Alternator Trouble?

[mainesail]

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.

By industry standards your batteries are dead. Anything less than 80% of new CCA or MCA is considered dead. WM uses Midtronics equipment and they should have let you know that 75% is not a passing grade for batteries. Dead batteries don't accept current like newer ones do so this could be part of your problem.. I still suspect a wiring/connection issue. You ideally should have your alt checked out too because you should have a higher voltage.. It is possible to lose some diodes but not all....

[2ndwish]

Spent a couple of days looking into this and here is what we found.
1) Had the alternator tested at a shop and it was bad. The max current they could get out of it was 20 amps and the voltage was 13.7 or lower. Replaced it with a Leece-Neville 65 amp unit with a 14.3 V set point. The batteries now charge at 9 amps and I was able to get the alternator to put out 21 amps (at 14.05 V as measured at the alternator). Also, the tach started reading correctly (it was off by some factor before).

2) The batteries are likely shot. I ran them down to 11.9 V and let it charge overnight on the charger and only got 13.5 amp hours into the batteries (should have been closer to 80 AH). One oddity is that when we operated the solar panels, we got more charge than this...so...

3) The battery charger (Lewco)does not seem to be able to charge beyond 13.7V, which may be contributing to the battery problem. As a side note- if we replace this, is there any particular reason to go with Promariner ProNautic and not the ProSport series- the specs are similar for our application and there is a big $$ difference?

4)While mucking with the batteries I discovered another issue which seems related. While there is a very good ground return from the batteries to the engine, the ground to the service panel comes through a single 10AWG wire. With 9A running, there was a drop of 0.3V between the battery and the panel, so the fridge say, sees this additional drop which makes it cut out early as a result of its battery protection circuit (the start current for a Danfoss compressor is 11A). This also explains why cabin lights dim so much when something is turned on. When I checked the wiring diagram in the owners manual, they confirm that this ground is provided by a 10AWG wire (although where it is connected is unspecified). 

We were expecting to find one problem, found several.

[Ted Pounds]

Definitely upsize the ground to the service panel.  It should be the same size as the positive feed since it will carry the exact same current load.

[Ron Hill]

2nd : Here is "rule of thumb" on battery voltage vrs % charge levels.  I published this in the mainsheet tech note long ago!!

12.75V = 100% charged and 11.75 = 00% charged 
( the % charge changes 5% with every .05 V -- ie. 12.25V = 50% charge).

[2ndwish]

Definitely upsize the ground to the service panel.  It should be the same size as the positive feed since it will carry the exact same current load.

Ted- It is not as simple as matching the gauges. The full starter current goes through the Perko on the panel, but the heavy ground is connected directly to the engine block. The panel itself requires a ground wire large enough to accommodate the service load, which is much less than the starter current. Still 10AWG seems quite small for loads which can reach 20A. I wonder what others have done?

[Ted Pounds]

Good point. I didn't think about starter loads.  But I definitely agree that #10 seems a bit small.  However, since I sold my boat a few years back, I don't remember how my boat was set up...

[Ron Hill]

2nd : As the replies have said "You can't have too many grounds" !!

I ran a #4 wire from the grounded alternator case (along with my 2 positive wire) direct to the batteries. 
Then to insure there was a good ground at the main electrical panel I ran a #8 ground wire from the batteries direct to the DC buss bar just above the Perko selector switch.

A few thoughts