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Old 16-01-2011, 11:30   #1
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Unequal Battery Condition

Last spring I bought my sailboat with 4x 6 volt Trojan 105s on board. They are wired as 2 in series, then the two pairs are in parallel. The effect is a 12v system with nominally 450 amp hours. This was their sixth season. By the end of this season I knew they were not feeling well; they couldn't seem to hold a charge. When I took them out of the boat I labeled them so that I would know which batteries were paired on the boat. One battery in each pair will not hold a charge and their rested voltage is 4.5 - 5.5 volts. The remaining two batteries when in series can be charged and will hold their charge for a week or more.

The previous owner had a professional set up the system and I know that we have both been conscientious about checking and maintaining the water level in the batteries.

So, why did one battery in each pair come to the end of its useful life before the demise of its mate?

I ask this question because I plan to replace all four and don't want to see the same thing happen again.

Thanks for your insights.
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Old 16-01-2011, 13:15   #2
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How old are your batteries now?

What are your hydrometer readings for each battery after putting a long charge on them and then letting them rest for a while with no load?

For whatever reasons not all batteries are the same, even off the same assembly line on the same day. If one is significantly worse I would go ahead and change it out. If you see some minor differences then I would not spend the money.
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Old 16-01-2011, 13:33   #3
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Take the two good batteries and wire them in series and buy two new batteries for the other series pair. In no circumstances put an old battery with a new one. The two old batteries probably won't last long so be prepared to replace those fairly soon.

Have you load tested the batteries, btw. Really the only way to test a battery. Most battery shops will load test the batteries for you.
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Old 16-01-2011, 14:07   #4
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I feel like a broken record!

The ability to "hold a charge" has very little to do with the health of a battery in terms of its CAPACITY, i.e., the ability to accept and deliver energy at anywhere near its rated quantity.

The only way to know if the batteries have useful capacity remaining is to load test them, preferably at the 20-hour rate. The "industry standard" way these days is to use a(n expensive) testing device like the Midtronics series testers which measure internal resistance and conductance.

If you have six year old T-105's, replace them. Treat the new ones well, i.e., keep them fully charged as much as possible, don't discharge them below about 50% capacity, occasionally equalize them, exercise them, keep the water level above the plates, add only distilled water, don't allow anything to contaminate the electrolyte, etc., etc.

RE: why one battery in a pair goes before the other, simple matter of math and aging. As they age, batteries deteriorate at differential rates for lots of reasons. In a pair, it's inevitable that one will go before the other.

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Old 16-01-2011, 15:16   #5
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Rereading your set up, your set up is 2 series batteries to parallel for maximum amperage in 12v. In that setup, best to replace all four batteries. If it was my set up, would wire two sets of series batteries to a battery switch so I could separate the banks or combine them via the switch.
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Old 16-01-2011, 15:56   #6
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One dead battery in a pair

Quote:
Originally Posted by KestrelBuck View Post
Last spring I bought my sailboat with 4x 6 volt Trojan 105s on board. They are wired as 2 in series, then the two pairs are in parallel. The effect is a 12v system with nominally 450 amp hours. This was their sixth season. By the end of this season I knew they were not feeling well; they couldn't seem to hold a charge. When I took them out of the boat I labeled them so that I would know which batteries were paired on the boat. One battery in each pair will not hold a charge and their rested voltage is 4.5 - 5.5 volts. The remaining two batteries when in series can be charged and will hold their charge for a week or more.

The previous owner had a professional set up the system and I know that we have both been conscientious about checking and maintaining the water level in the batteries.

So, why did one battery in each pair come to the end of its useful life before the demise of its mate?

I ask this question because I plan to replace all four and don't want to see the same thing happen again.

Thanks for your insights.

Could it be that the previous owner has rearranged the position of the batteries ? i.e. He had one weak bank of sulphated batteries so he switched one battery from his good bank into his stuffed bank?...cheers
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