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Old 11-17-2014, 12:03 PM   #1
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Separate house / starting banks, or not?

I know... this is probably a question like "Which religion is best?", but I can't find any threads that deal with it specifically, so I'm asking. (If such a thread exists and someone can point me to it, that would be fantastic!)

Bought our 1974 Gulfstar 43' Trawler in May. Been learning about it ever since, while learning about boat electrical systems - we're newbies. Recently replaced a totally fried Xantrex inverter / charger with new from Magnum Energy, and I'm trying to get it set up optimally.

The boat has 8 6V GC2 batteries (wired to make 4 12V pairs), totaling about 830 Ah of capacity. There are three battery switches - two that can tie together, or isolate, pairs 1 and 2; another that can tie together / isolate pairs 3 and 4; another at the DC panel that can tie together / isolate the output from the other two switches.

Since we got the boat, we have always had all three switches set to BOTH, so all 8 batteries are all being used as the house bank, and also the starting bank.

We have an 8kw Westerbeke with a totally separate starting battery, and the genset provides 120v to the AC panel, which includes power to the inverter / charger to charge the 8 batteries. (We don't have, but intend to add, some kind of trickle charger to the genset start battery - probably solar powered.)

Each engine has a Balmar high output alternator connected to a Balmar three-stage regulator, tied together with a Balmar Centerfielder, to ensure equal output of the alternators.

For most of the year, the boat is in a slip with shore power, so the inverter / charger keeps the batteries topped up all the time. We occasionally take it out for 1 - 3 days at a time on the hook, but so far, haven't been away from shore power longer than that. Next summer, however, we want to cruise the Bahamas for a month, hoping that much of that time will be on the hook.

So, the question: since we have a completely separate way to charge the 8 batteries if we ever discharge them below their ability to start an engine - is there any compelling reason to use the 8 batteries as two separate banks - house and starting? (As you can probably tell, I'm inclined to continue to operate them as one big bank - and Calder, in his latest edition, leans that way - but I'm open minded!)
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Old 11-17-2014, 12:14 PM   #2
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Sounds like you have it covered but I much prefer to have a separate engine start battery. Yes, if the bank is ran down you can use your generator to charge and then start, but you need to wait till it is charged up enough. Often that waiting is the difference from making a weather opening or not. Second even if you can start the engines off the main bank but it turns them over slow because of the bank being ran down, I would think it would be hard on the starters etc. I could see adding one appropriate size battery to start both engines, with the option of starting off the house bank if needed. That way you have everything. Just my thoughts and preferences.
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Old 11-17-2014, 12:20 PM   #3
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I'll throw something into the mix...


I have a house, start, and gen bank all with isolating/tie switches. My genset does NOT charge the gen battery so I rely on my Inverter/Charger to take 120v and replenish all batts on board.


So why do I need a seperate gen starting battery? If I can isolate the generator 12v circuit to keep from possibly drawing down the starting batteries, I do not see any function in having one more starting battery on board. If the starting batts are dead, then I can use the house batts to crank the generator or Perkins to recharge. If both banks are dead, then I probably forgot to isolate everything and left a draw on.


Just seems like too much redundancy. Yes, this is the second dumbest thing I have said
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Old 11-17-2014, 01:42 PM   #4
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I like redundancy and fool-proof simplicity. Brian, sounds like you have a nice setup with quality components. You didn't mention a Battery SOC meter which would give you fuel gage-like readings on your battery bank. I also favor combiners or ACRs over manual switches to provide charge to the appropriate banks, but not share the load when the charge is absent. Your centerfielder provides charge from 2 alternator sources to the single house bank which is great, and probably overkill (not a bad thing, IMO).

I like having a single start battery for both my engines separate from the house so it's always charged and ready for start with no other loads while at anchor. I do have my windlass on my start battery because it's only operated while the engines are running. My battery switches, mounted outside the ER, allow me to tie all banks together for load sharing but I have never needed to do that. I currently have an 8D start battery which is beginning to show its age. I will be replacing it very soon with a single Group 31 start battery for both engines to retain this redundancy.

