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My (Volvo TAMD41B diesel) engines came with 37 amp alternators. This seems low. I believe the factory original was 50 amp and in later years 60 amp. Is the amperage rating too low? Is bigger better as I have seen 106 amp alternators available too? Do these higher rated alternators use more horsepower?
That is low. Add to that if your drawing anything close to the rated amps that the heat generated will lower the output from the max rating. IIRC alternators draw something like 1 HP for 25 amps when the alt is at full output. The HP needed by the alt is more dependant on alternator load than the max rated output so when loaded at 50 amps or less there will be little difference in the HP needed to run it between a 50 and 100 amp alt .
The drive belt has to be adequately sized for the intended load. One horsepower is 746 watts, but there's alot of heat (waste energy) generated by the alternator, so one hp per 25 amps is probably pretty close. The wire FROM the alternator has to be adequately sized also.
Replacement depends what ampherage you want to draw on just one engine idling. You could be fine if you are not running a lot of radar, sound, lights, and livewell pumps at the same time, but most outboards put out more that what you have on your diesels, so go figure.
Thank you for the information. 25 amps used 1 Hp. The alternator is a Prestolite Leece Neville 8MR2050FA. They make various sizes to 105 amps that all use the same outside dimensions so upgrading to a larger one should not be a problem. The key is the rpm at which it makes the amperage. I plan to run at 3000- 3200 rpms. The 65 amp unit produces the most output at that range although the 105 amp unit produces the most at lower rpms and ramps up the fastest too.
Bigger isn't always better. If your usual daily house load is not so much....say you're only going out for a few hours or a day to fish but return home the same day...then you probably are running the engines the whole time and aren't pulling out of the battery "bank" more than you're putting in. Keep in mind that at idle an alternator is not doing any charging. It has to be spinning at the engine's max to even reach it's rating and only then if conditions allow it (i.e. it's not too hot.) Your 37 amp alternators probably (I'm guessing) provide somewhere around 15-20 amps under most circumstances. This may be plenty though, so don't go out and drop $1500 on Balmar alternators that you don't even need.
If you go out for extended periods and rely only on the engines to (A.) start, and (B.) charge the batteries, then you should do a careful analysis of your battery usage, storage capacity, and charging needs. You may find that you want to add more house reserve and have dedicated starting batteries isolated from the house loads. Dunno, only you can answer that. There are plenty of websites and systems available to help you out.
Could you let us know what type and number of batteries you have and what kind of trips you take? Do you have radar, navigation electronics, hot and cold water, pumps, livewells, etc?
__________________ Bill Adams
66 109 2A Safari Wagon
93 LWB
Allmand 23 Ticonderoga Chevy 350 EFI Cobra IO
Carver Mariner hybrid cuddy twin jet drives Ford EFI 5.0 HOs
saltwaters - 12/14/2008 4:35 PM Thank you for the information. 25 amps used 1 Hp. The alternator is a Prestolite Leece Neville 8MR2050FA. They make various sizes to 105 amps that all use the same outside dimensions so upgrading to a larger one should not be a problem. The key is the rpm at which it makes the amperage. I plan to run at 3000- 3200 rpms. The 65 amp unit produces the most output at that range although the 105 amp unit produces the most at lower rpms and ramps up the fastest too.
One of the important factors to getting good performance from your alternator is pulley size. Leece-Neville has very good graphs on output curves. If you measure your crank pulley diameter and look at your normal crusing RPM you can easily figure what alternator pulley size you need to maximize your alternator output. The only limiting factor is too ensure that at your WOT engine RPM your not overspinning the alternator. Also make sure the alternator is getting cool air as much as possible. Heat lowers your output and shortens alternator life.
A plug and play replacement shouldn't affect horsepower to any noticeable degree. I gotta ask since you didn't say, have the two 37 amp units been problematic? Ever find them not up to the task? If not, bigger may only be more expensive ...
Bill, You bring up some good points regarding load. I have the usual assortment of items, 2 GPS chartplotters, 2 fish finders, 4 VHF radios, 4Kw Radar, stereo, 2 bait tanks, fresh and saltwater washdown. A wide array of this stuff is in use depending on what's happening and if I'm running at night.
I have two batteries, one for each engine. They are 4D WM AGM. The starboard one doubles as the house battery but I plan to install a separate house bank sometime this winter now that I have room for them.
My trips are usually single day but that might change to multi-day trips this year. We go anywhere from a few miles close to home to 100 miles away from home depending on where the fish are. It varies widely.
Sickcat, Good information on the pulley size. I will stick with the standard Volvo two belt system and size. I didn't know people changed the size of the pulley when changing alternators. If I upgrade to a higher amp unit I will stay with the same outside dimensions and pulley.
Yes, I am having problems with new units. These are not original equipment Volvo alternators although I think they are supposed to be direct replacements.
They both generate battery alarms (audio and flashing lights) when I start the engines and they both stay on the charge cycle putting out 14.2 volts even though the batteries should be fully charged before being started. I have a Zantrex True Charge 20 hooked up to them. I also have to goose the throttles a little to get the tachs to work right after starting the engines.
I can silence the audio alarm but the light continues to flash. The wiring harnesses and gauge panels are brand new from Volvo. I didn't expect to have problems with them.
I put a post on the Prestolite forum but have not received a response just yet. I think there could be a possibility they are wired incorrectly or I need to modify the Prestolite wiring to make it compatible with the Volvo way of doing things.
If there are any wiring experts around that are familiar with Volvo diesel, these units have the following connections.
D+
B+ Battery output
B- Battery negative
AC
Separate connection for Excitation
Is the AC or D+ used for the warning light and which one is used for the tach?
I recommend that the alternators be upped to at least 65 amps. Your house bank should be two group 31 AGM deep cycle in parallel. The alternators should be connected to Balmar MaxCharge regulators with a Centerfielder dual alternator controller. The output of all alternators will go to the house bank only. The start battery (you only need one) should be connected to the house bank via a Voltage Sensing Relay.
I would agree to you should have a least 65 amps per engines , with an alternator that does well in the low end. As to the house batteries, I'd use those two 4 D's and buy a couple 27s or 31s as start batteries( those Volvos are pretty easy to spin).
Alternator AMP Rating 
