Is it the Batteries or the Charger

[DavidBolger]

Hi,

I need some help with my batteries and charger.

I have three batteries on board. 2 for the domestic and one for the engine. They were with the boat when I bought it and were replaced in 1998. They seem to be heavy duty commercial batteries. There is no amp hour rating on them but it says 700 amps cranking power and reserve of 180 minutes. I also have a 15amp merlin shore charger which is on all the time I am in the marina. The owner before me was also in a marina and had the charger connected at all times. I have maintained the batteries, topping up with water and cleaning the posts. Over the last six weeks I have noticed that the shore charger is "cooking" the batteries. They get very hot and you can hear the electrolite bubbling. My initial conclusion was that the batteries were shot and that this was a classic case of seeming like persistant overcharging. I did a specific gravity test and the results were just in the red zone. When charged the batteries read 13.7v and when I turn the charger off (I brought them home and used a car 6amp charger for 24 hours), the voltage drops to 12.5v. However, when I did a load test I cranked the engine for 15 secs and the voltage drops to 11.3v and stays steady (which I believe is a sign that the batteries are ok).

The merlin shore charger is one of these three phase chargers, red light - bulk charging, yellow light - equalisation, green light - float mode. I have noticed that when the batteries are hot and electrolite bubbling the red light and the green light are both on but both are less bright than when they are on separately.

So whats the answer, are my batteries shot? If so how do we explain the healthy load test? Or is the charger acting up? If so how do I fix it?

Any help is gratefully accepted but please be technically gentle as the above is pretty much the limits of my electrics knowledge.

regards

David

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[Robin]

The hot batteries plus bubbling suggests overcharging but this shouldn't really happen at 13.7v which would be the float voltage of your charger, so something seems wrong. Did you test every cell with the hydrometer? I would suspect that maybe you have one cell gone in one of the 2 batteries. If I am right (and 5yrs is not a bad life for a simple HD battery depending on how far/often it has been charged) then you will have to replace both batteries. A fully charged and rested (not on charge) 12v battery will show 12.6v maybe even as much as 12.7 or 12.8v.

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[DavidBolger]

Thanks Robin,

I tested all cells in each of the three batteries and they all show similar readings so I don't think it is one cell giving the problem. The SG result I get for each cell is just in the red range on the hydrometer.

David

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[andyball]

Have you checked the charging Voltage when they're actually bubbling....? it might be overcharging then. Have the red & green lamps always come on together?....cd be worth checking with the makers.

You mentioned the voltage dropping to 12.5V after the 6Amp charger was switched off....how long afterwards?. If only a few minutes then that's unusually quick from my exp.,although your load test suggests they're quite useable.

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[DavidBolger]

Thanks Andy,

The charging voltage when they are bubbling (well the voltage off the batteries) is 13.4-13.5. Before, when everything was working fine, only one light at a time would ever be lit at one time on the charger. The voltage drops to about 12.5 after about 5 minutes.

regards

David

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[ccscott49]

Pull them, get them checked with a high discharge tester, I think the batteries are duff. But I would also check out the charger, on a known good battery. IMHO

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[DavidBolger]

Thanks Colin,

Is the load test which I performed not a "high discharge test"? If not, what is a high discharge test?

regards

David

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[andyball]

hmm 13.5 shdn't cause excessive bubbling in an OK battery. But the lights coming on when they didn't used to may indicate a charger fault- check with merlin.(is it power-store sell these?)

Did they still bubble/heat up when left on your car charger having reached a similar voltage?....if not , the marine charger seems to be playing up.

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[DavidBolger]

Thanks Andy,

No the batteries didn't heat up from the car charger. However the car charger is a 6 amp charger whereas the shore charger is a 15 amp charger. I wonder if this is the difference. Also, the car charger showed that after 12 hours charging it was charging at 1 amp having started at 6 amps. Am I correct that to read the charging voltage, I should read the voltage reading from the batteries?

Is the conclusion that I may have a duff charger which has messed up my batteries? Any ideas on how to find a fault with the charger and then to repair it?

regards

David

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[Robin]

The voltage dropping to 12.5v after 5mins is suspicious unless there is some load on them. Under no load if I switch off our charger the batteries will show 13v +/- only slowly dropping to around 12.7 after say an hour or so.

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[andyball]

Quite right about the quick drop to 12.5V... 1hr+ is more like it & even then 12.5 seems a bit low....but if they still start the motor?

David: It's possible that your merlin charger is overdoing it....(almost?) any AC driven mains charger doesn't produce a dead flat DC output but more a series of pulses, & if something's wonky inside they can still show xxxVolts on a meter across the battery, but still be shoving too much in (very technical explanation!).

You're right about measuring the voltage at the batt. terminals & the drop in Amps on your car charger sounds quite normal.

Never looked inside a merlin charger, so can't advise on fixing/testing it, but to be "technically gentle"; ring up the makers....the change in the lamps might be well known to them.


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[duncan]

Re: so where have 14.20 volts come from

in the spirit of this thread I have just run around the odd batteries lying around the place (everyone has a few surely?)
2 came out of the boat when it sank (uw for around 2 hours) and were put on charge straight away and then left. 2 years later they were rediswcovered under a caravan bed, charged with a Halfords diesel engine charger (bigger than a can one) and recorded 12.74 and 12.75 volts a day after being taken off charge.
Today the smaller one is at 11.45v and the larger at 14.20v!!!!!!!!

wtf?

Its a 12v 175Ah 600amp - any other tests I can do with a normal multi meter?

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[saturn]

bin the batteries, 5 yrs is good service for lead acids on a constant charge in my experience.
my local caravan shop has 110 ah for £45
try the charger on your car battery.

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[andyball]

Dunno....

put a load on it & see if it's any use, the 11.45 Volt one is a duffer (asuming months & months haven't passed since the charge), but I've not come across one reading as high as 14.2V.

With a normal meter all you can do is observe the voltage drop when on load & compare it to a similar known battery with the same load/duration....maybe connect it to your car with jump leads (isolate the car batt.) & switch the lights on for a bit.



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[duncan]

Re: Dunno....

I will try that tommorow Andy.

Both have been sitting in the shed for around 4 months - tried again today and still 14.19 v - scary!

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[muchy_]

Have you tried taking the charger off both batteries and putting it on one at a time?
That way if one of the batteries is knackered the charger will playup just on that battery and work fine on the other one, but if the charger is knackered it will playup on both batteries.

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