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Old 22-07-2012, 11:24   #1
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Inconsistent ammeter & alternator

This last week, unfortunately, I had to do a lot of motoring in my Niagara 31 sailboat.

One day, the 0-200 amp meter stayed at zero, whereas my battery monitor showed the alternator was providing amps, albeit at a lower level than it should considering the state of the batteries.

The next day, after an overnight anchorage, all was OK !

Explanations gratefully accepted. Thx.
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Old 22-07-2012, 12:29   #2
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Re: Inconsistent ammeter & alternator

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The next day, after an overnight anchorage, all was OK !
The trouble with a broken meter is you never really know anything even the next day. When the readings make sense it isn't a sign that it's not still broken. You need to find out what is wrong. There really is no way to compare the readings from the battery monitor to the amp meter unless the two of them have been calibrated. The readings become less reliable as the batteries advance in age. Battery monitors assume the condition of the batteries are good even if the charge state is low. Charge controllers do go bad too. I've replaced my external regualtor and my solar panel regualtor in the past 9 months.

So is it a bad meter or something else? From your limited description I can't even guess.
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Old 22-07-2012, 12:38   #3
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Re: Inconsistent ammeter & alternator

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So is it a bad meter or something else? From your limited description I can't even guess.
Where do I start ??!!

I had the same thing happen last year.
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Old 22-07-2012, 12:47   #4
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Re: Inconsistent ammeter & alternator

As a start, buy a clamp ammeter and a decent quality digital multimeter so you know what is factual. It is very difficult to diagnose or trace many electrical problems without having all the facts.
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Old 22-07-2012, 13:03   #5
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Re: Inconsistent ammeter & alternator

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As a start, buy a clamp ammeter and a decent quality digital multimeter so you know what is factual.
Have those ! Next ?! Also have a hydrometer. Thanks so far.
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Old 22-07-2012, 14:17   #6
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Re: Inconsistent ammeter & alternator

A 0-200 ammeter will have an external shunt. The voltage drop (E) is taken across this known, small and stable resistance (R) and is related to current by Mister Ohm's Law:
I = E/R.

So the ammeter is really a milli-voltmeter with a face painted in amps. Isn't electricity fun?

The first thing to look for is corrosion at the wiring connected to the shunt and to the meter. If that is bright and shiny, then check the fuse holder, if installed, to make sure its contacts are bright and shiny.

While at the shunt, check the integrity of the heavy conductors to ensure that those connections are bright and shiny.

After the above remedial action, take your clamp meter and put it around the big cables going into or leaving the shunt. Start charging and see what the clamp meter records vice what the ammeter is showing. They should be close with but the clamp meter, if it is of reasonable quality, will be the more accurate.

If the readings are not close, then take your DMM across the two terminals on the ammeter, and record the milli-volts during a charge. Most shunts are 50mV for full deflection. This means that for your 200A ammeter, you should see about 12.5mV for 50A, 25mV for 100A and so on. The relationship is linear.

If there is a mis-match between actual (clamp meter) and DMM millivolt readings (converted to amps), then the shunt has failed. If there is agreement between actual (clamp meter) and DMM millivolt readings (converted to amps), and you cannot adjust the ammeter with the front adjustment screw, then the ammeter has failed.

Hope this helps.
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Old 23-07-2012, 08:17   #7
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Re: Inconsistent ammeter & alternator

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Hope this helps.
Definitely ! Thanks. Next time I'm on the boat, I'll check this out.
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