Mondo 50 amp alternator

[prh77]

Just installed this unit as Motorola was getting tired. It has a "sense" terminal that gives the internal regulator voltage input. This terminal has a jumper to the B+ terminal. If I were to wire the sense terminal directly to the house bank ( where the B+ terminal is wired ),the regulator would "see" actual voltage delivered to the house bank and adjust as required. I know in a perfect world, voltage at B+ and the pos term at house bank would be very close, but are there drawbacks to wiring as I describe? Only problem I see, is if the sense wire broke, alternator would run away. Any thoughts?

[Stu Jackson]

Peyton,

I assume by "B+" you mean the Alternator Output (AO).

What you suggest would work, but all you're doing is connecting what is now a very short wire, to make it a long wire, which will simply connect to the other end of the AO (B+) wire.  Electrically it's just the same. So why bother?  You should only have a 3% voltage drop in your wiring, so it's probably oversized even with #4 for a 50 Amp alternator whose output would be expected to be 25 to 30 A max.  You'd go from 14.8 V to 14.7 V not enough of a difference to the regulator.   

What's happening is that the "sense" is essentially reading it's own output.  Sometimes it's called an "excite" wire."  Same thing.

If the batteries are low in voltage, it will up the output.  As the bank gets full and the voltage rises to the regulators simple single setpoint, it'll cut back on the alternator output current at a constant voltage (the pre-set voltage).  Once the voltage is met, the battery acceptance alone will reduce the current and "push back" on the AO.  The AO current keeps trying and on long motors you can overcharge your bank.

That's how internal regulators work.  They're called tapering chargers.  The taper comes from the battery acceptance as the voltage rises.  Which, unfortunately with tapering chargers, means the current dramatically decreases pretty much right away.  And it's current you want, which is why internal regulators are not the cat's meow when it comes to new alternators.

For external wiring and a wiring diagram of the back of old OEM Motorola alternators connecting to new external regulators, see:  http://c34.org/bbs/index.php/topic,4548.0.html

[prh77]

Thanks for the quick reply Stu, I guess my issue is the voltage drop between alt and batteries. I have more like .4V drop. I guess if I am going to the hassle of running a longer sense wire, I should just run a larger AO wire.

[Stu Jackson]

Absolutely.  Since you want all the "juice" going to the batteries, the size of the AO is essential. 

I've reported on the battery acceptance rate of a 360 AH bank in the Acceptance topic (here: http://c34.org/bbs/index.php/topic,4787.0.html), and others.  Coupled with the fact that the alternators have been reported to only "produce" about half, if maybe just a little more than half, of their rated output, and fully confirmed with our old Spa Creek Auto Mac ammeter, you should be looking at maybe 25 to 30 A out of your 50A alternator with internal regulation with a half full (i.e, an "empty") house bank.  That's at startup for a tapering charge regimen, the amps drop off alarmingly after just a few minutes. 

#4 (4 AWG) wire is good for 30 to 40 amps up to about 32 feet, so if your negative grounding system is adequate, the #4 OEM wiring should be OK for you (3% voltage drop table).  [Remember the distance is "round-trip" ] The next sized wire is #2 and it's good for 40 to 70 A for the same distance, which could be considered overkill, but which is what I installed on Aquavite, because we have a 100 A alternator with external regulation and we see it producing about 50 A after two nights on the hook.

If your AO is sized adequately, then extending the field wire to the batteries is not necessary.  I haven't seen it done.  And it's not done on the new external regulators either.

[jmnpe]

Hi Peyton,

While extending the sense wire may give you some incremental increase of the working voltage you will see at the batteries, there is another source of voltage drop that often goes unnoticed and is often a bigger problem than the size of the alternator output feed. The "engine ground" point where the ground wire to the batteries originates is located in a terrible spot for maintaining a clean connection. On every older boat I have looked at that has not had previous electrical system upgrades and/or TLC applied, the ground connection on the engine is generally seriously corroded, especially if it has been operating in salt water.

When I went though our boat electrically, I moved the common ground point off of the engine and to a Blue Sea Power Post about a foot away, then ran a single large ground cable from the common ground Post back to the engine after thoroughly steel brushing and cleaning the attach point on the engine. I then sealed the very tight ground connection on the engine with Boeshield T-9. That process all but eliminated any significant voltage drop at the ground connection. Also, any alternator should have a dedicated suitably sized ground cable from the generator case directly to the common ground point: depending on the numerous nuts, bolts, and movable elements to get the case back to the real ground point is folly.

John
1988 C34 hull 728, standard rig
Otra Vez

[prh77]

Very good point,here I am concerned about the positive side and I have never checked the neg.

[Stu Jackson]

The OEM alternators did not have a separate ground wire, as John said, they used the bolted connections.  Our new Blue Circle alternator is now grounded with a separate ground wire.  We, too, used a bus bar to "collect" the grounds before connecting to the engine block.