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justonemoreproject 02-18-2009 12:27 PM

alternator and battery recommendations
After rebuilding my alternator it still was not charging my batteries. At full throttle it only put out 12.26 volts. I hired a tech to come on board because I thought I had a wiring problem. He used an amp meter to determine that my alternator was only was putting our .5 amps! The reason being that I have a low RPM diesel that cannot not spin the alternator fast enough.

My motor is a 30 year old 7 HP Petter Diesel paired with a 28 amp Lucas ACR15 alternator with internal regulator. Only 600 hours on it so its hardly broken in. My batteries are also getting to the end of their life. There are 3, 85 amp hour gel cells that have relied on my solar charger for charging, now the solar power can't keep them up. One was discharged down to 10 volts when I checked it yesterday.

The tech suggested that I get a high output at low RPM alternator of around 50 to 70 amps and some new batteries. Balmar alternators are very expensive. Can anyone suggest a good less expensive one?

My energy budget is modest. Just instruments, and occaisionally laptop, cabin lights and stereo, but I want to do more distant coastal and offshore cruising and racing and therefore need a reliable power supply.

Any thoughts or suggestions would be helpful.

Thanks - Larry

JohnRPollard 02-18-2009 12:44 PM


It sounds to me like you do not have adequate charging power for your battery bank. The VERY BASIC rule of thumb is that your alternator should be rated to about 25% of your battery bank, i.e. you should be running a 65 amp alternator to properly handle your 255 amp hour battery bank. This may explain why your alternator is dying and your batteries aren't charging.

I also think a 65-70 amp alternator would put a pretty heavy load on your (nominally) 7hp engine. If you go that route -- and I don't recommend that you do - be sure to incorporate a switch that can shut down the alternator when you need to put all the power to the prop.

My suggestion would be to both increase the size of your alternator and reduce the size of your battery bank. Maybe bump up to at most a 45 amp alternator and downsize the bank to two Grp 27s or even Grp 31s if you can boost the charging via solar.

This should still be enough reserve capacity for the kind of loads you describe, and should allow the batteries to get fully charged. Two fully charged batteries are much better than three anemic/undercharged batteries.

Let's hear what other folks think...

erps 02-18-2009 12:49 PM

You can get a standard delco externally regulated alternator for under a $100 that will put out 50 amps or better. You'll need an external regulator which vary in price from $20 for an automotive regulator to a couple hundred for a smart regulator.

justonemoreproject 02-18-2009 01:14 PM

Alternator and battery choices
Hi John,

Thanks so much for your response. I have owned this boat for 5 years and I don't think the alternator has charged the batteries during that time or the previous 25 years either. The solar panels have been solely responsible for charging the batteries. Originally, there were just 2 grp 27s on board but I was inspired by Nigel Calder to make some changes, but at that time I did not realize that the alternator was not charging.
4 years ago I replaced all the battery wiring; adding a new ACR battery combiner and selector switch and fuses. I also replaced the batteries. I went from 2 batteries to 3 batteries: 1 for a starter and 2 for the house bank. They were 3 year old 85 amp hour gel cells that came from a bank UPS system. My solar charger kept them topped up year round with no problem. (My boat lives in Richmond on the San Francisco Bay where there is lots of sunshine.)

Since the solar panels were able to keep the batteries fully charged, wouldn't if I got 3 new batteries for the bank?

Thanks - Larry

justonemoreproject 02-18-2009 01:33 PM

Pulley sizes
One problem is that my alternator works fine on the bench but not on the boat because of the slow Engine RPM. (When at low low idle I can actually count the belt RPMs as the belt revolves! ) The pulled on the alternator is 2.5" and the engine pulley is 3.5" If I went to a 2" pulley on the alternator and a 5" pulley on the engine would that help? There are 2 belts on the engine. One belt drives the alternator and the other drives the 2 water pumps. If I increase the size of the engine pulley, would it damage the water pumps by making them spin too fast?

Any suggestions on where to find a 2" pulley for the new alternator?

My alternator has 2 feet. Can I use a single footed one like this one from JC Whitney? MARINE ALTERNATOR : JC Whitney: Auto Parts & Accessories

Thanks for your help.


erps 02-18-2009 01:39 PM

Some engines allow you to bolt on different size pulleys, a small one for your water pump, a big one for your alternator. The pully on our MD17 that runs the primary alternator must be 10 to 12 inches in diameter. Even then, I run the motor at a high rpm when its putting out the amps to reduce belt wear. I don't know how hard it would be to modify your bracket to switch to a small case single foot alternator. Perhaps you could borrow one and see how well it matches up?

justonemoreproject 02-18-2009 02:42 PM

I think I will try this approach:

justonemoreproject 02-18-2009 02:49 PM

I think I will try this approach:
1. Buy a new 45-50 amp alternator
2. Place a 5" pulley on the engine and a 2" pulley on the new alternator.
3. replace batteries with 2 100 amp hour batteries .
4. eventually add a another solar panel.

ANY suggestions on who has the best pricing for alternators? I could not find a Delco alternator with an external regulator for around $100.00.
I did find this one. MARINE ALTERNATOR : JC Whitney: Auto Parts & Accessories

rocinante33 02-19-2009 08:15 PM


I really like my Leece-Neville 90 amp marine alternator. It was very reasonably priced ($167 2 years ago, if I recall correctly). I think that you should be careful to not put too big an alternator load on your small engine, though. 45-50 amps is probably the absolute maximum that your 7 HP could power. I think 70 amp would be too much for 7 HP.

I think the JC Whitney marine looks like a fair price. It might be worth a try.

erps 02-19-2009 08:43 PM

The nomenclature for the small case delco externally regulated alternators is 10DN. I got mine two years ago at an alternator repair shop in Bellingham Washington for $75 IIRC. That did not include the regulator. I just googled Delco 10DN and I don't see any that cheap right now. Here's a hit for $114. An auto wrecking yard would be another source. There were a lot of those alternators built.

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