Brian, with the addition of a single start battery and maybe a SOC, you have all the components needed for full redundancy and monitoring. If you add the start battery and a Blue Sea ACR, you can have your charge which is currently going to your GCs shared with the start battery without you moving switches. Then when you arrive at your anchorage, it will split and remain isolated until the charge voltage is detected.

An 8-GC battery house bank should be great on anchor for a couple of days. I only recently achieved the similar capacity on my boat after years of planning, buying and modifying, but mine is a 6-GC battery house bank. When I replace my 8D start with the Group 31 start, I hope to have the room for 2 more GC batts to add to the house bank.
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Old 11-17-2014, 02:42 PM   #5
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Brian

Had your exact same set up for 10 years on Bay Pelican. 12 6-volt batteries as one large house bank which all severed as the start batteries for the main and wing engine, thruster etc. Five years ago I switched the generator start battery to the helm as a totally isolated battery operating only the SSB and backup VHF radio. Generator is now started from house bank and its alternator charges the house bank.

Happy with this set up. The isolated battery can be moved to start the generator if need be, but then I haven't run the batteries down in 16 years.

Somewhere I was convinced that a above water line battery for the radio was a good thing to have.

As I have reported a half dozen times my isolated battery has been removed from Bay Pelican on several occasions to start generators on other boats. (Marty's jump start service).
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Old 11-17-2014, 02:51 PM   #6
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Chances are that the 8 gc batts is a big enough bank so you can drain them down to no lower than 50% and still get a good start on the mains. Your backup is the gennie to charge up. Sounds fine to me. Depends on how much draw and for how long you are asking of the bank.

My preference is a dedicated engine starting batt, but with that much capacity in the bank, you should be in good shape unless unattentive.
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Old 11-17-2014, 05:34 PM   #7
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Separate starting and house batteries is standard (most typical) arrangement for obvious reasons. Thankfully, that's what we've got.
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Old 11-17-2014, 07:42 PM   #8
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I assume that this discussion is aimed at lead acid batteries, can anyone confirm if the discharge percentages etc are the same for AGM batteries? As I have just replaced my old lead acids with AGM's it would be good to know
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Old 11-18-2014, 06:50 AM   #9
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What you have is fine , IF you add a SOC meter and stay in warm climates.

If the engine is cold soaked , below freezing, it may not spin fast enough with 50% dead batts.

If you are careless (or have no SOC meter) the std of having a dedicated start type batt , isolated from house , is best.
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Old 11-18-2014, 06:52 AM   #10
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I think I would keep the combined 8 GC batts as a house bank. Then add a two GP 31 batts dedicated to the engines and genset.
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Old 11-18-2014, 09:35 AM   #11
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To fill in a few blanks:

1. We are warm weather boaters. The boat is in Melbourne, FL, and all of our travels will be to the South.

2. We do have a SOC meter - brand new, as part of the new Magnum Energy Inverter / Charger. It has a neat feature that allows the inverter to be shut off if the battery voltage falls below a pre-set figure. The default (factory) setting for that is 10.5 volts, but that seems awfully low to me. I set it at 12.0, but that seems overly conservative. Given that the batteries are also my engine start batteries, what's a safe "Low Battery Cut-off" voltage?

I will definitely look into a separate start battery and the Blue Sea ACR - sounds like a really good idea - belts AND suspenders! (Or maybe just isolate 2 of my 8 GC2's and make them the start batteries, leaving 6 for the house, but with the ACR to keep them charged but not discharged, and the ability to tie all 8 together to start the engines if ever necessary.

