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  #1   IP: 73.35.200.244
Old 08-17-2020, 01:29 PM
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MMI 120A single wire Alternator and Firefly batteries

Hi all. I recently got a smokin deal on 2 firefly group 31 batteries. We cruise and anchor alot in the summer and really like the batteries.
things seem to be operating well. I have a couple questions for the group to see if my setup needs tweaking

i have the MMI single wire alt. it is the 120A model? excite lead is to the positive side of coil. MMI coil

electronic ignition

i use a dedicated start battery group 27 deep cycle.

i have a blue sea auto ACR

xantrex link lite battery monitor with shunt ect.

alternator wire goes to house bank. 2 group 31 fireflys mounted paralell

pro mariner shore power charger 2 bank

after being on shore power, the volts jump right away, to 14.2 sometimes 14.3
if we are on the hook and the house bank goes to 80 percent depleted, i torch off the motor and it does take 30 seconds or so for the alternator to fire off. it will charge at around 44 to 45 amps at 13.8 or so. as batteries fill this creeps up to 14.0 or so.

the information that i am finding on the fireflys is so confusing and varying that it makes it impossible to figure out anything. my shore charger is the bulk and absorption rates for firefly batteries according to the manual. so im just going with that for now. i really want to keep the system reasonable as this is not the forever boat.

any comments? advice? should i run the alt wire to starter? excite wire to starter? external regulator? leave it alone because it works fine?
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Last edited by Boat; 08-17-2020 at 01:34 PM. Reason: typo
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Old 08-18-2020, 02:39 PM
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45-55 amps is the most anyone seems to get out of any alternator unless they make a custom setup driven from the crank. You are probably doing pretty well as is.

If you read this thread you can gain some info:
http://www.moyermarineforum.com/foru...ad.php?t=11470
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Old 08-18-2020, 09:21 PM
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Agree, sounds like you're doing well. House bank & start battery separate, etc.
Your charge voltage could be a bit higher, which would raise the current. But you're limited by the internal regulator. The 30 sec delay is a surprise to me - usually you don't get that until you go to an external regulator.
I think you could improve your charging with an external regulator. Keep an eye on eBay for a Balmar ARS-4 (older technology but effective). Mod to the alternator to remove the internal regulator and bring the field lead out isn't a big deal. I did this to a 60A Delco that I bought on eBay.
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Old 08-21-2020, 12:25 AM
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thanks guys, i know the alternator is not the most ideal one. a week after we got the boat the original one grenaded. i found this site and got a new one figuring more is better right? lol i am looking for an external regulator and then do the mods to the current one.
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Old 08-21-2020, 01:24 AM
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Quote:
Originally Posted by joe_db View Post
45-55 amps is the most anyone seems to get out of any alternator unless they make a custom setup driven from the crank. You are probably doing pretty well as is.

If you read this thread you can gain some info:
http://www.moyermarineforum.com/foru...ad.php?t=11470
very true. after being on the hook a few days and then start the motor, the belt does alot of screetching. it gets under control with RPM around 1000 to 1300 or so.
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Old 08-21-2020, 07:35 AM
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Quote:
Originally Posted by Boat View Post
very true. after being on the hook a few days and then start the motor, the belt does alot of screetching. it gets under control with RPM around 1000 to 1300 or so.

Screeching at startup, when the charging load is highest, is a pretty good sign of a worn belt. Replace it. Our 120A alternator puts out about 60A when starting with depleted batteries, which quickly tapers off to 30-40A. After 200-300 hrs of engine runtime, the belt will begin screeching at startup. A new belt cures it 100%. I always carry a spare.

Our big alt is only running at 50% of capacity (max), and even less normally, so it should run cooler and last longer.

Our external Balmar 612 regulator also has a "soft start" feature which ramps up the load slowly instead of slamming it on all at once, which helps the belt not squeal and last longer.
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  #7   IP: 137.103.82.227
Old 08-21-2020, 02:30 PM
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Quote:
Originally Posted by Boat View Post
thanks guys, i know the alternator is not the most ideal one. a week after we got the boat the original one grenaded. i found this site and got a new one figuring more is better right? lol i am looking for an external regulator and then do the mods to the current one.
Adding a regulator to 10si based alternators is a bit of a pain, their native setup has the regulator in the negative lead from the field.
I kind of kluged a way to do it myself, but if I were doing it again I would start with this:
https://store.alternatorparts.com/partnod7044.aspx

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Old 08-21-2020, 10:21 PM
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Quote:
Originally Posted by edwardc View Post
Screeching at startup, when the charging load is highest, is a pretty good sign of a worn belt. Replace it. Our 120A alternator puts out about 60A when starting with depleted batteries, which quickly tapers off to 30-40A. After 200-300 hrs of engine runtime, the belt will begin screeching at startup. A new belt cures it 100%. I always carry a spare.

Our big alt is only running at 50% of capacity (max), and even less normally, so it should run cooler and last longer.

Our external Balmar 612 regulator also has a "soft start" feature which ramps up the load slowly instead of slamming it on all at once, which helps the belt not squeal and last longer.
the belt is fairly new (May) however it is the 26 inch. even with the longer bracket i cant seem to get it as tight as i would like 1/2" play with my finger. I tried a 25" belt but seems like it will go with some coercion but didnt.
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Old 08-22-2020, 09:36 AM
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My alt is not the Moyer one (Mando?). I believe its an Amp-Tech. It has a slightly smaller pulley. I use a 25" belt. If its too hard to get on, get it started on, then bump the starter. It'll walk right on.

I have also found that, because of the small curvature radius of the alt pulley, the notched belts last much longer than the solid ones.
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Last edited by edwardc; 08-22-2020 at 09:38 AM.
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Old 08-22-2020, 08:00 PM
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From an old post:

I have a Balmar and a Motorola alternator. The standard A4 belt that the Motorola uses is too short for the Balmar and the next bigger one is kind of loose even at full adjustment
I was getting annoyed by this and decided to really cram the short one on. I managed to get it halfway on and it was stuck. Not coming on or off
It was fully on the alternator and half on the engine pulley. The part that was on was towards the outside of the engine. I decided that I would crank the engine to get the belt to pop off and go to plan B. Well to my happy surprise it went right ON instead and once in the pulley has about 1/2" slack if you press on it
So far working great and no slipping.
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