current leakage and anode erosion

[gjgm]

I have found a small current leakage on my starter battery circuit..with the isolator off, I am still seeing a 6 MA current flow. I have not looked into where as yet (bilge pump, perhaps?) but is 6 MA significant, or not at all ? I think somewhere I read more than 1MA was an issue, but I cannot find where I might have read that. The house batteries read zero with their isolators switched off.

 

[lw395]

The leakage which eats anodes is current flowing from your circuits to the seawater.
What you seem to be seeing is a current flowing from the battery despite the isolator being 'off'. ???
How are you measuring this?
It would imply current is flowing from the battery + to battery - without going through the isolator.
Where can it go, without going through the isolator?
It could be flowing through damp or dirt. Or there is a load which bypasses the isolator.
It's only if it goes vis the sea and back into your anodes that you will have a problem.
But you should sort it anyway, or a tleast understand what you have.

 

[gjgm]

The leakage which eats anodes is current flowing from your circuits to the seawater.
What you seem to be seeing is a current flowing from the battery despite the isolator being 'off'. ???
How are you measuring this?
It would imply current is flowing from the battery + to battery - without going through the isolator.
Where can it go, without going through the isolator?
It could be flowing through damp or dirt. Or there is a load which bypasses the isolator.
It's only if it goes vis the sea and back into your anodes that you will have a problem.
But you should sort it anyway, or a tleast understand what you have.

Thanks, and agreed...but the bilge pump works with islolators off (not unusual) but I havent yet looked at the "in" side of the isolator to see if anything is connected there.
Measured by in series with multimeter...ie battery cable removed and meter put between disconected cable and battery terminal.
Still..while I would prefer a zero reading, is 6ma too much does anyone know?
Actually, I think on one of the house batteries I found a 0.3ma reading..which I guess is very small indeed !

 

[VicS]

I have found a small current leakage on my starter battery circuit..with the isolator off, I am still seeing a 6 MA current flow. I have not looked into where as yet (bilge pump, perhaps?) but is 6 MA significant, or not at all ? I think somewhere I read more than 1MA was an issue, but I cannot find where I might have read that. The house batteries read zero with their isolators switched off.

to put this in perspective some simple arithmetic will tell you that if you have a 75Ah battery if will loose 10% of its charge, in additional to the natural loss, in 52 days.

( alternatively compare with my car in which when just standing idle has a discharge current from the battery of approx 10 mA )

If you are worried about loss of zinc, if this current is flowing into the water via your anodes a little bit of chemistry and a chunk more arithmetic will tell you that in 100 days you will lose approx 17.5 g of zinc.

6mA therefore not large enough to panic over but I think I would want to know why there was a 6mA discahrge ( I know what's taking 10mA in my car, it's the clock)

 

[gjgm]

Thanks..well something is munching my anodes and starting on more..in about 10 weeks. Marinas says it isn't them coz they have tested..ok..lets go with that for now...we don't use shorepower much and don't stay connected..so I am checking the boat, which we leave unconnected and isolators off. So 6ma isnt going to account for two prop anodes and a good bite into two prop cones in this space of time...
Which leaves...? Stray current in the marina from,say, a nearby boat? That's alot of stray current then....

 

[DavidMcMullan]

Does your boat have a flexible coupling? If yes, does it have an electrical connection across the coupling, so the prop shaft and propeller are electrically connected to the boat's earth? If not, this may lead to accelerated anode depletion.

 

[VicS]

Does your boat have a flexible coupling? If yes, does it have an electrical connection across the coupling, so the prop shaft and propeller are electrically connected to the boat's earth? If not, this may lead to accelerated anode depletion.

Why ? Surely the only purpose of bridging a flexible coupling is as part of the circuit between the prop and a hull anode.

If the OP has a hull anode fitted to give some protection to the prop then agreed the coupling should be bridged but otherwise what reason is there to do so ???

 

[gjgm]

It is outdrives, and the prop anodes are the ones under attack.
I guess I am trying to trace, by what cause? If I am rarely on shorepower, that ought to exclude the usual issue via pontoon earthing, and I seem only to have a very minor current leak on my own boat. Not enough to have eaten two prop anodes and part of the prop cones in a matter of weeks.

 

[Jabs]

I lost 11kg of anode and a saildrive in six months.

I firmly believe the cause was a floating ground connection in the marina.

So after huge expenditure, I fiiitd an isolating transformer!

There are as many opinions on galvanic corrosion as there are experts!

Work your way through the issues and come to your own conclusion.

It is a long, frustrating and contradictory journey!

Tony

 

[VicS]

I lost 11kg of anode and a saildrive in six months.

I firmly believe the cause was a floating ground connection in the marina.

So after huge expenditure, I fiiitd an isolating transformer!

