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Old 23-07-2021, 22:44   #1
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Question 2x6v Deep cycle start bank and 2x12v regular house bank

Hey all,

Our boat came with two banks and each had a single 12v battery. There are your typical group 29 "marine" (but not really) batteries. Obviously the capacity is OK for a day sail, but not for spending a week on anchor.

Due ty space limitations, my only option is to move 12v engine battery into the house bank. The space made available will fit 2x6v real deep cycle batteries, but they are wired as "Starting/engine".

The engine is a Yanmar 2GM20F with a stock alternator (probably around 30-35AMP i would guess).

I know this is not ideal and it would be great to swap 6v deep cycle bats into the house and 12v "regular" into engine bank, but it's just not an option without rewiring everything.

We are about to take off on a week long trip where we will only be able to charge from the engine. I don't mind running it as needed, but know that it's unlikely to recharge deep cycle ones fully. My thinking is that I will run the engine about 2-3 hours in the morning and another 2-3 in the evening. Hoping this will get us through the 6 days we plan to stay without shore power.

As far as loads
Under way:
- small fridge/cold plate
- chart plotter
- VHF
- LED nav lights if at night

On anchor:
- fridge/cold plate
- occasionally VHF
- anchor light at night (not LED as I didn't get to climb the mast yet).


So the question is
- given that I don't have a choice to do it differently, is this too crazy of an idea? Is this setup likely to last at least a a year or two?
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Old 23-07-2021, 22:58   #2
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

George,
need to know what capacity yr 6v batteries are.
We run a pair of 6v 225amp deep cycles for both house & starting on our Yanmar ysm8 but found our stock 35 amp Hitachi alternator not much chop for charging as it has a thermal cutout. It cuts out a lot when you need it most. I doubt our batteries are any where close to nominal capacity as they were old freebies but we survive with a similar loading as yours but we have about 160 watts of solar & a 55amp Bosch alternator.

Cant you put a couple of solar panels on?
My guess is that you would survive with 4-6 hrs per day motoring for 6 days but I would be a lot happier with some solar.
I would just go on the trip & monitor the battery voltage.
Our deep cycles still easily start the engine at 12v & we probably have the same starter.
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Old 23-07-2021, 23:10   #3
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

George, what have you got in the way of battery monitoring. This will at least tell you what is going on. Cheap and cheerful with a nice big screen is the NASA Battery monitor, or Victron but more expensive.

https://www.nasamarine.com/product-c...tery-monitors/

Cheap copy that will work:

https://www.amazon.co.uk/Battery-Pro...%2C156&sr=8-13
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Old 23-07-2021, 23:36   #4
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Compass790
6v capacity is 210ah, which wouldn't change since they will be run as a single 12v bank. I looked for a way to add solar, but honestly it would be tough. There is no good spot for it and that's outside of what I can do prior to the trip happening next weekend.

Pete7
Nothing really in terms of monitoring. I have an original analogue voltage gauge with a needle and a simple voltmeter built into a 12v USB plug (really just a USB charging port more than anything).
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Old 24-07-2021, 00:19   #5
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Ok, that volt meter isn't really going to help you. When there is a load on the batteries it will read lower and won't tell the truth until the batteries have been left standing for some hours, hence the need for a battery monitor. They are not perfect but give a good indication of state of charge (SOC) and what is going on in terms of current going in and out.

For solar how about a flexible panel for use when you are stationary?

We have one of these in addition to the panel on an arch, so use it in harbour or at anchor. It won't survive rough weather and the controller isn't the best so we changed it, but it gives us power, is light and easy to store away when not in use. By moving the panel a couple of times a day around the yacht to face the sun it really increases the power generated.

Pete

https://www.amazon.co.uk/Foldable-Po...110951&sr=8-59
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Old 24-07-2021, 00:19   #6
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

George, hi, given that you acknowledge that there are limitations with your existing arangement I would suggest that the purchase of one of those small emergency start jumper battery packs would provide you with a security blanket and go and enjoy the journey.
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Old 24-07-2021, 00:53   #7
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

George_SD, you are overthinking this.

Fit the largest capacity house battery you can physically cram in.

