High Output Alternators

[Stormer_2003]

Can someone help me clarify some issues surrounding high output alternators? I''m not sure I see the logic in having one unless one does a lot of extended cruising where your battery bank is discharged by more than 50%. It seems with three stage chargers, the charging rate is almost always in the range of 15 to 30 amps. If that is the case, the standard 35 amp alternator should be fine.

The only time I can see when a high-output alternator is when the batteries are down at the 50% level. At that point, you might get at higher charge rate. So - if you are off crusing somewhere and cycling the batteries between 1/2 capacity and 3/4 full, you will need to run the engine less. But for someone who doesn''t deplete the batteries below 3/4 full charge - I don''t see it making sense.

Hope that was clear...

Do I have it right?

Thanks.

[svzephyr44]

In a word - yes. In another word - no. (Sorry, I just had to do that tonight!) If you don''t care how long it takes to recharge your batteries (because you only use them a little bit and then put them back on a shore power charger) then the answer is no - there is no need for a high output alternator. The more complete answer is - the output of the alternator needs to be matched to the size of the battery bank + the load you run during charging (e.g. navigation lights, helm instruments such as wind, speed, depth, radar, chart plotter; refigeration) + the amount of time you are willing to run the engine to charge the batteries + any other potential charging sources (e.g. Wind, Solar, Towed Generator.) Replacing the charge of 50% of a 50 amp hour battery bank takes a lot less time than replacing the charge of 50% of a 2000 amp hour battery bank. So you are really juggling four parameters: output of the alternator, size of the battery bank, how much of the alternator output is not available for charging as it is running other things and how much you deplete the charge on the battery bank.

Confession: Someone responded to one of my posts with the suggestion that I check out Nigel Calder''s Boatowner''s Mechanical and Electrical Manual (second edition.) Well, you know, I''m a guy and a trained engineer and have owned boats for years - what would be the point? But I took it out from the library and it is fantastic. Read Chapter 1 on "Establishing a Balanced Battery-Powered Electrical System. You will be glad you did.

Fair winds and following sea.
Zeph

[GordMay]

As Zeph said - yes & no.
There are several conditions that limit the output of an alternator, including: Alt’ Speed, and Batt. Acceptance.

Alternator speed is a function of engine speed and pulley ratio. Most Alternators are designed to provide full output at about 6,000 RPM, so you won’t get full output at anything under about 2,500 RPM engine speed (assuming 2:1 pulley ratio). Alt’ current output varies as the inverse of the square of speed - so a 35 A rated (@ 6,000 RPM cold) alternator, turning at 4,000 RPM (66%) will only put out a maximum of about 15 Amps (0.66 x 0.66 = 0.4356%) & (35A x 0.4356 = 15.246 A).

Acceptance rates vary proportionally with battery size. A larger battery (more A/H capacity) will accept more charging current. A small battery bank will not benefit (much) from a High Output alternator.

HTH
Gord

[sailnaway]

I was crusin this summer and I bought a new battery bank three batteries and an alt.
the batteries were two AMG 31 and a led acid 31 I know they should be the same but the led acid is isolated and not the house. Anyway I bought a Balmar with the brain it is an expensive little bugger,the problem with it is it never worked I left port and while it looked cool and showed voltage by that evening it was deadand so were my new batteries. Back on went the old 35 amp alt. I installed a disconnect on the cranking battery so I can put all the charg to the house batteries as the cranking bat needs very little charging. The Balmar may be very goog but why a company would make a system to charge only if the brain box works is beyond me it puts out no voltage at all without the box so if that dies so do your batteries. I returned the thing and have had no problems since and I went for almost two weeks without a dock.I am careful to always leave one battery full the cranking battery. The AMG batteries are great so far about three months old and holding well.