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Old 18-03-2014, 17:07   #1
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Question Alternator not charging issue

I'm hoping to get some ideas what might be causing this problem.

The boat is an 80's vintage Beneteau that has gone through several owners over the years. The engine is a Perkins 4108 and somewhere along the line, somebody replaced the stock 3 wire, 55 amp alternator with an over sized single wire 100 amp alternator. Who ever did the "upgrade" did a miserable job and I'm just trying to get it to work. The batteries are in 2 banks: one group 31 deep cycle and the other bank is 2x group 31 deep cycles in parallel. So total battery capacity is around 350 ah. The wire run from the alternator to the batteries is about 10 feet and my best guess is that it's 8 AWG (wire insulation diameter is just over 7 mm). With the engine running, the voltage at the alternator is 14.4 vdc, but the most I'm seeing at the batteries is 12.4 vdc. I've had the alternator and batteries tested and everything passes with flying colors.

I tried running the engine at higher RPM's (over 2000) to see if that might make a difference, but no luck. My guess is that the wire gauge is too small, but from what I've read, 8 awg should be fine.

Any other ideas? Who ever did this "upgrade" really did a horrible job. The alternator is too big for the battery compartment (so you can't tension the belt properly) and he snipped all the original wiring short so I'd have to re-wire everything if I decided to install a 3 wire alternator in the future. He also buggered the starter and instrument wiring, but that's a whole other headache that I've (at least for now) gotten to work reasonably well.
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Old 18-03-2014, 17:15   #2
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Re: Alternator not charging issue

First, try to find a shorter v belt so you can tension it correctly. Use a Green Stripe or Dayco belt also. A small increment in belt length seems to make a huge differnce though so try just slightly smaller.
For a 100 amp alternator... you should probably be using maybe #1 cable or so. Bigger the better. What type of regulator drives it? or is it just another internal regulator? If it's a Delco style internally regulated one those are pretty good... but I dont believe the 100 amp case is any bigger than the 45 amp case..? not sure.
Not sure you should see that much voltage drop in 120 ft even with just the 8 ga wire though... clean the ends very well by talking them off and cleaning them.
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Old 18-03-2014, 17:31   #3
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Re: Alternator not charging issue

It has an internal regulator. No name or label on it at all. Took it to an alternator shop and they bench tested it to 100 amps.

The installation is a mess. The bracket they made to hold the alternator doesn't really keep it rigid, and if I tension the belt, I can't close the compartment cover.

I might have some 4 awg or larger wire I can use for a test, but what about the ground? I don't remember seeing anything that big for a ground either. I'm more than tempted to dump it and replace everything, just really don't want to spend the dough on this right now.
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Old 18-03-2014, 18:01   #4
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Re: Alternator not charging issue

If there's that much voltage drop, either your batteries are pretty dang dead (sucking a lot of current) - or there's something wrong with the wiring.

Too small, corroded, bad connector, etccc...

#1 - you measure higher than battery voltage at the alt - i.e. the alt is working
#2 - Low voltage at batteries. 12.4 is pretty much a half-discharged battery

Based on those 2 facts, I'm guessing that wherever you're measuring that voltage at the battery is not connected to the alternator at all. Or barely so. Trace that. Look for any fuses, switches, etc... And yup, ground is the other half of the circuit. But if you can start a diesel, ground is likely hooked up well enough to let high currents through. Possibly you're getting excite voltage from another source (like an ignition switch, who knows what PO's do to boats)

So trace how current gets from the alt to the batts.. I'd guess that even with 8ga you'd pull the voltage to the 13's... (But that's a guess... didn't do any math or anything.)
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Old 18-03-2014, 18:08   #5
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Re: Alternator not charging issue

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Originally Posted by jeepbluetj View Post
If there's that much voltage drop, either your batteries are pretty dang dead (sucking a lot of current) - or there's something wrong with the wiring.

Too small, corroded, bad connector, etccc...

