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Old 01-13-2017, 08:29 AM   #1
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Change a couple or change all?

Surveyor said a couple of the 6V agm house batteries look distorted like they may have gotten hot. There are 8 batteries in the bank.

Should all 8 be changed or can you get away with changing the ones that don't load test?
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Old 01-13-2017, 08:52 AM   #2
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Bad batteries drag down good ones. Old batteries drag down new ones. If you can keep them in separate banks you could get away with it. Just don't mix them. They won't be happy.
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Old 01-13-2017, 08:58 AM   #3
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First, check to see if they actually are damaged. A battery load test is one way to find out. It is recommended that all batteries in a battery bank be of the same size, age and condition. Mixing old and new is going to lead to charging problems. If you split them into two banks, old/new, you still have a charging problem if both banks are being simultaneousy charged off the same charger.
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Old 01-13-2017, 09:57 AM   #4
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Given the above, why does it make sense to use 8 batteries rather than 4 Ds or 2 8 D's...aside from the weight issue when changing time comes?
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Old 01-13-2017, 10:13 AM   #5
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Given the above, why does it make sense to use 8 batteries rather than 4 Ds or 2 8 D's...aside from the weight issue when changing time comes?

If they're 6V golf cart batteries (e.g., GC2s), most are said to be better able to handle deep cycling, compared to most 12V batteries. Better for house loads.

Reading suggests most 12V batteries that are labeled "deep cycle"... aren't really. Rolls and I think Lifeline have some 12V products that seem to get decent reviews, but most 12V "deep cycle" batteries apparently aren't very "special."

Could also have been space related. Maybe the boat builder could insert more capacity in that particular shape. Sometimes happens. (For example, I have room for 3 Group 31s (~300 Ah), where only a single 8D (~245 Ah) would fit).

And then there's the weight issue; moving several smaller batteries, especially if there's lifting in confined spaces is often easier than humping 4Ds or 8Ds around.

Anyway... depending on your house loads/needs... and if you only have a couple distorted batteries ("distorted" is bad) or a couple that don't pass a decent load test... another, maybe temporary, solution is to maybe just remove some (pairs, if they're 6V) from the bank. Run that way 'til you're ready to really fix it, assuming the remaining batts will service your house loads for a while.

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Old 01-13-2017, 10:34 AM   #6
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Surveyor said a couple of the 6V agm house batteries look distorted like they may have gotten hot. There are 8 batteries in the bank.

Should all 8 be changed or can you get away with changing the ones that don't load test?
If you have some batteries that are failing, then it may be a good time to review your electrical setup. You have 8 x 6v golf cart batteries it sounds like. You didn't mention if they were in one bank or two. Depending on where the batteries are located, and if you need to replace a bunch, I would consider looking at replacing with 6v wet cell batteries. They are typically the vest value as far as $/amp hour. However, if they are not easily accessible for maintenance, then not a great option.
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Old 01-13-2017, 11:02 AM   #7
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Like a rabid fox, I'd want to know why those two have failed, and not the rest?

How has the bank been assembled? Meaning, the various connections loop from where to where? More batteries are assassinated by poor wiring choices...

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Old 01-13-2017, 11:05 AM   #8
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Surveyor said a couple of the 6V agm house batteries look distorted like they may have gotten hot. There are 8 batteries in the bank.

Should all 8 be changed or can you get away with changing the ones that don't load test?
Actually load testing the batteries would give you a better understanding of their condition than just looking at them.

Conventional wisdom says all batteries in a bank should be the same type and same age. If some check bad, I would replace them all.
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Old 01-13-2017, 11:54 AM   #9
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Change a couple or change all?

Right, I don't think an external inspection is very reliable or conclusive. Load testing individual batteries would be great, but probably means removing them which I wouldn't want to do. Assuming a 12v system, you have 4 strings of 2 batteries each. An easier way to test would be to change them all up, then do your own load test on each string. You will need to disconnect all but one string, and apply a more or less steady load of maybe 10 to 20 amps. Record the voltage drop over 2-3 hrs. Then do the same for the other strings.

The recorded voltage declines over time should be about the same. The exact numbers aren't as important as differences between strings. This will usually flush out a failing battery.
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Old 01-13-2017, 12:07 PM   #10
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I just load test each battery in place. You just have to disconnect the cables if you cant otherwise isolate.


