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Discussion Starter #1
Hello,

There is a very strange problem in 24V DC system of a sailboat. There are three 24V battery bank (domestic use) which consists of two series 12 Volt batteries (There are 6 batteries). Three banks are connected in parallel.

Captain says that one of the batteries in each bank gone dead after 3 months. In each bank the battery which connected as – 24VDC was dead.

+ Batt1 - + Batt2 -

24VDC loads are supplied from (+) side of Batt1 and (-) side of Batt2. (I could not upload a figure, sorry)
In each bank Batt2 was gone dead.


He replaced dead batteries with new ones but he made a fatal mistake: He purchased smaller capacity (AH) and starter type batteries. He installed 182 Ah starter battery in series with 225Ah service battery to have 24 V seervice bank. Now I check the 182Ah starter battery and it is dead, inevitably. He admit his mistakes but, again batt2 (starter batteries) of each bank is gone.

There is a Mastervolt Mass 24/100 charger, one of previous models. Batteries are gel type batteries. I suspect if mastervolt is adjusted for wet type batt. I have checked electronic board but could not find any dipswitches where you can make adjustments.

There is also another battery which gives 12 VDC for electronics. There is a DC to DC converter to charge this battery from the 24VDC system. This battery is OK. Minus of 12 VDC is connected to minus of 24 VDC system by a circuit breaker. I don’t know if this breaker is necessary but captain said that one of his electrician told him that batteries in 24 V bank are decharging over this connection so there must be a isolator (diode). This sounds me nonsense.

I am sorry for my incapable English but hope you understand the situation.
Is there any comment on the problem?

Thank you in advance.
 

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Don Radcliffe
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My first step would be to look at the connections of your batteries in series, as it appears that there may be a drain on Batt1 that is not on Batt2 or vice versa. There should be ONLY ONE connection from the - side of each Batt1 to the + side of each Batt2, and that should be the heavy cable which combines them in series. ANY other wires will cause the problems you describe. It is not clear whether the Batt2 were discharged more and never recharged, or whether Batt1 was discharged more which resulted in overcharging the Batt2, but the second is more likely if you have gel batteries. Remember that gel batteries can sit around discharged for months without problems, but can be ruined overnight by overcharging, while the opposite is true of wet cell batteries.

Once one battery in a series goes bad, you must replace both batteries with the same type, capacity, and age. They really don't make gel starter batteries, so your captain may have put in a non-gel battery. The drain problem I described above would have killed the replacement batteries anyway.

The latest Mass 24/100 does have a dipswitch setting for gels, which just increases the float voltage.

The 12v system should be isolated by the DC-DC converter, but you can test for unexpected current drain
 

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Discussion Starter #3
Thank you for your informative reply.
I have checked the wire between - and + and it was ONE heavy wire.

You are right, starter batteries were not gel type. I checked after reading your reply. So this means that captain wired completely two different batteries in series. One is gel type, deep cycle and 250 AH, the other is wet, starter, 182 AH.

Is it possible that batteries can burn? When I was making checks and measures on batteries, they were very hot.


Remember that gel batteries can sit around discharged for months without problems, but can be ruined overnight by overcharging, while the opposite is true of wet cell batteries.
It is completely true what you have said.
 

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██▓▓▒▒░&
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Let's get this clear:
One wet battery, one gel battery, different AH capacities on both, and that's what he bolted up as as single 24V bank?

Does this guy wear a colorful fright wig, big round red nose, and floppy oversize shoes? Because, that's about as surefire a recipe for killing batteries as you could find. They won't charge evenly, they won't charge correctly, you might as well separate them now.

Rebuild as 2x wet in one bank, 2x gel in the other. Or else throw out the gel batteries, because the odds are that the charging system is set up for wet cells and gel cells usually (not always, but almost always) are destroyed when they are charged at the same voltages as wet cells.

It really is that simple. And extensively documented.

Oh, your buddy needs to call (703) 749-5538, that's the parent company for Ringling Bros. They always need clowns.
 

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Don Radcliffe
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Batteries can get very hot when they are charging at too high a current, which can happen if one cell is shorted. In any case, the high temperatures are damaging to the battery.

I spent some time looking for gel batteries in the Med--They were either not available or too expensive in Marmaris, Athens, Rome, and Mallorca. I finally found some duty free at a reasonable cost (only 30% higher than in the US) in Gibraltar.

I would say that the next step is to separate the batteries and do individual charging and 20 hour load tests on each one (as a standalone 12v). If you have 4 good batteries left, you might combine them and buy two new gels, but if you have 3 or fewer good batteries left, you are probably best off buying 6 new matching batteries of whatever good deep cycle brand you can get locally.
 

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Telstar 28
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I'd point out that if you have two batteries of different capacity connected as a single 24 vdc battery bank, even if they're the same chemistry, one is going to be overcharged when the other is charged properly... and that will generally lead to it dying prematurely.
 
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