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Old 03-12-2012, 08:13 PM   #1
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group 31's in parallel?

I've got to replace my house bank which is currently 4 6V interstates.* I've been very unhappy with them, they won't take a charge above 11.9v no matter what I do, even when they were brand new.

I had 2 8D prior to them and had much better luck.* Only problem is that they are a bear to install.*

In perusing the defender catalog today I compared the group 31 to an 8D.* Less than 1/2 the cost, well more than 1/2 the AH, and less than 1/2 the weight.*

What would be wrong with wiring 4 of them in parallel?* I have plenty of room.* Am I missing something obvious?

*

*
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Old 03-12-2012, 08:22 PM   #2
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RE: group 31's in parallel?

Accurate voltmeter?
Specific gravity?
Temperature?
Battery charger output?

If all that is in order then you are missing about a volt.
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Old 03-12-2012, 10:29 PM   #3
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RE: group 31's in parallel?

An old diode-style battery isolator could contribute to the low charge voltage, will drop about .7V. If the boat is wired in the old waywith a ten-gage wire from the alternator up to an ammeter at the helm station and back to the starter terminal, that may very well drop .5V.
If you have room above the batteries to use a glass hydrometer with temperature compensation it would tell you the true charge state and may show a bad cell. (Sears sells them.)
If all the cells show an SG of about 1.250 the voltmeter may be fault.
If the true voltage at the battery bank while charging is only 11.9V the batteries will probably show an SG below 1.200 and may be badly sulphated from the low charging voltage.
If you install new batteries with a low charging voltage they will likely perform no better than the 6V batteries and may have a short life, so it's probably worth checking the batteries and using an accurate voltmeter to check voltage at the alterator output terminal and at the battery bank. The readings should be very close.
Yes, you can parallel the group 31s.
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Old 03-13-2012, 04:15 AM   #4
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group 31's in parallel?

Group "31" is a size , not a definition of what the battery is usefull for.

I am sure deep cycle G31 are made , but most I have seen are simple truck starting batts .


If you have the room I would consider 4 - 6v or more deep cycle batts* for* the house bank.

Compare the "20 hour rate" if its for the house , and remember its the lead that does the work , and heavier is more work .

If you return to 8D , be sure to get DEEP CYCLE , not starts or the BS "everything for everyone " in between version.

Before purchasing any batts , I would suggect you install a SOC meter, to learn to live with the new set.

Best under $200 you will ever spend ,

Second choice 6v Gollf Cart Batts from Sams Club , no testing , they will swop them for new quite often.


-- Edited by FF on Tuesday 13th of March 2012 04:21:04 AM
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Old 03-13-2012, 05:17 AM   #5
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group 31's in parallel?

I've done quite a bit of work on my 6Vs, I really wanted them to be the right solution, I'd heard good things from so many people I trusted.

My set up is pretty simple, the Alternator on my port side, replaced last year, (Ac Delco, internally regulated, 100 amp), is wired directly from the alternator to the amp meter to the house bank. I put in new cables as well, I can't remember the size, but I upped it about 3 sizes from the original, #2 maybe?

The AC charging is handled by a xantrex true charge 40, again with upgraded wiring.

I had zero problems with the deep cell 8Ds that I had before, they held a longer charge than the 6V, I got more years out of them (4 compared to 2), and the 6V cause a weird dimming of the lights after about 20 minutes (I hear an audible click under the dash, then the lights dim). As soon as I switched from 12V to 6V I started having these issues.

I used a hydrometer and a load tester to check each battery after they were about a year old. One tested just a little bit low, so I thought I may have a bad cell, and replaced it. No change.

It is possible that that batt was bad from new I suppose, but I'm hesitant now to spend $600 replacing the bank and have the same issues when I could go back to what was working before.

I understand the difference between deep cells and combo/cranking batts. I was looking at deep cell group 31, they are available. I should have been more clear on that, sorry.

The boat is in the Bahamas, and I will have to do this before our trip back, so I won't have a lot of shopping options.* interstate is available there, so that is likely what I will end up with.


-- Edited by Dougcole on Tuesday 13th of March 2012 05:39:12 AM
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Old 03-13-2012, 05:27 AM   #6
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RE: group 31's in parallel?

(Ac Delco, internally regulated, 100a)

First change should be to either have the regulator removed or a better alternator with external regulation and then install a 3 or 4 stage V regulator.

The internal auto brained charge profile will seldom charge House Batts., unless you operate many, many hours at a time.
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Old 03-13-2012, 08:13 AM   #7
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RE: group 31's in parallel?

Quote:
Dougcole wrote:
I've got to replace my house bank which is currently 4 6V interstates.* I've been very unhappy with them, they won't take a charge above 11.9v no matter what I do, even when they were brand new.

I had 2 8D prior to them and had much better luck.* Only problem is that they are a bear to install.*

In perusing the defender catalog today I compared the group 31 to an 8D.* Less than 1/2 the cost, well more than 1/2 the AH, and less than 1/2 the weight.*

What would be wrong with wiring 4 of them in parallel?* I have plenty of room.* Am I missing something obvious?

