Over performing battery charger

[Amulet]

Sterling 10 amp battery charger. Charges two 110 amp hour batteries from shore power. Does me well, and I never have the batteries below 50%. Can do several days without shore power and a week if I don't use the fridge.

After some work on the boat, the fridge was on but my mains RCB tripped - batteries flat as a scone. Now: the charger asks for a 10 amp fuse on the negative side. Does this mean it can deliver a sum of 10 amps to both batteries. Well: I had put a 15 amp fuse in, and, with the dead batteries, it was delivering 5-6 amps to one and 13 to the other. (I have battery monitors.) Blows the fuse, doesn't it! Well, I want the batteries charged, so the obvious solution is to put in a 20 amp fuse. Looks good, 5-6 amps to one and 13 to the other - now we are cooking.

I go home.

Two days later a bout of anxiety causes me to abandon my professional responsibilities and visit the boat. Fuse blown and batteries flat. Melting around the fuse holder. Solution, a bigger fuse, 25 amp, carved away the holder to aid cooling. Total charge sums to over 20 amps. Not bad from a 10 amp charger. Had to work again today, but I guess the marina would have called me if my boat was on fire or the the batteries had exploded. Fuse blown again?

Am I in trouble?

 

[dt4134]

I've got a 30A charger with three outputs of 10A each. Two are tied together for the domestic batteries and I have seen it peak at about 25A.


That said, I suspect you have a bigger problem than blowing a fuse on the charger if one of a pair is taking much more current than the other - I had that with 15 year old batteries when one of them had a duff cell. Ended up replacing both.

If it were me I'd make sure there was no load on the batteries then try to charge them one at a time. Even if it means isolating the other whilst you do it. Check water levels too.

Once they've both had a good charge then put things back together and see how they get on.

Do you know why the trip switch tripped?

 

[Barry Jones]

Lots of current but at what voltage ?

 

[Amulet]

Do you know why the trip switch tripped?

Not really, byt the RCB is incredibly sensitive and often trips when I first plug in. It then resets fine. I have assumed (limited real understanding) that there is some capacitance in the set-up, meaning that at start-up it needs to charge up, causing an input/output current inequality.

 

[Amulet]

Lots of current but at what voltage ?

The lowest of the two batteries had dropped to about 7.5 volts. The charger was able to stuff 15 amps into it at a voltage of about 12.4.

 

[boatone]

Am I in trouble?

YES !!!! For gawds sake stop putting higher and higher value fuses in!

The fuse is rated at 10 amps to protect the circuitry in the event of a fault that might damage the charger. I am amazed that you have been able to ramp up the fuse value so high.

Solution - get the charger looked at.

 

[agurney]

I recently fried the electronic innards of an expensive heavy duty charger by trying to charge a completely flat battery. Google turned up lots of similar stories with a plethora of possible explanations.

In future I'll stick to my trusty 1970s trickle charger if the battery's been completely run down.

 

[Amulet]

YES !!!! For gawds sake stop putting higher and higher value fuses in!

The fuse is rated at 10 amps to protect the circuitry in the event of a fault that might damage the charger. I am amazed that you have been able to ramp up the fuse value so high.

Solution - get the charger looked at.

It claims to be able to self-regulate the current it gives. It has routinely given 15 amps over the years. But of course you are right. I will feel a bit daft if it has fried the charger. (And then I'll buy a 25 amp one as replacement.)

 

[boatone]

It claims to be able to self-regulate the current it gives. It has routinely given 15 amps over the years.

The fuse is not concerned with the current it gives - it protects the charger in the event that it takes too much due to a short or whatever.

The 15 amps or wotever you quote it 'giving' is not really what the charger gives - its what the batteries take and will vary depending on the state of charge.

 

[halcyon]

The lowest of the two batteries had dropped to about 7.5 volts. The charger was able to stuff 15 amps into it at a voltage of about 12.4.

Buy a new battery.

Try taking that one out of circuit and just charge the other, see what you get. A flat battery on charge is nominal 13 volt, 7.5 and 12.4 V are a dead and buried battery.

Also as said do not uprate fuses, sean to many burnt out boats.

Brian

 

[Amulet]

Buy a new battery.