Thanks, all!
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Old 11-18-2014, 09:38 AM   #12
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To answer your question about the cutoff voltage what type of loads (current/wattage and type) are you running off of the inverter.
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Old 11-18-2014, 10:50 AM   #13
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I think what you have is absolutely fine and I would not complicate it at this stage with a separate House bank...UNLESS you have cranking needs over and above propulsion engine starting: for example, do you have an electric bow thruster? In that event, a case could be made for a separate cranking/start bank using a different battery technology. But otherwise, I like you set-up.
You also commented that your genset needed a trickle-charge and that you were contemplating solar for this. Doesn't your genset have a small alternator already in place for charging the genset start battery? Why would it need more than this? A dedicated genset start battery is likely to be fully recharged by the genset alt about 4 minutes after starting.
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Old 11-18-2014, 01:44 PM   #14
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I think what you have is absolutely fine and I would not complicate it at this stage with a separate House bank...UNLESS you have cranking needs over and above propulsion engine starting: for example, do you have an electric bow thruster? In that event, a case could be made for a separate cranking/start bank using a different battery technology. But otherwise, I like you set-up.
You also commented that your genset needed a trickle-charge and that you were contemplating solar for this. Doesn't your genset have a small alternator already in place for charging the genset start battery? Why would it need more than this? A dedicated genset start battery is likely to be fully recharged by the genset alt about 4 minutes after starting.
No bow thruster. There is an electric windlass, but as someone noted above, that's never used unless the main engines are already running, so no fear of it robbing me of starting power.

The genset does have its own alternator to charge its starting battery. The trickle charger is because the boat sits in the marina for long stretches w/o the genset running - it would keep that battery fully charged all the time. Overkill?
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Old 11-18-2014, 01:48 PM   #15
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To answer your question about the cutoff voltage what type of loads (current/wattage and type) are you running off of the inverter.
The fridge and freezer run off the inverter, but they don't seem to take much power. (They're old and I can't find actual load info about them, unfortunately.) There are two outlets that run off the inverter, but those aren't used much at all. Except first thing in the morning, to make coffee. I know the wattage of a coffee maker is high, but I don't THINK the overall draw on the batteries is all that much, since it runs for only about 10 minutes before the coffee is made and only the warmer is running.

So... what's a general range for a safe low voltage level before the inverter automatically stops drawing power from the batteries? Is the factory setting of 10.5V OK, with my battery bank? (About 830 Ah capacity.)
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Old 11-18-2014, 02:41 PM   #16
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That should certainly handle the surge current of the fridge compressors and the load of the coffee maker. If you wanted to be safe you could change it to 10.75 or so but if you go with a separate start battery then the 10.5 would be fine. You might want to do a test with the fridge and freezer compressors running and the coffee maker running and see what it drops your voltage to. That way you would know what approximate voltage your battery would be without those loads in case you decide to not do a separate start battery.
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Old 11-18-2014, 02:58 PM   #17
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Try a dockside experiment. Disconnect shore power and run things on inverter like you would on the hook. Let batts run down til inverter shuts down. Try a start on the engines.
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Old 11-18-2014, 03:14 PM   #18
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The genset does have its own alternator to charge its starting battery. The trickle charger is because the boat sits in the marina for long stretches w/o the genset running - it would keep that battery fully charged all the time. Overkill?

There is no stand-by draw by the generator, so assuming you return to the dock with the genset battery fully charged, no problem leaving it just sit, even for extended periods (like a month or 2).
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Old 11-18-2014, 03:15 PM   #19
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I assume that this discussion is aimed at lead acid batteries, can anyone confirm if the discharge percentages etc are the same for AGM batteries? As I have just replaced my old lead acids with AGM's it would be good to know
I have all AGM's and as far as I know the 50% rule applies to them too.
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Old 11-18-2014, 03:20 PM   #20
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The
So... what's a general range for a safe low voltage level before the inverter automatically stops drawing power from the batteries? Is the factory setting of 10.5V OK, with my battery bank? (About 830 Ah capacity.)
I am not familiar with your Magnum inverter/charger, but MasterVolt & Victron models -- & I suspect most others -- are pre-configured to disconnect their inverter circuitry when V drops below a certain point: I think on my set-up, this is 12.0V....but that point is programmable by you. I would have thought 10.5V is much too low...there is something clearly wrong with the battery bank if inverter loads draw the V down to that level and you want to cut out well before that. Do you leave the inverter circuits on when you are dockside and on shorepower and away from the boat...or can/do you switch to 'charger only' mode? (I do. And I have the same house bank size as you do.)
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