Alternator Questions

[Rick_Powers]

Hi Folks,

I'm experiencing that my battery is not re-charging very well after motoring for several hours.

Questions:

1) What should be the output voltage from a good alternator? I am measuring 12.6 volts. I have heard that 14 volts is expected. Is that true?

2) Is it worth getting the high amp output MMI alternator for $330+ ?

Thanks!

[rpowers]

More info

OK, that is 12.6V at idle, but I am getting 13.3V at 2000rpm, cruising speed.

Maybe i'm OK after all...

How much power drain does a CD player cause?


--------------------
Rick Powers
Palo Alto, CA
1976 Catalina 27

[Marian Claire]

Others know much more about the electrical stuff than I. I do believe you should be reading close to 14. Mine reads 13.7 +-. Your planned use would determine the need for a high output alt. Long periods on the hook, lots of toys, refrigeration etc, would draw down the battery and a high out put would cut down on the recharge time. I think. Dan S/V Marian Claire

[sastanley]

13.3 will slowly kill the batter(ies) by chronic undercharge. Optimum for wet cells is in the 14v +- range. Our old Motorolas don't put that much juice out with their stock regulator. I found an aftermarket regulator for the 35 amp Motorola that allows me to push 14.18 v into both batteries thru a simple mechanical isolator. Unregulated, the 35 amp alt. puts out almost 15v...which is too much.

if you do some searching around this site, I have detailed part numbers & a website that carried the regulator in another thread.
Sorry my response is short. Shoulder surgery has left me with only one typing hand for a while

[Don Moyer]

While it's true that most modern internally regulated alternators are biased to regulate system voltage at 14.2 volts, I would with utmost respect for all that has been recently posted in this thread (and only out of concern that we might unintentionally mislead some of our newer members) modestly submit that thousands of sailors have crossed oceans with OEM Motorola alternators for decades while maintaining a charging rate of 13.2 to 13.4 volts.

I think that what we're all saying here is that while more modern alternators with their 14.2 charging rates have clear benefits in terms of charging rate versus time spent running your engine at anchorage, if you have only a modest amount of electrical gadgetry on your boat, and are willing to spend a little more time motoring, there's really no need to rush out and turn in a well maintained OEM Motorola for a new high-output model.

Don

[67c&ccorv]

In regards to wet cell batteries:

"...once the battery nears full charge, 14.4 volts is too high, gassing the battery vigorously and causing the positive plate to oxidize (corrode). If you continue to run the engine - perhaps motoring - a charging voltage of around 13.2 volts is ideal for maintaining full charge without overcharging. For years automobile manufacturers compromised on 13.8 volts - high enough to replace the shallow discharge from starting without later resulting in serious overcharging...using a 13.8 volt regulator in a sailboat charging system nearly eliminates any risk of overcharging (the engine compartment heats up enough to derate the regulator to 13.3 volts) but charging times with with a 13.8 volt regulator will be twice that of a 14.4 volt unit. Unless you motor a lot, expect your batteries to be chronically undercharged."

Sailboat Electrics Simplified, (p.102) Don Casey, 1999.

[Rick_Powers]

Thanks All!

Excellent responses.

This forum is worth it's weight in gold!

[rigspelt]

I'm seeing 14.5 with the new API alternator from MMI, but I have a note that Don had advised on the forum somewhere that API does not recommend altering this alternator to use an external regulator, that it should put out 14.2 volts, and that 14.2 volts should be OK with extended runs. Sounds like I need to have the alternator's regulator checked.

[Baltimore Sailor]

I'd say it'd be worth pulling the alternator and taking it to a good auto electronics shop -- NOT an Advance Auto Parts-type place, but a good old neighborhood place that specializes in alternators -- and let them analyze it for you.

Mine was failing last season, and I took it to a little shop in the outskirts of Baltimore and for about $90 they completely rebuilt it into a 55A alternator that's like brand-new and charges at 13.5 volts at idle.

One more thing I don't have to worry about any more.

[Wes]

The only thing I think everyone has missed here...Is... How old are the Batt. Maybe they no longer will take a good charge. Just a thought. Wes