Try taking that one out of circuit and just charge the other, see what you get. A flat battery on charge is nominal 13 volt, 7.5 and 12.4 V are a dead and buried battery.

Also as said do not uprate fuses, sean to many burnt out boats.

Brian

I'll let you know. I actually don't believe the batteries are dead. As Boatone says, it is a constant current charger, which works out what current to shove into the batteries based on their charge state. The lowest battery is at 7.5 volts on open circuit. When I switch on the charger it comes up to about 12 volts and 15 amps, which is presumably what the charger reckons it can stuff into this battery on the bulk phase of its charge. It's 12 volts because that's what's necessary to charge at the required rate. (The capacity of the batteries is programmed by hand into the charger.) While it stays on the bulk phase, which is 'til the batteries are 75% full I think, the necessary voltage will rise to maintain constant current. It then moves on to the next phase of lower charge and finally on to the float stage which seems to be at about 13.4 volts.

I admit the recklessness of my ramping up the fuse. In truth I had left the manual at home and mistakenly thought that the ten amp charger could give ten amps for each of its three outputs and therefore that a bigger fuse in the common negative was still asking it to work to spec. I then got home and read that it asked for a single ten amp fuse. Foolish I agree. When I left it it had been charging for an hour and a half without problem.

I should be taking bets on what I'll find on Sunday when I go to the boat:

1. Boat burned to a frazzle (unlikely - the marina would probably have called)
2. Battery charger dead
3. 25 amp fuse blown
4. RCB tripped
5. Batteries exploded (probably would have done enough damage to alert the marina staff)
6. Batteries nicely charged up and everything hunky dory.

I'm betting both 3 & 4 with the batteries still flat and the charger still functional.

Will post a photo of the burned out hulk that once was my boat on Monday!

 

[halcyon]

. When I switch on the charger it comes up to about 12 volts and 15 amps, which is presumably what the charger reckons it can stuff into this battery on the bulk phase of its charge. !

The 12 volt you see is set by the battery, the only voltage regulation you see is top end voltage. At this point you limit amperage, to control the voltage.

A flat battery on charge will be 13 volt, you will never see 12 volt.


Brian

 

[Barry Jones]

If one or both of your batteries is reading 7.5V as an OCV its not flat - its dead.

Even one dead battery will drag down and kill the other in time.

You need to address the problems with your batteries and then look at the charger.

Some smart chargers will detect a dead battery within the first minute or so of trying to charge, but I don't think this is happening here.

Generally speaking, chargers will generally try to ramp up the voltage to 14.4V in the bulk phase and I suspect this is what is trying to happen here.

If the terminal voltage is abnormally low, then it will see a very low battery resistance and apply a high initial current.

 

[charles_reed]

You have at least one defunct battery and are on the way to having a defunct charger - if you don't manage to burn your boat before admitting the truth.

 

[Phoenix of Hamble]

Agree with above... Your '7.5v' battery is dead.

No charger will handle that.

 

[Amulet]

Thanks for all the wisdom. You have convinced me that I am wrong and lucky that my boat is still intact. Whilst the risks may have been foolish and high, my foolishness seems to have gone (relatively) unpunished. It looks like I might need new batteries and a new charger, which will hurt, but not as much as a new boat. If everything does turn out OK, then I can only assume that the monitor is misreading the voltage, which is possible.

The symptoms (and maybe the foolishness) are more complex than I have described. In the first instance it took battery one down to just below 10 volts Battery 2 was at 12.7. I put the load (fridge) on battery two and put both to charge. Battery 1 was brought up to a credible but not good 12.4 (on open circuit). After leaving the boat I returned to find the fuse blown and battery one reading 12.4 and battery two (now carrying all the load) reading 7.5. The fridge should have cut out at 10 on its battery protection gizmo. I realise that the bilge pump and the navtex are on ... but 7.5?... and the bilge pump was not running???

I am surprised, because, prior to this incident, the batteries were performing excellently (three years old and never taken below 50% in their life).

The truth is that, if both batteries are at rock bottom, they could each actually want 15 amps bulk charge, so my charger is underspecified if I want to cover that eventuality. I should have a 30 amp charger, not a 10 amp one.

I will let you you know what the outcome is, how lucky I've been, and how unjustified is my wishful thinking. Thanks again for the technical input.