Replacing an externally regulated alternator, with a 1 wire internally regulated alternator

[okawbow]

Here's the story. I have an ancient Albin diesel with a dead Bosch 35 amp alternator. I sent the alternator to Spyder marine for a rebuild. They said it was beyond help. Spyder sent me a new Delco 94 amp marine alternator with the same mounting bracket.

The Bosch alternator had 4 wires leading to it. It also used an external regulator. The Delco alternator has 1 terminal marked Batt. It also has a ground lug. Obviously, I hook the large red cable to the Batt. stud, and put a heavy ground wire from the ground screw to the engine block. What do I do with the other 3 wires? Will switching these alternators cause problems?

 

[kenn]

If you're certain that the Delco has a regulator built-in,then what you suggest should work OK.

Assuming you have no further use for the external regulator, you can simply remove it and its wires.

 

[okawbow]

Sounds good. Spyder said the new alternator is internally regulated, and self excited.

Now I wonder if the tach will still work?

 

[Old Skool Neil]

I installed the same alternator on my Atomic 4 and it works great. I was particularly impressed with the low RPM performance, charging at barely above idle. Assuming your tach was driven by the old alternator, you'll need to have the alternator shop install a tach wire to one of the internal windings. It's an easy job so it should be cheap. If the pulley on the new alternator is the same diameter as the old one you probably won't need to do a calibration exercise - although it wouldn't hurt either.

You mentioned the "heavy wire." It needs to be a helluva lot heavier than what was on the original 35A alt. I wired mine with #4 cable figuring I would never tax the new alt to its maximum.

 

[Benny]

Very nice boats. You do not mention the engine's horsepower but being the fact that it had a 35 amp alternator it seems that a 94 amp alternator could be ill suited for the engine.

 

[Old Skool Neil]

The load on the engine is inversely proportional to the state of charge of the connected battery bank. If the batteries are less than fully charged, the alternator will put out only the necessary amperage to maintain ~14 volts (approximated). That output decreases (as well as the load on the engine) as the battery state of charge increases.

For example, if the batteries only require 15 amps due to their state of charge, the load on the engine will be the same whether the alternator is a 35 amp or a 100 amp. Either way, it's only putting out 15 amps.

With heavily discharged banks, the bigger alternator can decrease charging time dramatically but the higher the charge rate, the greater the load on the engine, albeit temporarily.

My point is under normal circumstances there's little difference between the 2 alternators as far as engine load. But when you really need some punch, you've got it with the bigger alt.

 

[Maine Sail]

Sounds good. Spyder said the new alternator is internally regulated, and self excited.

Now I wonder if the tach will still work?

With a two wire alt, and no AC Tap, it will not work if it was originally sensed off the tach, unless you have them re-wire it. It may also not have the same number of poles as the original so your tachometer reading may be off anyway. Ask your supplier how many poles in each alt..

 

[Maine Sail]

My point is under normal circumstances there's little difference between the 2 alternators as far as engine load. But when you really need some punch, you've got it with the bigger alt.

With a bank of 400 Ah you could be asking for all a 100 amp alt has. Basic rule of thumb is that wet cell lead acid batteries can accept roughly 20-25% of their rated Ah capacity in bulk charge mode. The big concern is the belt and can it handle the 100 amps if the op has a bank that can accept it.

100 Ah bank = 25 amps
200 Ah bank = 50 amps
300 Ah bank = 75 amps
400 Ah bank = 100 amps


P.S. If he has gels or AGM's the numbers above don't matter as AGM and Gel batts can accept 40-100% of the Ah capacity..

 

[okawbow]

Thanks for the info, everyone. The motor is 20hp, and my batteries are 2, 27M, 575 cca.

I need to update my guages and tach, so I'll stop by the alternator shop.

 

[larryw]

Lots of advice here; ask the shop if the internal regulator is a "smart" 3-stage regulator, or does it function like an automotive regulator that turns the alternator off much too soon for any real charging.