You are missing nothing except you may be using the wrong vendor for your batteries.* Shipping batteries is pretty expensive.

Multiple 12 volt batteries*can be wired in parallel to increase the available current.**It's best if they are the same*brand chemistry, size and age.

I am using four group 31, 12 volt*Sears Diehard Marine AGM batteries wired in parallel for my house bank.** They were not cheap, but they are maintenance free and have a three year non-prorated warranty.* Pretty much every town has a Sears store.
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Old 03-13-2012, 08:56 AM   #8
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RE: group 31's in parallel?

Quote:
rwidman wrote:You are missing nothing except you may be using the wrong vendor for your batteries.*
There is a great deal missing. If that 11.9V was measured with the battery on charge then there is a problem with the charger. There is no charging happening at that voltage.

If the voltage immediately drops from 13 or 14 volts on charge to 11.9 when off charge then the battery has failed.

If the battery was left at low voltage or left "on charge" with such a low voltage for an extended period then they are probably not worth saving.

With the information provided, it sounds like the voltmeter is wrong.

*Regardless of deep cycle, AGM, start battery, or other form, the chemistry in a lead-acid cell produces 2 volts. It takes more than 2 volts to charge one and if yours never exceeded something less than 2 volts per cell then your charging system or your voltmeter have failed and in that case it is difficult to believe that unless you bought the batteries yesterday (the "since new" implies that you have used them for some time) means they either went dead in a few minutes of use off charge since you installed them and that never bothered you until yesterday*or we are missing a few other bits of information.

What is the SG when you see the 11.9 volts and under what condition is that voltage obtained?

How quickly and to what level does the voltage drop under say a lighting load with the battery off charge?

How and where are you measuring voltage?
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Old 03-13-2012, 11:44 AM   #9
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RE: group 31's in parallel?

+1

plus

Can you show us how you wired the 6V batts in parallel/series?*

Have you checked the battery voltage with a DVM to verify the voltmeter readings?
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Old 03-13-2012, 12:40 PM   #10
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RE: group 31's in parallel?

Quote:
FlyWright wrote:
+1

plus

Can you show us how you wired the 6V batts in parallel/series?*

Have you checked the battery voltage with a DVM to verify the voltmeter readings?

Two*6 volt batteries, wired in series (positive of one to the negative of the other) are for all practical purposes, a 12 volt battery.* The only difference is, it's in two seperate cases.
*
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Old 03-13-2012, 12:49 PM   #11
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RE: group 31's in parallel?

As others have said, you have something else going on here besides your batteries. They may be bad at this point but something made them that way. Even a bad battery will take a charge of over 11.9 volts and a bad battery will take a charge until it cooks the entire battery if it is bad in most cases. You need to figure this out before any other batteries get ruined. Chuck
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Old 03-13-2012, 01:41 PM   #12
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group 31's in parallel?

Quote:
rwidman wrote:FlyWright wrote:
+1

plus

Can you show us how you wired the 6V batts in parallel/series?*

Have you checked the battery voltage with a DVM to verify the voltmeter readings?

Two*6 volt batteries, wired in series (positive of one to the negative of the other) are for all practical purposes, a 12 volt battery.* The only difference is, it's in two seperate cases.
*

There are accepted practices and then there are best practices.* Many boats are not wired optimally.* If not wired optimally, the battery performance deteriorates.

This battery bank designer tool will show how to connect them for optimum battery performance.

*Here's the best discussion I've seen about battery bank connections.* Assume each battery shown is really a pair of 6Vs in series for this discussion.


-- Edited by FlyWright on Tuesday 13th of March 2012 03:08:49 PM
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Old 03-13-2012, 08:25 PM   #13
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RE: group 31's in parallel?

Cool!
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Old 03-13-2012, 08:36 PM   #14
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RE: group 31's in parallel?

OK Guys, thanks for all the help with this mystery, I will do my best to answer all your questions.* And sorry for the lack of earlier clarity.* Some points:

1. I know my current house bank is cooked.* They will take a charge up to about 12.5, and hold it for maybe 20 minutes under a very light load.* After that they drop way down.

2. I use my fluke multi meter directly on the battery posts to get readings.*

3. I have an analog volt meter on my dash which serves both my house bank and start bank through a momentary toggle switch which directs the current from one bank or the other to the meter.* I find this gauge to be highly suspect as it reads correctly for my start battery, but for my house bank it reads about a volt lower than what my fluke reads at the terminals on the back of the gauge.

Even more perplexing, I can switch the lead wires from the start bank batt post to the house bank post.* The house bank still reads a volt low even though it is now on the other side of the switch and using the other wires that work fine with my start batt.* If I combine the banks both read correctly on the meter.

I know this sounds impossible, I've been trying to figure it out for two years with no success, it is the only problem on my boat that has ever toatlly stumped me.* I replaced the gauge and the switch but it still acts the same.

4. When charging with either the alternator or the xantrex the voltage goes up to 14V or higher.

5. The start battery (4d) takes and holds a charge perfectly.* It charges from the stb engine alternator and the xantrex.* When I combine the banks my lights brighten right up.

