Alt Output Current Wire Size Question(s)

[dcicirelli]

In the threads that I have been reading there are some suggestions to move to a larger Alt if you add batteries. I have a set of 3 Grp 27 batteries and one Grp 27 battery (isolated as a starter battery as recommended). I went from the (1 and 1) to the (1 and 3) configuration and I did not change anything else. Questions: 1, If I had gone to a higher output alt, would I need to replace the 10 AWG wire going to my alt meter and then to my battery. I know that the 10 AWG is rated around 30 AMPs so I was wondering why there was no mention of changing this wiring when going to a110 AMP HR alt. QUESTION 2, after aprox 20 hrs of operation my amp meter became eratic going from 0 -30 amps. I "tapped" on the regulator and that stablized the output to just over 0 amps which I considered normal, but the next day the current stayed high at 30 amps and no amount of tapping (I know, not the best fix but hey what do you want from me...) would change the draw...so... I disconeected the orange wire from the alt at the alt meter and ran like that all day. Question 3, Could I or did I damage my alt? When I reconnected it There was no indication of current draw.

Thanks,
Dave

[Al Schober]

Dave,
The reason for going to a larger alternator is to cut down the engine run time required for recharging. But, there's no way you're going to put 110 amps into your batteries. First, the start battery will never get run down. Second, the 3 house batteries will accept (at most) about 25 amps each if fully discharged. My house bank is 2 Gp27s, and my 65 amp alternator never sees even 50 amp output.
Whether your 10 AWG wire is adequate depends on several things; such as the actual current you're putting out, how far it is from the alternator to your house bank, and whether your regulator is sensing voltage at the alternator or at the battery bank. Don't forget that the ground return wire also figures into the voltage loss equation.
What are you running for a regulator? The days of 'tapping the regulator' went out 40 years ago when things changed from relay-type regulators to the solid state ones. Tapping a transistorized regulator does nothing - making me think you have something else going on.
Do you have a digital multimeter? Recommmend that you take some measurements while charging - voltage at alternator, voltage at battery, voltage drop from alternator + to battery +, voltage drop from battery - to alternator case. Should be instructive. One thing I found was that I needed a ground strap from the alternator case to the block - the mounting foot of the alternator was giving erratically high resistance and significant voltage drop.

Al

[tenders]

True a big alternator will never hit capacity while charging the batteries but if you're planning to run an inverter or a windlass with the engine running you might hit 50-75% of a big alternator's theoretical capacity, which probably exceeds what you'd want running through #10 wire.

Around that point the stock belt becomes the limiting factor--there's only so much energy it can transfer before slipping. If you upgrade the belt with something wider or add two belts, which of course require different hardware on the alternator and engine, I would definitely err on the high side with the wiring too.

I have a 110A alt on my engine, stock belt WIDTH (the larger alt requires a slightly longer belt), and I put a #8 wire on the output. It may even have been #6, I don't recall exactly, but it is the third-beefiest wire in the engine compartment after the starter wire and the ground to the batteries.

The inverter does get taxed when I run a vacuum cleaner (short bursts; frequently) or heat stripper (long runs, infrequently) in season while on the mooring.

[ndutton]

One of the reasons for going to a beefier alternator is to charge without maxing it out. Consider the relative ease of a 100 A alternator putting out 30 amps operating at 30% as compared to the stock 35 amp alternator putting out the same 30 amps operating at 85%. In either case, the load on the engine is roughly equal.

Wire length is also a factor and cockpit mounted engine panels with ammeters can have a current path of 50 feet or more before the current reaches the battery. A heavier wire gauge compensates for this length and associated inefficiency (voltage drop). This is in part why many of us have dumped the ammeter in favor of a voltmeter.

In my case, my 100 A alternator is a single wire connection, one #4 wire less than 6" in length to the starter post and #4 battery cables from there through the battery switches to the batteries. It's not that I think I need that gauge all the time but with an upcoming windlass installation I'll be set if I ever do.

As always, this all depends on your particular power needs. I need a lot.

[hanleyclifford]

If you are running 4 group 27's you have stepped up to the big league, electrically speaking. The direct ammeter needs to go. Neil got rid of his. I believe both ammeter and voltmeter are desirable. Consider going to the shunt type meter. #4 wire is good for the 100 amp alternator but remember the negative path back to the batteries must also be #4 or better. I like the Blue Sea stuff but there are other options. The picture shows my current set up. BTW, I have a multi unit that might be sold if you're interested. I took it off becauseI like to see both volts and amps simultaneously.

[sastanley]

I just relocated my ammeter into the engine compartment between the alt. output & the battery isolator, while adding a voltmeter in the gauge pod. The thought there was, if I really need to know how many amps I am putting out, I can do so (without lengthening the charging circuit!!), but as long as the volt meter in the cockpit says 14 or so, I am confident the alt. is working.

My new set up has a ammeter & volts in the nav station panel..I suspect I'll ignore the ammeter for now, and maybe some day install something else in there, unless it is a shunt type & I may hook it up.

[ArtJ]

IF you really want to know what the current is get one of the blue seas
current shunts which goes in the negative lead nearest the batteries
or where they meet at the a/b switches. The voltmeter Calibrated as
a Ammeter can be located remotely with small gauge wires, but viewing is difficult in direct
sunlight. Definitely remove the old ammeter which required the current
to flow all the way to the cockpit, besides the wire gauge could never
handle the current load .

Regards

Art

[CalebD]

...snip...
...so... I disconeected the orange wire from the alt at the alt meter and ran like that all day. Question 3, Could I or did I damage my alt? When I reconnected it There was no indication of current draw.

Thanks,
Dave

I think it is possible you could have fried the diodes in your alternator. On some engines a wire connection is made from the alt. to a tachometer. I think it really depends on what type of alt. you are using; self regulated, external regulator etc. A good resource for wiring and engine alternators is the Nigel Caulder book, "Boat owners mechanical & electrical manual" http://www.amazon.com/Boatowners-Mec.../dp/007009618X
If you disconnected the main positive (+) charge output I'd expect that to be bad somehow.
I've seen the diodes get fried in an alternator when the batteries were hooked up in reverse polarity.
For your sake I hope everything is fine.

[hanleyclifford]

From the description in post #1 it seems likely that something was going wrong before you disconnected the wire. You should take the alternator to a shop for testing.

[dcicirelli]

Many Thanks and Final (almost) Results

First of all thanks to all of you for your informative responces. Heres is what I have found out. I had the original 1976 35 amp motorola alt so I can not complain about service life, and in discussing the use of the battery switch with my co-owner, we both agreed we had probly moved the battery switch with the motor runinng. I talked to Dom Moyer and he indicated this was the begining of the end for my old unit. I purchased a 55 amp unit and installed it this week in the original configuration. I is charging the batteries thru the alt meter and seems to be working fine. I do not have any large current draw on the motor the house battery bank is for lighting and a 2000watt inverter used for as microwave and an expresso maker (very important to have hot donuts with your coffee!). That said, I like the idea of increasing the wire size to the batteries and changeing the amp meter in the cockpit out for a volt meter and having the amp meter (non-series type) in the cabin. As for how much laod a dead battery will place on a charger I would like to know where that info came from I was looking for it with no luck. Anyway after the install of my new alt the RPM gauge did not work and the temp gage was maxed out. Most likly unrelated to anything but my fat fingers.... we'll see.

Thanks again for the information, I will start making my material list for rewiring direct from the alt to the batteries.

Best Regards
Dave C