If I found a bad one and the rest were pretty good by load check and by specific gravity..... I would pair up the batteries with the closest performance specs, and just replace the one or pair in a bank.


Another good test is like TT mentioned, isolate and try the 20 amp hour rating.

I have been adding battery banks to existing ones for years and as long as the older batteries were in decent shape, haven't noticed any great issue. Maybe shortening the whole banks life? Possibly?....try and prove it.


You can buy 20 new batteries and have one go bad almost instantly....you gonna replace the other 19?
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Old 01-13-2017, 01:44 PM   #11
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How old are the batteries?
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Old 01-13-2017, 02:05 PM   #12
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If you have one or more "old" batteries in a bank and add new ones, the new ones will discharge into the old ones id the voltage has dropped in the old ones. The voltage has to be equal in all parallel connected batteries. Higher will discharge into lower until they are all equal.


No, if you buy 20 new batteries and one goes bad almost instantly, you don't have to replace the other 19.
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Old 01-13-2017, 05:27 PM   #13
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You can buy 20 new batteries and have one go bad almost instantly....you gonna replace the other 19?

Best and most profound answer I've read in a long time about replacing batteries

Thanks.
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Old 01-13-2017, 06:30 PM   #14
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If you have one or more "old" batteries in a bank and add new ones, the new ones will discharge into the old ones id the voltage has dropped in the old ones. The voltage has to be equal in all parallel connected batteries. Higher will discharge into lower until they are all equal.
I *think* you could say the exact same thing about a brand new set of batteries with different voltages but I don't think you've learned anything here about the performance (capacity) of the bank.
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Old 01-13-2017, 06:42 PM   #15
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Surely, if you have a bank of 8, after not very long one will be not as good as the other 7, but not bad enough to need replacement. But, if you do replace it, the replacement will be newer and ? better than the other 7, = continued inequality.
Seems to me perfection is hard to attain and you have to accept some degree of inequality in a bank of 8. You would be lucky if they all age and decline at the same rate.
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Old 01-13-2017, 06:45 PM   #16
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The question really can't be answered until you know the reason and extent of their damage and the condition of the other batteries. If something happened to the two that didn't impact the others and the cause has been corrected, then I would replace two or eight depending on the condition of the remaining six. If they have more than 50% of their life remaining, I'd keep them then.

I look at it that the old will pull the new down to their level. So, you shorten the life of the new ones but not as much is lost as is lost throwing away good batteries. If the old are only six months behind the bad two when it comes to probable failure then I'm not going to pair new ones up with them.

Always replacing all batteries is just not logical. Now, don't complain because the new ones don't last as long.

Most of the time replacing all makes sense, only because their condition isn't much better than the failed ones.
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Old 01-13-2017, 06:48 PM   #17
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I have been adding battery banks to existing ones for years and as long as the older batteries were in decent shape, haven't noticed any great issue. Maybe shortening the whole banks life? Possibly?....try and prove it.

I think worst case your new battery won't last longer than the remaining bank. I just replaced a cell on a 6-7 year old battery bank, and would do it again without hesitation. Beats the heck out of replacing the whole bank.

I think where replacing everything makes sense is when you start to see multiple cells failing in short order, or multiple cells that have lost measurable capacity. In other words, an indication that they are all nearing end of life. Then you might as well bite the bullet.
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Old 01-13-2017, 06:58 PM   #18
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This discussion bring up the question as to how long batteries will last. I have used AGM batteries on my last boat and current boat and both have an automatic charger hooked up to the. I have gotten up to nine years on them, that the most. Wet cells of acid and water would give me perhaps five years.

What have you guys been getting?
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Old 01-13-2017, 07:17 PM   #19
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I *think* you could say the exact same thing about a brand new set of batteries with different voltages but I don't think you've learned anything here about the performance (capacity) of the bank.
That's why you buy them all at once and the same brand and model. They should be very close to each other in voltage, internal resistance and capacity.

Remember, a twelve volt battery is already made up of six two volt cells connected together at the factory.
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Old 01-13-2017, 07:38 PM   #20
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The safe advice is to tell you to replace everything from your alternator, batteries, battery boxes, cabling and your charger/inverter with the most expensive stuff West Marine will sell you. No discount allowed, full price only is acceptable.

Myself, I'd just pull the two questionable batteries and see if I could live without them. If not head to Costco/Sam's Club and grab two off the shelf.
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