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Old 03-02-2014, 08:15 PM   #1
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Checking alternator output

What is the proper way to check the alternator output with 12v parallel batteries? Is the multi-meter run from the positive on battery one battery to the negative on battery two or is it acceptable to just use battery one for the check?


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Old 03-02-2014, 08:26 PM   #2
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You can read the voltage on just one battery's pos and neg. REad each battery separately to verify all are getting equal charge volts.

'02 Ultimate Advantage 40J Spartan MM - Cummins ISC
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Old 03-02-2014, 08:26 PM   #3
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Since the batteries are parallel, it should not matter where you check it. There is likely cables from + on one battery to + on the other and the same for the -. Not sure what chassis you have but usually the easiest way to check it is turn on everything that is powered by those batteries and bring the engine idle up to over 1000 RPM. You typically want to see a steady 13.2V or higher.
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Old 03-02-2014, 09:15 PM   #4
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checking alternator output

Thank you for your replies, gentlemen. That clears up the questions I had. I will check each battery separately and check them in parallel. I would have noted the make of the chassis (Freightliner) but I figured that electricity was a universal thing.

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Old 03-02-2014, 09:25 PM   #5
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That 13.2 VDC is when lead-acid batteries are nearly fully-charged. When they are at, say 60-70% capacity-or less, you should see 14.6 VDC charge voltage. For better battery longevity.
2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG retired;Good Sam Life member,FMCA."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution. "Abraham Lincoln"
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Old 03-02-2014, 10:18 PM   #6
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The proper way to be sure the correct amps are being suppied as well is to find out what the Alt. is suppose to put out and then have the correct tool to put a draw on it to assure its doing its job, even a bad Alt. could show 13.8, but not give enought amps to keep a battery charged! Something to pounder for sure!
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Old 03-02-2014, 10:36 PM   #7
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The simplest way to check charging current, is with a DC clamp meter like this one:

MASTECH MS2108A 4000 COUNTS AC DC Current Clamp Meter Backlight | eBay

Just clamp it around the cable, and it will tell you the charge current.

Due to temperature compensation in the alternator regulator, voltage will vary from about 14.5V cold, declining to about 13.5 - 13.8V hot when the alternator is hot.

Stan Birch
1999 Winnebago Adventurer 32T Ford V-10
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