Fit a small starting battery of whatever size is available. The 2GM20 requires very current (say 150A max) to start and should start with within 2 seconds of cranking. This equates to less than 0.08 AH. The alternator will fully recharge the start battery within minutes.

Use the start battery for starting only. It will last years!

You can reduce the electrical requirement for starting massively if you use your decompression levers for starting. Crikey, it will start this way on a handful of flashlight dry cells - OK, not really but you get the idea.

Use the house battery for house loads and recharge with the engine alternator as required.
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Old 24-07-2021, 08:46   #8
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Yes, I am overthinking it probably. Normally, I would add a Victron battery monitor, try to find a space for solar panel and MPPT controller, add DCDC charger.

But as mention we are leaving in a week and are likely to sell the boat in a couple of months anyway.
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Old 24-07-2021, 09:21   #9
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

I agree with UncaBob. Given the price of these gizmos, every young girl should have one!

I also agree with Wotname. The amount of juice used to start a 2-banger 15 horse, Yanmar is so little that your batteries will hardly feel it, and your dinky little 35Amp alternator can put that amount back into the batteries in just a few minutes.

Your problem, if you have one, is profligate use of juice for "housekeeping". However, if the list of uses you gave us is accurate and complete, then you are not likely to have a problem at all. Nevertheless there are obvious juice consumers that can be eliminated. Frinstance, why waste juice on a chart plotter? Is there any reason that a paper chart cannot serve you just as well? Or better?

You haven't mentioned cabin lights. Are yours LED? An incandescent bulb draws 1.5 A. A led bulb of equal luminosity draw 1/10 A, i.e. only 1/15 as much. Cabin lights of the antique kind can kill the batteries very quickly.

I assume your stove uses propane. If so, you kill the power to the solenoid valve when you are not actively using the stove, don't you?

Power management in a small sailboat is all about CONSERVATION :-)

Being in SoCal you have a temperature problem, of course. You can minimize the problem by NOT OPENING you icebox unless you absolutely have to. Provision for the six days with foodstuffs that DON'T require cooling.

But much more importantly: Sit down and make a spreadsheet of ALL the electrical devices that USE juice in your boat (other than the engine' starter motor, which is separate issue), opposite the items put their rated draws which you can get from spec sheet and other sources, then the number of hours in any 24 hour period you think you will be using them. Multiply out the lines and add up all the products. What you now have is a "power budget" that tells you how many ampere hours you will use in 24 hours.

Now you do the same thing again for all the PRODUCERS of juice you have aboard. If the total ampere hours so calculated is less than the total requirement indicated by a properly minimized power budget you have a problem. Else you don't.

If you do have a problem, then add more producer capacity. In SoCal solar panels may be the way to go. For those of us who sail in the Salish Sea in winter in the pouring rain, the ONLY proper solution to such a problem is a bigger alternator :-)!

As for knowing the state of your batteries: NAPA Autoparts (or kindred) will sell you for a mere ten bux something called a hydrometer specifically meant to test the charge in batteries. It's far from hytech, but it will do the job. I keep one in a fixture I made in my battery box.

Going back to these basics will teach you a lot that will be a sound foundation upon which to build more esoteric knowledge.

All the best :-)

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Old 24-07-2021, 11:35   #10
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

So I just talked to the folks at the battery store and they said:

1) if I have 2x6v engine and 2x12v house - my alternator is unlikely to recharge the 2x6v well since they take much longer than 12v and once 12v are mostly charged, alternator will reduce output to the point where it's not going to benefit the 6v.

2) I should just add a third (same type as existing) 12v battery instead of 2x6v. Yes, it will be less capacity in total, but all of them will charge equally and I'll be draining them equally. This does make sense.

So maybe I'm overcomplicating it with the 6 volt batteries and should go for less capacity, but equal size/type with 3x 12v?

I thought batteries were straight forward - add more, have more, use more. But looks like it is actually way more nuanced than that.
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Old 24-07-2021, 13:32   #11
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Quote:
Originally Posted by George_SD View Post

But as mention we are leaving in a week and are likely to sell the boat in a couple of months anyway.
Considering this I would just bring a small generator, extension and portable charger as a back up.