#1 - you measure higher than battery voltage at the alt - i.e. the alt is working
#2 - Low voltage at batteries. 12.4 is pretty much a half-discharged battery

Based on those 2 facts, I'm guessing that wherever you're measuring that voltage at the battery is not connected to the alternator at all. Or barely so. Trace that. Look for any fuses, switches, etc... And yup, ground is the other half of the circuit. But if you can start a diesel, ground is likely hooked up well enough to let high currents through. Possibly you're getting excite voltage from another source (like an ignition switch, who knows what PO's do to boats)

So trace how current gets from the alt to the batts.. I'd guess that even with 8ga you'd pull the voltage to the 13's... (But that's a guess... didn't do any math or anything.)
That's pretty much what I've been thinking. I'll have to dig around and see where that poor connection may be. The batteries are only 5 feet from the engine, but the wire runs through some spots I can't access for a visual inspection. As long as I'm getting 13.7+ vdc I'm happy, but at 12.4 it's running down the batteries instead of charging them. :P
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Old 18-03-2014, 18:34   #6
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Re: Alternator not charging issue

FWIW, 8awg will have 1.3v drop on 10' @100a. 4awg is .5v @ 100a
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Old 18-03-2014, 18:51   #7
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Why isnt the alterntor + conected to the startor solenoid on the battery side? Why run a separate cable from alt to batt? If your engine starts the wire size shouldnt be a problem?
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Old 18-03-2014, 18:54   #8
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Re: Alternator not charging issue

Single wire alternator conversions (on cars) don't come on until about high RPMs. Usually a quick rev will start it up assuming that it has sufficient voltage. I agree with your idea of switching it back to a 3 wire. It sounds like a lot less trouble.
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Old 18-03-2014, 18:59   #9
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Yea the alt on my engine requires a quick rev and then back to idle to start up
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Old 18-03-2014, 19:10   #10
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Re: Alternator not charging issue

With a slipping belt, it may not be getting the required RPMs to turn on.
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Old 18-03-2014, 20:40   #11
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Re: Alternator not charging issue

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Originally Posted by Raider Curt View Post
With a slipping belt, it may not be getting the required RPMs to turn on.
I thought of that, but the voltage at the alternator is 14.4+ vdc, so it -is- turning on, right?

Quote:
FWIW, 8awg will have 1.3v drop on 10' @100a. 4awg is .5v @ 100a
That's about what I thought, so I should be seeing at least 13.1 vdc at the battery. Could a dirty terminal be the cause for such a big voltage drop? I guess I'll have to clean it to see.

BTW, are there any spray on liquids that can be used to clean terminals? Or is emery paper the only way to go? How about a liquid to protect the terminals from future corrosion?
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Old 18-03-2014, 20:49   #12
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Re: Alternator not charging issue

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Originally Posted by prof_mariner View Post

BTW, are there any spray on liquids that can be used to clean terminals? Or is emery paper the only way to go? How about a liquid to protect the terminals from future corrosion?
Wire brush. Vaseline grease.
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Old 18-03-2014, 20:57   #13
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Re: Alternator not charging issue

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Originally Posted by d design View Post
Why isnt the alterntor + conected to the startor solenoid on the battery side? Why run a separate cable from alt to batt? If your engine starts the wire size shouldnt be a problem?
That's how the engine was originally wired, but the idiot that "upgraded" the alternator snipped all the wires short and ran a dedicated 8 awg to the alternator.

He snipped off about 8" at the alternator, about 6" at the starter and a couple feet at the batteries. The wires are bundled in a mesh sleeve, so pulling them and replacing them will be a PITA. Adding terminals to extend the wires is an option, but all those terminals are sure to add RFI noise and power loss.
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Old 18-03-2014, 22:00   #14
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Re: Alternator not charging issue

if you're sitting at the dock testing stuff your batteries are probably already full and your alt is probably only going to put out a few amps. so you shouldn't be seeing any drop even through the small 8 gauge. only under high amps. sounds like bad connection(s).

but yes the 8 is too small for when it is actually running at 100a. which with your small batteries will probably be never. run a big ground too. run the wires straight to the house bank. you should have something like an ACR to share the charge
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Old 18-03-2014, 22:03   #15
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Re: Alternator not charging issue

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Originally Posted by prof_mariner View Post
That's how the engine was originally wired, but the idiot that "upgraded" the alternator snipped all the wires short and ran a dedicated 8 awg to the alternator.

He snipped off about 8" at the alternator, about 6" at the starter and a couple feet at the batteries. The wires are bundled in a mesh sleeve, so pulling them and replacing them will be a PITA. Adding terminals to extend the wires is an option, but all those terminals are sure to add RFI noise and power loss.
all those wires are live and most likely unfused. and best to be removed. shorting againist the engine would be disastrous

with smaller alts going to the starter post is great. with bigger alts you're better off going to the house bank first. as 90% of it is going there anyways. then you only need to share a bit back to the starter battery. instead of running it all to the starter battery and then 90% having to get over to the house.
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