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Old 12-10-2014, 09:22   #1
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Alternator Problem?

Basic Info:
- Volvo Penta (Mitsubishi) 115amp alternator, internally regulated, N-type
- External voltage sense wired to the house battery bank
- House Batteries: 4 Grp 31 AGMs (105amp-hr each, 420 amp-hr total) - only 2 months old, no deep discharges, fully charged almost daily with solar and wind power, good efficiency.

Problem: alternator output appears low and/or not apprpriately responsive to battery voltage

Data
House bank fully charged, engine running, no house loads - battery monitor = 14.2 volts, charge rate 2.0 amps
House bank fully charged, engine running, 10 amp house load - battery monitor = 13.95 amps, charge rate negative 7.0 amps

After allowing the house bank to discharge for 2 days (live aboard), the battery monitor with no loads showed 12.5 volts and 67% SOC, (-140 amp-hrs)

House bank at 67% SOC, engine running, no house loads - battery monitor = 13.15 volts, charge rate 38.2 amps
House bank at 67% SOC, engine running, 22 amp house load - battery monitor = 12.90 volts, charge rate 20.0 amps

Questions:
1. Shouldn't the alternator produce (and the batteries accept) more than 38.2 amps (9.55 amps each) at 67% SOC?
2. When the house loads are increased, and the battery voltage drops, why doesn't the alternator increase its output, especially when it's not working very hard?
3. Does this point more to an alternator problem (poor voltage regulation? bad diode?), or something else entirely?

Notes:
- Before collecting the data, I verified the basic function of the voltage regulator at various engine speeds, and I checked the external voltage sense by running the engine with the voltage sense wire connected to both sides of the isolation diode (the alternator increased its voltage to compensate for the .7 volt loss across the diode).
- I cleaned and checked connections. I verified minimal voltage drop across the circuit, and the battery monitor voltage matched my meter readings (diode voltage drop aside),
- Before collecting data, I re-routed the alternator output directly to the house battery side of the isolation diode to eliminate this variable.
- After fully charging the batteries for 48 hours I disconnected the solar panels, wind generator, and shore power.
- There were no signs of belt slippage, and the engine / alternator did not even get hot during the following tests. "Engine running" means enough rpm to maximize alternator output.
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Old 13-10-2014, 11:02   #2
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Re: Alternator Problem?

Is this at idle or full rpm? 40a at idle sounds normal to me.

Lower voltage is normal with a maxed alt, which it would be at idle

Rev it up

If you already are revved up. The. I would get the alt pulled and tested

You battery monitor and charge rate numbers are strange. are you reading from a panel gauge or a battery monitor (net). as the battery monitor should have read 2a in both the first 2 cases. you're ether confusing terms, or your battery monitor is wired weird a d not getting net. Or you are calling your panel amp meter a battery monitor which it is not
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Old 13-10-2014, 11:32   #3
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Re: Alternator Problem?

I had a similar thing a couple of weeks ago; it was a loose belt even though there wasn't any other signs of it slipping.
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Old 13-10-2014, 12:03   #4
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Re: Alternator Problem?

How is it connected to batteries. Isolator? What size and length of wire?
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Old 13-10-2014, 12:06   #5
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Re: Alternator Problem?

Quote:
Originally Posted by sailorboy1 View Post
I had a similar thing a couple of weeks ago; it was a loose belt even though there wasn't any other signs of it slipping.
My belt wasn't loose, it was glazed. I changed belts after sandpapering the pulley sheaves. End of problem.
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Old 13-10-2014, 12:08   #6
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Re: Alternator Problem?

Solar, wind, shore, isolation diode...Why is that last one even in there, that's obsolete long ago.

I'd disconnect all other charging sources and just look at how the alternator works without all the potential complications. And reality-check your readings with a "known good" handheld ammeter and voltmeter.

"Divide and conquer" still works well. You can also do a fast diode test on the alternator, from the output to ground with engine off. It should measure nearly zero with proper polarity, and infinite with reverse polarity. If you don't get the infinite, you've got one or more blown diodes, which will reduce alternator output when it is running, but also should be draining the batteries even when it is not.
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Old 01-11-2014, 06:11   #7
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Re: Alternator Problem?

Thanks for the suggestions on my alternator problem, everyone. I followed your tips: checked belt tension, checked connections, and "divided and conquered" by running the alternator output directly to the house battery bank.

I ended up getting the alternator bench tested - the internal voltage regulator was not up to spec. It's now fixed and working like a charm.

HelloSailor, I agree that the diode isolator is ancient tech. However, the wind and solar are wired directly to the house bank, so they are not affected. Typically we only rely on the alternator for longer, downwind passages, so replacing the diode isolator has been a low priority. Plus, the alternator's internal regulator uses the input from its sensing wire (from the house bank) to "bump up" the voltage, compensating for the voltage drop across the diode. It's not perfect, but it works ok.
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