Alternator Trouble?

2ndwish · April 29, 2013, 08:00:20 AM

Spent the weekend at Catalina Is which gave us a chance to evaluate some things. We had noticed last year that the house bank would run down quickly (~5 hours) and our rule o'thumb of an hour of engine in the morning and evening was not keeping up. We had the batteries checked at the end of the season and they were fine. The voltage at the battery with the engine on is 13.7 V which seemed fine. I installed a battery monitor for this trip to see what was going on (Found a pretty good V, A, AH meter with a shunt made in China, cheap ~$40, on Ebay).
Our alternator is an 8MR2049 (50 amp?). When the engine started and the house batteries very low, it charged that bank at 7.5A, but within a few minutes dropped to 2A. The alternator was only seeing the house batteries at the time. This does not seem like normal behavior, but it does explain what we've been seeing on our cruises. Any ideas?

As an aside, we also purchased a 120W solar panel and PWM charge controller ($150). That worked great. It put out a steady 5-6.5 A with just a couple of adjustments during the day. It was enough to run the fridge all day with a couple of amps excess to the batteries.

Stu Jackson · April 29, 2013, 08:08:34 AM

Alternator trouble? Maybe not. Please advise what engine rpm you were running when you got those amperage readings. Running at idle, or even 1500 rpm doesn't always produce output even with low batteries from a night of use and the batteries able to accept high amounts of current. 13.7 is a low voltage for acceptance charging, but also please advise when this voltage was seen: at startup at idle or when full out running, etc.

2ndwish · April 29, 2013, 08:53:32 AM

Should have been more specific... We charge at ~2000 RPM. The house batteries are 2ea 80 AH Group 27 maintenance free wet cell, deep cycle. When under way, at full throttle, it exhibited the same behavior.

I did not see an adjustment for the set point on the alternator. Is there one I'm missing? What is the proper set point?

Ron Hill · April 29, 2013, 02:05:07 PM

2nd : As Stu said 13.7V at 2000rpm is low if the batteries are discharged. It would be better to see 14 - 14.2V at that RPM.

You said you installed a battery monitor. When I am charging batteries I primarily look at Amperage and then look at Voltage.

What was you amperage reading?

Stu Jackson · April 29, 2013, 05:04:55 PM

Ron's on the path...

What amperage at what voltage at what rpm is the information we need. From what you've said so far, it appears you have a stock 1987 or so OEM internally regulated alternator on an M25XP engine. Is that right?

2ndwish · April 29, 2013, 06:02:50 PM

When I started the engine, I switched to the house battery bank. Ran the engine up to ~2000 RPM and observed ~13.7 V(approximately) and 7.5A current. Within a few minutes (less than 5) it was down to 2 A at the same voltage. The engine is an M25XP with a stock alternator and internal regulator.

I should also mention that with the solar panel alone I was seeing 14.2-14.3 V @ 6 A on the monitor- also suggesting the set point of the alternator is too low.

Stu Jackson · April 29, 2013, 06:18:43 PM

It could very well mean that your batteries are FULL.

2ndwish · April 29, 2013, 07:00:48 PM

Most definitely NOT full. The batteries had been running the fridge all night and the voltage had dropped to ~11.7V. Also, the batteries were able to take everything the solar panel could put out. BTW the fridge will not actually run properly below 12V , it will just cycle the fan repeatedly and the compressor shuts off as soon as it starts-another interesting tidbit.

Stu Jackson · April 29, 2013, 07:21:22 PM

So, then, unless there are wiring issues, you need to check the alternator. Right?

2ndwish · April 29, 2013, 07:25:33 PM

Yup. I'll check the wiring by looking for a voltage drop under load. Can the regulator setpoint change? Is that a failure mode? Somehow seems unlikely.

Stu Jackson · April 29, 2013, 07:28:30 PM

2ndw,

Unless you have changed your regulator, we have to assume you have an OEM setup. Unless you take it to an alternator shop, those old OEMs aren't "user adjustable."

Since your monitor is new, you may not have been in a position to know what USED to be going on with your alternator before this trip.

You might also have battery issues, but your solar system seems to be working.

Sounds more like an alternator issue. Maybe as simple as a regulator failure.

How's your belt tension?

2ndwish · April 30, 2013, 09:00:05 AM

Hmm. I think the belt tension is ok. If there was a lot of slipping I'd notice it. But you raise a good point about the rule of thumb. There are two additional variables in recent years. 1) The batteries were replaced about three years ago (could their acceptance be different than the old batteries?) and 2) the refrigerator compressor was also changed at that time. If the cycling of the fridge is a "feature" to protect the batteries, they may not be discharging as deeply as the old ones and therefore not charging as fast at the low setpoint. Now that I have the monitor, I will do a thorough discharge this weekend and report back next week. Either way, it seems that with the current situation, the alternator is not charging efficiently and should be addressed.

Ron Hill · April 30, 2013, 09:12:04 AM

2nd : It sounds as though your voltage regulator is seeing fully charged batteries?!?

Dumb question - do you have the "wiring harness upgrade" installed?? (see critical upgrades)

Ron Hill · April 30, 2013, 03:34:50 PM

2nd : Also rereading your #7 reply, 11.7v is EXTREMELY low (infact they maybe flat!!). I'd even guess that the voltage part of your "new" monitor isn't correct.

I'd recommend that you check the "new" monitors voltage readings against a good multi-meter.
My thought

2ndwish · April 30, 2013, 11:20:35 PM

Thanks for the ideas. The new monitor was checked against our panel mounted voltmeter and a handheld digital multimeter (also used to calibrate the current readout). All agreed within 0.1 V. Our house bank has two, 80 AH batteries. When they were tested at WM last fall, they had ~75% life left, so 2 batteries run down a little more... 0.7x160AH =112AH capacity. The fridge +other things are about 5A so 10 hrs would be 50AH or 60% of remaining capacity. I do not have the discharge curve for these batteries, but using an old curve for a lead acid battery I found on the web, the state of charge could be between 25-50% for 11.7 V for a discharge rate between C/20-C/10, so 11.7 V is low, but not out of the realm of possibility, particularly if the batteries have lost some more capacity or are out of balance.

ps. We have upgraded the harness