician from Blue Sea Systems and he suggested that I leave everything the way it was. I canít remember his explanation on why it was best left that way but I do remember that it made sense to me at the time. I think your suggestion that it would allow you to isolate half of your bank in the event of losing a battery makes sense. Perhaps a marine electrical engineer will chime in here. Or maybe not.

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Old 06-06-2018, 16:42   #3
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Re: Two house battery banks - necessary?

The deeper you discharge a battery bank the shorter the life span. If you leave them combined the discharge depth will only be half during the same time period.
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Old 06-06-2018, 21:15   #4
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Re: Two house battery banks - necessary?

If it's already like that. I would leave it. And always leave the switch on both.

The big downside is you need 2 battery monitors if you want to get one.
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Old 06-06-2018, 22:20   #5
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Re: Two house battery banks - necessary?

I have a similar set up but no separate starter battery. Actually the second bank is the starter bank but I believe 225ah deep cycle isn't what I'd call a starter battery.
I have the two banks combined mostly. If heading offshore I separate them giving the redundancy of a second bank. I like having the option of being able to isolate the banks.
The great think about the extra capacity as a result of combining the banks is very rarely are my batteries below 12.6v at dawn.
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Old 07-06-2018, 09:27   #6
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Re: Two house battery banks - necessary?

Remember the isolation switch is also a fire switch. In the event of a fire or major electrical fault you should be able to isolate the batteries. There is enough power stored in there to weld quater plate steel! It is also useful if you want to equalize the banks, sometimes this can create voltages over 15v that can damage nav electronics
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Old 07-06-2018, 09:57   #7
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Re: Two house battery banks - necessary?

I have 3 banks (6GG2) each on a switch. The switches allow me to individually test each bank (engine starting, equilization, bad bank isolation). Can not imagine having it wired any other way.
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Old 07-06-2018, 10:09   #8
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Re: Two house battery banks - necessary?

Quote:
Originally Posted by sailingunity View Post
I have about 960AH capacity on my boat, split between 2 house banks. They each have separate switches so they *can* be isolated, but is there really a need for this? The only reason I can think of is in case one bank has a battery that shorts, I could isolate it and continue on. But is that common enough to merit the trouble?

I also have a separate starting battery, separated from the house bank with an automatic charge relay.
I think you have nailed it. You probably use them in parallel anyway but have the option to turn one loose should it have a shorted cell.
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Old 07-06-2018, 10:38   #9
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Re: Two house battery banks - necessary?

Two banks was the old school way to manage batteries. How old is the boat?


meridian28 is right about DOD and battery life. Keep them joined.


Battery management has evolved since the two bank method was popular. One bank will be discharged less and will have a longer life. The catch is that you need an engine start battery in case you let the main bank discharge to low to start the engine.



If the two bank are physically separate the switches may be helpful in an emergency. Otherwise it is just more components to fail. Of course you still need a battery on-off switch and a fuse or circuit breaker within a foot of the battery per ABYC standards..
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Old 07-06-2018, 16:33   #10
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Re: Two house battery banks - necessary?

Quote:
Originally Posted by sailingunity View Post
I have about 960AH capacity on my boat, split between 2 house banks. They each have separate switches so they *can* be isolated, but is there really a need for this? The only reason I can think of is in case one bank has a battery that shorts, I could isolate it and continue on. But is that common enough to merit the trouble?

What trouble? Being able to separate the batts makes so much sense in a lot of ways: in case of fire or other disaster, being able to charge or discharge some, but not all batts for testing purposes, separating the batts for diagnostics or other purposes, like equalising while not on the bus and much more.




I'm basically treating every house batt as its own bank, separated by one relay per batt (6 house batts, then the 2 starter batts as one bank, no relay there but a 1-2-both switch). The batts are in 4 different locations, close to the biggest consumers like windlass, inverter and fridge.



This also alllows us to combine different chemistries, switching from the 4 LFPs to the 2 lead-acid batts. Every batt is being monitored for temperature, voltage and currents going in and coming out.


So no trouble at all to be able to separate them.
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Old 07-06-2018, 23:22   #11
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Re: Two house battery banks - necessary?

Apart from safety advantages above.
If I wanted to measure Ahr capacity on half the bank. And still have normal use. Certainly a big advantage to wait the enormous long time periods for the Battery Volts to settle.
I would think measuring Ahr capacity every 6 month or 12 is a advantage.
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Old 08-06-2018, 00:10   #12
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Re: Two house battery banks - necessary?

Quote:
Originally Posted by stormalong View Post
Two banks was the old school way to manage batteries. .
This

So that you could isolate off the radio in an emergency. May still be a useful way of doing things if you still have HF or SSB on board.

Personally, I'd leave as is and you have the best of both worlds: Isolate if you need to but for day to day use as one bank.

Quote:
Originally Posted by meridian28 View Post
The deeper you discharge a battery bank the shorter the life span. If you leave them combined the discharge depth will only be half during the same time period.
and this

Cycling and uneven use of each bank would be a race to the bottom
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Old 08-06-2018, 04:31   #13
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Re: Two house battery banks - necessary?

Thanks for the responses. They are combined 100% of the time. I hadn't thought about equalization, that's a good one.

The boat is from 74 but the previous owner completely re-wired it in 2006 - so I'm guessing it didn't come with 2 house banks (electrical requirements wouldn't have merited so many AH in 1974).
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Old 08-06-2018, 07:51   #14
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Re: Two house battery banks - necessary?

I play with a similar design, but call them Main and Reserve.

Eliminating all other dedicated batts for windlass / winches, thrusters, cranking engines etc.

Which loads, along with House, can be and are regularly switched as needed for continuous live testing that all is well.

One LFP the other lead.

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Another way, more similar to the design discussed here is one big heterogenous bank, but with the "Reserve" functionality accommodated with setpoint-adjustable LVD / VSR cutoffs that isolate a pack / segment of the bank if voltage drops.

But in normal daily operations, sized so it's always one big bank to get the Peukert capacity and lighter-cycling longevity bonuses.

Best combined with similar LVDs taking less essential **loads** offline as voltage drops.

For the belt & suspenders crowd, the small jumpstarter powerpacks kept charged on standby are now capable of cranking even large diesel engines in a pinch.
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Old 09-06-2018, 08:08   #15
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Re: Two house battery banks - necessary?

Just remember that those jump start packs donít last forever. They arenít buy and forget.
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