Hastily made improvements will not increase your value and you can keep the new stuff after.

Having a gas generator cluttering up the cockpit is a small price for worry free back up power.

A carbon monoxide sensor would also be highly suggested

Have a fun trip.
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Old 24-07-2021, 13:37   #12
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Quote:
Originally Posted by George_SD View Post
So maybe I'm overcomplicating it with the 6 volt batteries and should go for less capacity, but equal size/type with 3x 12v?
If you intend to sell the boat in a few months, then absolutely. The new owner sees 3 new house batteries and ticks that off the list of thing that might need replacing.

Pete
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Old 24-07-2021, 16:08   #13
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Do you have a battery switch that has both on it? I mean it connects to both start & house.

If you leave it switched to both the alternator will keep working.
The 2GM20 has hand start capability too IIRC.
or +2 for Uncle Bobs suggestion.
If you are going to sell the boat do nothing except Unc Bobs suggestion if you cant hand start.
I read your OP wrong as I thought the deep cycles would be starting & house.
If you have a small house battery you could get into trouble running your fridge/ anchor light overnite. If the voltage drops too much the current will get really high & maybe burn something out.

Whats its capacity? ( I mean the 12v house battery)

I still know you can monitor battery condition using your voltmeter. Its accurate to say it needs to sit for a few hours for a true reading but if it is being drained the voltage will go up after a while if you stop the drain & let it sit so you are erring on the side of caution.
We have monitored battery condition for years that way & havent been caught out with no start capability but we just use the 2 6v deep cycles in series for both start & house.
We have hand start in theory but not in practice as for some reason the sprag bearing wont engage. Test your hand starting capability before you rely on it.
You could put temporary jumper wires to combine the house/start banks if you have

double terminals on the batteries.
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Old 24-07-2021, 16:14   #14
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

Quote:
Originally Posted by George_SD View Post
So I just talked to the folks at the battery store and they said:

1) if I have 2x6v engine and 2x12v house - my alternator is unlikely to recharge the 2x6v well since they take much longer than 12v and once 12v are mostly charged, alternator will reduce output to the point where it's not going to benefit the 6v.

2) I should just add a third (same type as existing) 12v battery instead of 2x6v. Yes, it will be less capacity in total, but all of them will charge equally and I'll be draining them equally. This does make sense.

So maybe I'm overcomplicating it with the 6 volt batteries and should go for less capacity, but equal size/type with 3x 12v?

I thought batteries were straight forward - add more, have more, use more. But looks like it is actually way more nuanced than that.
You friendly battery shop guys are talking horse dung (unless there is some other detail missing from your report).

The time taken to recharge any battery is dependant on the degree of capacity (amp hours) which needs to "put back into " the battery. Engine starting takes hardly any amp hours out of the engine battery. Yes it does take a large number of amps which the battery must be able to deliver but very little amp hours.

I agree that there is no need for 2x6V deep cycle batteries for the engine starting and in fact, a regular 12V car (auto) battery is a better solution for starting a 2GM20.

In the perfect world, the house battery should be deep cycle(2x6V or 1x12V) and adding more batteries to achieve the necessary capacity as detailed by others upthread. However for your intended house use, 2x12V of any sort will be sufficient.

I am assuming your start and house banks are electrically separated by either a manual switch(s) or by an ACR or similar.
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Old 24-07-2021, 16:58   #15
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Re: 2x6v Deep cycle start bank and 2x12v regular house bank

the two banks are separated by 3 separate switches - House (+), Engine (+) and Disconnect (-). So i can run either or both.


The batteries are 12v Group 29 115ah (i have doubts about this rating and think it's probably around 70-80). These are not real, deep cycle robust batteries with super thick plates, but kind of "dual use" really. Your typical EverStart, Duralast, Diehard, etc found at auto parts stores or walmart. They typically range between 65-115ah claimed, depending on the brand.

Maybe I should return the two 6v and just get another Group 29 to make this real simple. Sure, it will be less capacity, but at least it will be equal type/capacity/amp/volt and makes it simple for alternator and shore charger.
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