There are as many opinions on galvanic corrosion as there are experts!

Work your way through the issues and come to your own conclusion.

It is a long, frustrating and contradictory journey!

Tony

An isolating transformer is the ultimate way of isolating the boat from the shore power supply and the troubles that can come with it.

A galvanic isolator is usually seen as the solution but they are not foolproof and in any case only block current from very low voltage, e.g. galvanic, sources. They don't block leakage currents from 12 volt sources, which are what are most likely to cause catastrophic electrolysis.

Even an isolating transformer wont offer any protection when electrolysis is the result of an on board electrical fault.

 

[Bobc]

Is your current leakage a battery monitor perhaps?

 

[gjgm]

Is your current leakage a battery monitor perhaps?

I need to see if anything is connected pre isolator,if so remove that and check again and hope I can trace the source. While things of course can go wrong, this is a new problem and nothing new has been fitted.
Is there a way to check on stray currents in the marina...or at least..around my sterdrives!?

 

[QBhoy]

Lots to consider hear and may be unrelated. Can you get a meter and test continuity between running gear protected by the anode ? You may have a break in the connections.
Another reason for anodes disappearing in some marinas is if the pontoons are not earthed properly and shore power causing the problem nearby. Worth checking all your earths and anode to gear continually. Will be fairly obvious on the meter if there is a break.

 

[VicS]

Lots to consider hear and may be unrelated. Can you get a meter and test continuity between running gear protected by the anode ? You may have a break in the connections.
Another reason for anodes disappearing in some marinas is if the pontoons are not earthed properly and shore power causing the problem nearby. Worth checking all your earths and anode to gear continually. Will be fairly obvious on the meter if there is a break.

The OP is talking about prop anodes on outdrives. It would be wise to check for continuity between the anodes and the props if there was corrosion of the props but that is not the issue here.

The Op says he rarely uses shore power and does not stay connected.

ITYWF that, like saildrive props, out-drive props have rubber bushed hubs so will be insulated from the shafts , the drives' the engines and the rest of the boat. They should therefore not suffer any corrosion due to current from on-board electrical faults. The OP does not say what engines, drives or props he has so it's difficult to confirm this.

 

[gjgm]

I now seem to get 25 ma on the starter circuits ?!..but if I disconnect the lead to the battery charger, it drops to about zero....think I need a a marine electrician!

 

[pandos]

I now seem to get 25 ma on the starter circuits ?!..but if I disconnect the lead to the battery charger, it drops to about zero....think I need a a marine electrician!

Could it be that the charger is absorbing charge from the batteries when not connected to mains,,, my home charger when connected to the battery shows voltage etc even if not connected to mains,

 

[VicS]

I now seem to get 25 ma on the starter circuits ?!..but if I disconnect the lead to the battery charger, it drops to about zero....think I need a a marine electrician!

My home / car battery charger draws 80mA from the battery if left connected but not operating!

You have not said what charger you have perhaps it is not suitable for permanent connection.

Compare with the Ctek chargers. Most draw less than 1Ah per month, equal to a constant draw of less than 1.4mA . Some of the larger ones draw double that but still <2.8mA

 

[gjgm]

Mastervolt, so I don't think that is it. Actually, I left the boat now for some weeks with no shore power and batteries disconnected..still disappearing anodes, so I kind of think the issue is not on board...

 

[RichardS]

Mastervolt, so I don't think that is it. Actually, I left the boat now for some weeks with no shore power and batteries disconnected..still disappearing anodes, so I kind of think the issue is not on board...

I'm not an expert in this ..... but I suspect that if you have no shore power connected then disappearing anodes can only be related to galvanic/electrolytic action occurring on board your own boat due to a mis-match of metals/anode materials. I'm struggling to see how other boats could be eroding your anodes as they will all be too far away.

Richard

 

[VicS]

Mastervolt, so I don't think that is it. Actually, I left the boat now for some weeks with no shore power and batteries disconnected..still disappearing anodes, so I kind of think the issue is not on board...

Mastervolt. Fair enough
If your batteries are completely disconnected, with no "always on" equipment such a bilge pump, still connected and your shorepower also fully disconnected we can just about rule out electrolysis due to current from the DC system and current carried via the shorepower. This leaves only the possibility of galvanic corrosion due to dissimilar metals on your own boat or corrosion due to the mysterious " stray currents" in the marina.
If nothing has been changed the former seems unlikely. Stray currents are only likely to cause problems if there are electrical connections between the drives and some other under water appendage allowing a current to take the route through the boat as the pate of least resistance.

How to check for "stray currents" ????
Multi-meter and two long leads with inert electrodes on the ends perhaps ???
Lower the electrodes into the water as far apart as possible and see what voltage you can find between then and what current will flow.