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Old 21-11-2013, 05:34   #1
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Alternator does not Fully Charge the Batteries

Hi, to continue with battery issues..

Transferred the boat on Saturday by driving some ten hours with the engine.

To my surprise, batteries were not full after that, only 12.5 volts. I also noticed only 14.1 volts charging voltage and very few amps in the Nasa battery guard all the way (btw, quite a nice peace of equipment).

Configuration is as follows: MD2040 year 2004 standard 60A alternator with a standard controller and a sensor cable sensing the voltage from the leasure battery bank directly, 2 x 105 Ah leisure batteries, 1 x 95 Ah starter and an automatic separator Surepower 702.

Batteries are ok as I can fully charge them with shore charger.

What's wrong/broken? Suggestions?
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Old 21-11-2013, 05:58   #2
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Re: Alternator does not fully charge the batteries

Bad rectifier in the alternator? Could be multiple diode failures, maybe 4 of the 6 are bad knocking the output by 2/3.
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Old 21-11-2013, 07:23   #3
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Re: Alternator does not fully charge the batteries

At 14.1V it will take a very long time to charge to 100%...
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Old 21-11-2013, 09:32   #4
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What are your bulk / absorption / float settings?
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Old 21-11-2013, 09:56   #5
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Re: Alternator does not fully charge the batteries

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What are your bulk / absorption / float settings?
Sounds like a factory alt, just bulk & absorption, no float....
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Old 21-11-2013, 10:16   #6
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Re: Alternator does not fully charge the batteries

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Bad rectifier in the alternator? Could be multiple diode failures, maybe 4 of the 6 are bad knocking the output by 2/3.
My experience is it only takes one diode for this voltage drop. Diodes are in pairs and I believe there are six in an alternator, so one diode means 1/3 of power is lost. Pull the alternator and have it tested. Some automotive parts stores have test equipment for free test.
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Old 21-11-2013, 12:28   #7
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Re: Alternator does not fully charge the batteries

Thanks folks, yep this begins to be my diagnosis, too. And I just realized I have started the engine several times with shore charger on which must be identical or even worse for the rectifier as taking the electric load off....... my bad.

I'll let you know the test results next week.
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Old 21-11-2013, 16:37   #8
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Re: Alternator does not fully charge the batteries

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Thanks folks, yep this begins to be my diagnosis, too. And I just realized I have started the engine several times with shore charger on which must be identical or even worse for the rectifier as taking the electric load off....... my bad.

I'll let you know the test results next week.
Shore powered battery chargers have circuit breakers so if you overload them for too long, all you have to do is hit the reset button. The problem is overheating. Really small chargers may not have circuit breakers, but can stand some overload.
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Old 21-11-2013, 16:42   #9
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At 14.1V it will take a very long time to charge to 100%...
Lead acid 12V batteries? I thought 13.8 to 14.2 was what an alternator output should be?
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Old 21-11-2013, 22:26   #10
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Re: Alternator does not fully charge the batteries

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Lead acid 12V batteries? I thought 13.8 to 14.2 was what an alternator output should be?
14.4 or a bit higher is normal far a standard dumb regulated alternator.
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Old 21-11-2013, 23:16   #11
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Re: Alternator does not fully charge the batteries

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Originally Posted by tuomas View Post
Hi, to continue with battery issues..

Transferred the boat on Saturday by driving some ten hours with the engine.

To my surprise, batteries were not full after that, only 12.5 volts. I also noticed only 14.1 volts charging voltage and very few amps in the Nasa battery guard all the way (btw, quite a nice peace of equipment).

sounds fine to me. how do you know they are not full? if there is only a few amps going into the batteries they are probably already full.
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Old 21-11-2013, 23:43   #12
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Re: Alternator does not fully charge the batteries

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Lead acid 12V batteries? I thought 13.8 to 14.2 was what an alternator output should be?
Correct - but these automotive internally regulated alternators are designed to charge starter batteries which lose less than 1% of their capacity when starting an engine. A deep cycle marine service/house battery loses 50% of its charge and needs a different charging regime with a multi-stage external regulator that may go up to 14.8v or higher. The OP said he had a "standard regulator" - whatever that is? Old regulators were as low as 13.8v, modern ones may be much higher and may have temperature compensation to reduce output voltage.

A starter battery may take 15 minutes to recharge, a deep cycle battery will take 15 hours or more to get anywhere need full. BUT at only 14.1 volts the batteries will not reach their gassing voltage of about 14.4v, so will not help to remove the Lead Sulfate that clogs up the plates and reduces capacity. Gassing also stirs up the electrolyte and evenly mixes the electrolyte to reduce stratification that concentrates the acid at the bottom of the battery causing plate damage. All this undercharging will prematurely kill a battery.

On a cruising boat that spends a lot of time away from shorepower a hot-rated marine alternator and an external regulator with battery and alternator temperature sensors will prolong battery life.

As Maine Sail says - "Batteries don't die, they get murdered"!
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Old 22-11-2013, 00:24   #13
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Re: Alternator does not fully charge the batteries

The op has a boat with a 2004 Volvo MD2040 engine that would likely have an alternator with a charging voltage of 14.4. It has been a long time since the set voltage was commonly 13.8 volts.

The same alternator with an external regulator at the same 14.4 bulk/absorption voltage, would not charge the batteries any faster. Only if the external reg was at a higher voltage would there be a time difference.

We haven't been told when and how the voltage of 12.5 was measured, or the accuracy of the measurement but if it was only a short time after the engine was shut down after a 10 hour run the voltage should have been much higher. If some time elapsed and/or there were loads on the battery the voltage could easily have dropped to 12.5 volts. A fully charged rested battery is considered fully charged at about 12.6 volts or higher.
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Old 22-11-2013, 08:13   #14
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Re: Alternator does not fully charge the batteries

The stock OEM Valeo internal regulator on the MD20x0 series engines is set for 14.0V as per the Volvo owner's manual and service manual.

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Old 22-11-2013, 08:18   #15
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Re: Alternator does not fully charge the batteries

Really. Rare to see that low on a newer engine.

My point was that an external 3 stage regulator will not charge any faster than a built-in dumb reg as long as the bulk/absorption voltage is the same. With a 3 stage set higher the charge will be quicker.
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