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Battery Chain


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I have been on my boat for a few days attending to some routine maintenance which included checking the levels in the batteries. There are four 125amp hour batteries in the bank, and it has become noticeable that the only ones that ever need water are the same two batteries. All the batteries were new at the same time and are only two years old, the two that require water are the ones nearest to where the charging lead connects. I know it should not make the slightest difference where the charging lead connects but it seems odd that these are the only ones that ever need attention, it also seems progressive - the first one takes more than the second one. All connections are tight and the draw off for the combi inverter is on the last battery in the bank. It appears that there is a chain effect here starting from the most used going on to the least used.

It is a fair assumption that in a battery bank one of the batteries will lead due to the fact that they will have varying voltages when fully charged, but this lead must interchange between the batteries, as the voltages in each battery change during discharge. I intend to check them with a high discharge tester because the plates in the two batteries in question look a little sad. As a qualified eletrician I should probably not be asking this question but DC battery banks act in peculiar ways :D

The question is - Is this a common occurrence in battery banks and if so why?

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The question you should really be asking is why one battery should die before another, if they are only two years old their demise is premature. Are they being over-charged.

 

The position they are in the series will have no significance. Generally there will be no voltage difference between one good similar battery and another.

 

In practise there will be probably one dud cell in each battery, the remaining good cells are evaporating the electrolite because they are working harder and generating more heat. Use the high current tester, hold it in place for more than a couple of seconds, you will probably see that one cell starts to 'boil'.

Edited by John Orentas
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The question you should really be asking is why one battery should die before another, if they are only two years old their demise is premature. Are they being over-charged.

 

 

John

 

If this was the case. I would have expected it to affect all the batteries.

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Colin.

 

Life is not like that, not all the Comets crashed.

 

Four batteries are not going to fail symulteaneously, having said that even with a small measure of abuse they should last more than 2 years, I suspect they were a bit dodgy when you bought them. Have you any more info, are they in a hot environment, do you have a charge controller.

 

I have said this before but a hand-held clamp type DC ammeter is the best tool you can have, you can see what every battery is doing.

Edited by John Orentas
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To avoid this I went for two 220 AH batteries, and have connected the two leads to different batteries.

 

It would be interesting to hear what the biggest load on your batteries is, ie. how powerful is your invertor and what do you use it for?

 

Phil

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Phil, I'm not sure I understand your setup - which two leads?

 

If the leads are +ve and -ve, then by connecting to 2 batteries, you are connecting them in series, not parallel. That won't work from standard chargers (assuming 12V batteries).

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The idea is that instead of having a voltage drop in both of the interconnecting cables en route to/from the second battery in the bank, that there is only one interconnecting cable from each battery to the output/input leads.

 

Phil

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Update on the battery chain. Checked all four batteries with high resistance tester, the two that have needed constant topping up are as flat as a witches t--t, and are short of water again after only a week on a trickle charge, replacements under guarantee required, which will be interesting as they are elecosol batteries and after the comments on this forum about customer care that this company offers I am holding my breath. Contacting them by telephone only gets a recording for you to leave a message, and I have had no response from an e-mail that I sent on Monday. Will update you all on the outcome.

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I think you should ask for 4 new batteries as they must have come from a faulty batch.

Worth a try. :D

 

 

I had considered that, but the faulty two have red cases, and the other two have yellow cases. I gave the good ones a fair test 25seconds each the resistor on the tester was glowing red hot and the needle on the meter did not flinch so I am happy with these at the moment.

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Why diferent cases ? are they the same etc . does the colour define made in diferent country's or are they a lower quality battery?

 

 

I don't really know, they are identical in size physically and I had always assumed they were electrically.As until now I have never had reason to remove them from the battery box, but asking this you have set me thinking, I have to admit I have sometimes wondered why there were two of one colour and two of another, but as they say if its not broke don't fix it. I took delivery of the hull as a sail away and they were already installed. I am on the boat again tomorrow and I will extract one of the yellow ones and check its output, the red ones I know are 125 amp hour.

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Just going down this route of different size batteries myself. I am assured by the electricians I have at work that it makes no difference at all if you have varying AH batteries connected together in parallel.

 

Dave R

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Dont know but was thinking along the lines that if you have a battery bank of 2 batteries giving a total Amp hr of lets say 400 you could put between 40-80 amps of charging in but if one of the batteries was 100 and the other 300AH and you charged at say 60 that would be to much for the 100 ah battery ... or does it even out.?

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It's all a question of the voltage. both batteries will come up at the same rate (volt wise) and so will take what they require. As the voltage increases so the charging rate will decrease.

 

It is a bit like having a 200 gallon tank and a 100 gallon tank side by side at the same height. They will both run down and fill up at the same level if the filling / draining lines are the same size.

 

Dave R

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The slight voltage variation you can see between two similar batteries is not significant. As with many electrical measurements a better result will be seen when the circuit is loaded.

 

If you can read a different voltage when 2 batts. are connected in parallel it means you have bad connections somewhere.

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Colin.

 

Life is not like that, not all the Comets crashed.

 

I have said this before but a hand-held clamp type DC ammeter is the best tool you can have, you can see what every battery is doing.

 

John

 

What do you mean by a hand held clamp DC ammeter it must need wiring into the circuit on DC, amprobes only work on AC.

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The slight voltage variation you can see between two similar batteries is not significant.  As with many electrical measurements a better result will be seen when the circuit is loaded.

 

If you can read a different voltage when 2 batts. are connected in parallel it means you have bad connections somewhere.

 

 

With batteries connected in parallel the battery with the higher voltage will become the lead battery until it becomes equalised with the next highest voltage and so on until all batteries are equal. Basic electricity, ohms law voltage = pressure.

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