6. I thought about getting an externally regulated alternator, but didn't at the advice of my mechanic, who may very well be wrong.* The Ammeter shows that the alternator pushes more than a trickle rate, it LOOKS like it is working correctly.

I replaced my house bank about a year before I replaced my alternator.* I had basically the same alternator before, just a little smaller.* I rewired the new one when i replaced it.

7. I'm pretty darn sure the 4 batts in my house bank are wired correctly.* I had my mechanic and a marine electrician friend help me.* I worked super hard on the jumpers, they are big gauge (0 I think) exactly measured, beautifully crimped, and heat shrunk.* I've reworked them twice just to make sure.

8. Everything was working fine until I changed from 12V to 6V, then all these problems cropped up.* I know it seems like everything should work, but I am at a loss, and going back to what was working seems like a logical step at this point.

Thanks again, any help is appreciated.
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Old 03-13-2012, 08:41 PM   #15
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RE: group 31's in parallel?

Quote:
FlyWright wrote:rwidman wrote:FlyWright wrote:
+1

plus

Can you show us how you wired the 6V batts in parallel/series?*

Have you checked the battery voltage with a DVM to verify the voltmeter readings?

Two*6 volt batteries, wired in series (positive of one to the negative of the other) are for all practical purposes, a 12 volt battery.* The only difference is, it's in two seperate cases.
*

There are accepted practices and then there are best practices.* Many boats are not wired optimally.* If not wired optimally, the battery performance deteriorates.

This battery bank designer tool will show how to connect them for optimum battery performance.

*Here's the best discussion I've seen about battery bank connections.* Assume each battery shown is really a pair of 6Vs in series for this discussion.



-- Edited by FlyWright on Tuesday 13th of March 2012 03:08:49 PM

*OK, very cool.* I think I had method 1 with my 4 6V.* I'm going to try method 2, or maybe 3.* Thanks.
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Old 04-20-2012, 01:41 PM   #16
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Just a follow up. I went over last week and replaced my batteries with two deep cell 8D ($900 in Green Turtle Cay, ouch).

It took my charger a long time to bring them up, but they eventually took and held a 13.5 V charge, on my fluke multi meter at the terminals. I'm no longer getting the lighting issues I had before.

The analog voltmeter on my panel still reads about a volt low. I'm thinking resistance in the wiring maybe? Maybe in the switch? I'm going to rewire it to see what happens.

This is not to say that the 6V were the problem, I think now that I somehow had a weak cell right from the start, and since all four were wired together it translated to the entire bank.

Nonetheless, it is working now. Thanks for your help, everyone.

Doug
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Old 04-20-2012, 03:51 PM   #17
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I've heard that with 6 volt batteries the first in the series sometimes gets cooked.

The solution was to ratate the batteries every so often.

Can anyone elaborate on this.

SD
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Old 04-20-2012, 04:27 PM   #18
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Quote:
Originally Posted by skipperdude View Post
I've heard that with 6 volt batteries the first in the series sometimes gets cooked.

The solution was to ratate the batteries every so often.

Can anyone elaborate on this.

SD
You heard wrong. The batteries we are talking about are made up of individual 2 volt (nominal) "cells" wired in series. In a 12 volt battery, there are six 2 volt cells wired in series. In a 6 volt battery, there are three. Wire two 6 volt batteries in series, and you have a 12 volt battery. Assuming the cable connecting the positive of the first to the negative of the second is large enough, there is absolutely no difference.
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Old 04-20-2012, 04:32 PM   #19
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Quote:
Originally Posted by Dougcole View Post
Just a follow up. I went over last week and replaced my batteries with two deep cell 8D ($900 in Green Turtle Cay, ouch).

It took my charger a long time to bring them up, but they eventually took and held a 13.5 V charge, on my fluke multi meter at the terminals. I'm no longer getting the lighting issues I had before.

The analog voltmeter on my panel still reads about a volt low. I'm thinking resistance in the wiring maybe? Maybe in the switch? I'm going to rewire it to see what happens.

This is not to say that the 6V were the problem, I think now that I somehow had a weak cell right from the start, and since all four were wired together it translated to the entire bank.

Nonetheless, it is working now. Thanks for your help, everyone.

Doug
Measuring the open circuit voltage of a battery (or bank) tells little. Measuring the voltage under load is the key.

If you have a volt meter that's reading low, it's not because of the resistance of the wire unless there's a pretty high resistance. A volt meter puts very little load on a circuit.

Want to know if your voltmeter on the panel is reading correctly? Put the leads of your trusty DVM on the terminals on the back of the panel voltmeter and observe the reading. Both should be the same, they are measuring at the same point in the circuit. If not, one is inaccurate, probably the analog panel meter.
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Old 04-21-2012, 01:40 PM   #20
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Great idea testing the leads on your panel mount voltmeter with a DVM. At least it'll tell you if it's the gage or the wiring that's the culprit.

Undersized wiring or resistance through old connectors can rob you of voltage. I have a 12V plug at my helm that well below battery voltage. It looks like it's original equipment from 1977. Replacing it is on my long to-do list.
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