Dead Cell ?

[ayms]

Hopefully there will be a quick answer to this.

During the round the island race our BEP battery monitor started alarming with low voltage 9.5v on the domestic bank.

Having come off shore power the night before followed by anchoring with 6ish hours sailing the next day this was a bit of a surprise.

Got my hands on a draper hydrometer and checked each of the 12 cells over two batteries. All of the cells read 125 g/ml with one reading 120. Is this a dead cell ? If it is, is the solution to replace this battery or the entire bank ? Would this have caused the low volts over short period of time ?

Each battery has 135ah. Not sure of age.

According to the monitor there was 60% capacity left when we had low volts, is this expected ? We have never got this low before. I know you are not supposed to deep cycle batteries ie below 50% but I thought the volts would still hold if the capacity got this low, is that correct ?

Perhaps the battery bank was discharging faster than normal due to the dead cell making the monitor get out of sync and therefore incorrectly reporting the capacity.

Thoughts would be grateful.

<hr width=100% size=1>Doing the Arc ? Me to on www.makathea.co.uk

 

[philwebb]

Hi, I think that you should take out the suspect battery, give it an overnight charge
with a car charger then take it to a car accesory shop and get it discharge tested.
This will show whether it is really duff or not. I`m not sure about your hydrometer
readings, I would have thought they should be 1.25g/ml not 125. On my hydrometer
it has three zones green yellow and red . A dud cell shows a big difference from
the rest ie they will all be in the green except for one which will show red. A bit of
variation from cell to cell is not unusual.

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[ayms]

If we are talking in colours.. all the cells are green apart from this bad one which is orange !

I was not sure whether a garage load testing a battery is a good enough indicator. This is based on what I have read on here before. Of course I could be wrong.

If there was a dud cell would it cause the battery to discharge ? If so then the bank would discharge.

<hr width=100% size=1>Doing the Arc ? Me to on www.makathea.co.uk

 

[PIGLETSDREAM]

Took one of my batteries to Halfords, they tested it for free.

<hr width=100% size=1>Must go, Matron is coming

 

[Talbot]

According to adverc, the cells showing 1.25 SG are at abt 85% charged which should equate to abt 12.6v. The bad cell is only 55% charged, so I suspect you have had some over sulphation from this cell, which has fooled your system into thinking that it is fully charged when it isnt, and then any load knocks it right back. You are in luck , there is a simple and relatively cheap solution to this problem


replace that battery /forums/images/icons/smile.gif

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[ayms]

Hey talbot. What is adverc ?

Should I replace both batteries in the bank? Got it in my head that replacing one is not a good idea. ie having a new one is great but if the second one is not as good will drain off the good one which makes the whole process almost pointless.

<hr width=100% size=1>Doing the Arc ? Me to on www.makathea.co.uk

 

[snowleopard]

if you have your battery tested at a garage they will put a low resistance across it to draw a big current. a dud battery will show a sharp voltage drop after a few seconds. while testing, remove the cell caps, a dead cell will bubble during the test.

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[Vara]

<A target="_blank" HREF=http://www.adverc.co.uk/index.asp>ADVERC</A>

<hr width=100% size=1>If it can't be fixed with a lump hammer dont fit it!

 

[Talbot]

at the moment you can get 110 amp hr deep cycle batteries from your local caravan shop at abt £50 so I would definitely replace both on the basis that if 1 has failed, the other may be just about too

Adverc is one of the top marine electrics places, along with Stirling for the supply of battery chargers and alternator regulators (amongst other things). Their web site has a lot of good data in it, and is worth a study

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[Trevor_swfyc]

You say you are not sure of the age of the batteries, either you have a poor memory or the batteries are old!
You can mess about with batteries for ages and then buy new or save a lot of time and effort and skip straight to buy new batteries.
Make sure you get deep cycle leisure batteries and then date the batteries, that way you will be able to see exactly when you purchased them. My new battery is now 5 years old doesn't time fly.
You are right it is better to have two new batteries else the bad one will drain the good and the bank will only be as good as the worst battery in it.

All the best.
Trevor

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[William_H]

It sounds like a dud battery and the short answer is replace it or both. However I respect anyone who wants to DIY or do it on the cheap. An interesting sport in itself. To confirm the battery is dud take it out and fully charge it using a cheap battery charger ( ie put atm least 135 Amphours into it) then connect it in place of the engine start battery. if it is dud it won't like starting the diesel. You could also connect a bank of light globes high wattage like old car headlights, calculate the wattage and hence amps or measure the amps and while monitoring the voltage see how many hours it lasts. If you get about 50% of rated amp hours before voltage drops below 10 volts it is good. If the voltage drops dramatically fairly quickly it is a dud cell. If the batteries can be isolated so that you discharge one at a time then you don't have to worry about matching the batteries new and old and even in paralell I am not sure there is that much concern about a new and old. I would risk it. Although as has been said one dud probably limited life expectancy for the rest.
How was the Round the Island race? I was discussing it with some friends (English) as we were departing on a winter race here at East Fremantle (about 2.5 hours) (despite horrible forecast it turned out OK). There must be some good tales out there of adventures on the RTI race. Regards Will

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