Alternator Short Causing Positive Charge on Ground

[Marshall Mahoney]

I was installing a battery in my boat and for some reason I had the idea to check for voltage between the the negative battery post and the negative battery cable (Positive cable/post connected). Selector switch off and all breakers off -- reading near 0 volts. When I turned the selector switch on, I got a full 12 volts! I had 12 volts on my grounding block, engine block bonding wires -- everything. I even had 4 volts in my bilge water! After a couple of hours of disconnecting wires and checking voltage, I traced it to my alternator. Crazy thing is, my alternator worked! This may have been the cause of my zinc electrolysis (could not have helped). Definitely a cause of my battery drain.
After a bit of research, it seems that failed alternator diodes are a common cause of stray current. Next time you are changing batteries, or are just curious, easy to check as described above...

[Tony Meola]

Marshall,

The alternator would only leak current when running. Rather than check voltage, check ohms. Not to say it would not work checking voltage but ohms is a definate teller of a bad electrical device.

When our boat was new, I mean as in 1 year old, we had an electrolysis issue that depleted our zincs in two weeks and actually burnt the prop nuts. We got lucky by hitting bottom coming in the inlet and Bent a prop. When we pulled the boat to pull the props the zincs were gone.

To find the culprit we checked the stray voltage. We had 2 volts in the bilge and a volt and half when we checked between the rudder and water at the dock.

We disconnected the batteries and hooked the ohm meter to the battery cables with ever device turned off including the breakers. We turned on the first breaker and took a reading, then turned on each 12 volt equipment and checked the ohm reading. Turned it off and moved on to the next one.

Culprit turned out to be a BILGE pump. We replaced the pump and float and the problem was solved.

Not saying you didn't solve your problem, but make sure you double check the ohms reading of every device you use on the boat.

[Marshall Mahoney]

Tony -- did you mean test with amp meter (by putting an amp meter between the negative cable negative battery terminal)? Not sure how I would test with ohm meter. The alternator had some sort of internal short because it was creating a path from the positive post on the alternator to the engine block. With the positive cable connected to the positive alternator post, I read 12v at the battery. Lifted the positive cable, voltage dropped to 2 volts. Pulled the fuse to my power panel, and voltage went to near zero. So I still have an issue, but only a fraction as bad. Replaced the alternator and still 2 volts at the battery. I will start checking my end devices with volts and amps tomorrow.

[Tony Meola]

Marshall

Attach the positive and negative ends of the Ohm meter to the battery terminals. Then shut everything off as I explained, and then go through each each item on by one. Turn and then turn off. Everything should show resistance if working properly.

Most electrical devices have the Ohm resistance printed on them. If you do not come up to the number shown on the device or if you have 0 Ohm reading you found part of your problem.

With my problem when the boat was new, the stern bilge pump put out 0 ohms. Changed it and problem solved.

Also check to make sure that you do not have any wire connections that may be bad and possibly leaking current. Remember we are in a damp environment and connections go bad. Especially watch the bilge pump connections. They tend to be low in the bilge and are always damp.

[Marshall Mahoney]

Thanks Tony. I should have thought it thru -- makes sense now. Checking it out today.

[Carl]

Funny I had to reread it myself. Electrical is not my gig, but always looking to learn.

Tony wrote:
"Attach the positive and negative ends of the Ohm meter to the battery terminals."


It's Before step one is where I got lost... FIRST Remove cables from the battery- - then Attach meter test ends to cable terminal ends...not the battery. Tony said terminals...In my head, I heard Battery Posts. I'm just thinking...boy, that cannot be right. I guess more a lack of thinking. Makes perfect sense now. Thanks

[Tony Meola]

Carl

Sorry I was clearer in my first post.

Must be old age setting in.

[Marshall Mahoney]

I hate electricity...

[Carl]

Carl

Sorry I was clearer in my first post.

Must be old age setting in.

No need to be sorry Tony, I am kind of dense in that area. Obvious at times escapes me…thinkIgot it now. Thx

[Tony Meola]

I hate electricity...

I guess you did not find the problem?

Electricity is not bad once you understand how it works and how someone wired the dam place.

[Ironworker]

If I ever sell my boat it will come with a updated wiring diagram and manual with all the pieces and parts documentation.

[Bruce]

Electrical wiring is logical...there's an electrical source, a protection such as fuse or breaker, some type of switch to turn off or on, and the final component that is energized by the electrical source. In some cases such as lights, outlets, etc the final load source is piggy backed together to operate multiple items with one flick of a switch.

You can do so much with a VOM meter and a 20' set of clip leads.

Unless you are working on British and Italian stuff. Then all bets are off and anything goes. This includes all avenues of transportation.

[Marshall Mahoney]

So... for a couple of years my neighbor has blamed me for his (and my) "quickly" dissolving anodes -- get a bit less than a year out of them. I keep my main circuit breakers open when docked, and the only thing live is shore power, battery chargers, and bilge pumps. My faulty alternator would not have been an issue since the circuit breakers are left open. I have a galvanic isolator.

My boat is currently on a trailer. I checked for stray current using an ammeter (could not seem to get my ohm meter to give me meaningful info). No stray current readings on ANYTHING except my two battery chargers, which showed .07mA and .02mA. I don't think they should be bleeding thru, but when connected to shore power, I don't think they are the issue.

At this point I don't think my boat is the issue -- but the readings will probably be all over the map when I check it again at the marina...