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post #1 of (permalink) Old 01-14-2005, 07:08 AM Thread Starter
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The right alternator

Whats the right alternator for a very slightly modified Cj7 with 4.2. I have a stock reman unit from Advance auto parts.
Which while running at night outs out about 13 volts per my volt meter..
Whem i hit the day lighters, BAM! go below the 12 volts.
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post #2 of (permalink) Old 01-14-2005, 08:06 AM
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Re: The right alternator

I'm considering one of these for mine:

135 amp

180 amp
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post #3 of (permalink) Old 01-14-2005, 09:13 AM
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Re: The right alternator

That's too low a voltage for even an inadequate one.

Try measuring between the (-) battery terminal and a body ground. It should say less than .1 volt with everything turned on.

If you have a bad body ground, even a 10,000 amp alternator won't help.
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post #4 of (permalink) Old 01-14-2005, 10:58 AM Thread Starter
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Re: The right alternator

Hey Rich,, Been a while.. Thanks..

given that i have a good ground, When shopping for right size alternator, what would be a good choice amp wise. i know they make these very high amp jobs. but i'm not sure what if any thing woudl i gain.
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post #5 of (permalink) Old 01-14-2005, 01:08 PM
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Re: The right alternator

Add it up. Normally a Jeep alternator is about 35 amps or so. It's adaquate as long as you don't have lots of accessories. A stock electrical system will draw about 20-25 amps with everything turned on. Whatever is "left over" goes to recharging the battery - IF the battery needs it. Otherwise the overage is not used.

Add it up.
Take the wattage and divide it by the volts
W/V = A 55W / 12V = 4.5A

Many driving/fog lites are 55 watts each = 4.5 amps each - allow 10 amps for a pair.
Big high intensity lites may be 10 each.
Stereos, amps etc draw heavy only on peaks, but look at the labels.
Allow 2 amps for your CB.

Winch? The alternator cannot keep up with a winch anyway - it will mainly serve to recharge the battery. A bigger alternator will help the battery recover faster, and it can help supply SOME of the power to the winch during use -- so big ones are helpful there.

THE MAXIMUM A BATTERY CAN STAND TO RECHARGE IS ABOUT 30 AMPS! More than that will kill it anyway -- that's why most automotive ammeters only go up to 30 amps. Too much charging current warps the plates.

So -- take the stock amount of 25 circuit draw, add up your accessories, add a few extra amps as a margin, then add 30 to that to charge the battery if it needs it. That's the most your rig needs. More capacity just goes to waste.

Remember -- the alternator supplies all the accessories and engine FIRST, then whatever is available can go to the battery - up to that 30 amps.

To get a battery to take more than 30 amps the charging voltage has to go way up - 18-20 volts? The battery has internal resistance that limits the current in -- Ohm's law. But 20 volts is very hard on lites, modules, and electronics.
You can measure that internal battery resistance. Put an ammeter in series with the battery cable on a low charged battery. Read the voltage at the terminals and the current going in. Use Ohm's law R=V/A.
Works for charging or discharging.

Then once you know the resistance, you can calculate how much voltage is needed to charge it with higher amps.
V=AR

The voltage regulator prevents that over-voltage, thus overcharging, from happening.

Now if you are planning to share it as a dual purpose welder, go as big as you can get for the money.
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post #6 of (permalink) Old 01-14-2005, 01:14 PM
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Re: The right alternator

My reason for looking at those alternators isn't for the maximum output but rather the capability at idle. My 63 amp has no problem keeping up above 1500 rpm but idling around town or a trail with the lights, heater, wipers and electric radiator fan pulls the voltage down under 11.
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post #7 of (permalink) Old 01-14-2005, 04:06 PM
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Re: The right alternator

The higher output types most likely won't help that.

There's 2 approaches -
Mechanical - the hard way - decrease the size of the pulley or increase the size of all the other pulleys. That way it spins faster. Probably not practicle.

Elecrical - first, check the grounds. Engine to battery, battery to body ground. A bad ground raises all kinds of troubles, not just charging.
Igf ity's a separate regulator, mounted on the splash panel, run a braided ground wire from the engine direct to the mounting screw on the regulator. And make sure the wires are in good shape - including the big output wire on the alternator.

Increase the field excitation current a little. Not difficult, but we need to know what kind of alternator and regulator you have.
And, if it's a GM conversion, how you have it connected.
That's a typical symptom of GM conversions into Jeeps - easy to cure.
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post #8 of (permalink) Old 01-14-2005, 05:27 PM
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Re: The right alternator

I have a 100Amp Delco....works like a champ, even at idle. I was running an electric fan....with that on, lights, and everything else, voltage stayed above 14 even at idle.

Don't need it now...it won't fit on my 5.7L SBC. It's for sale if interested.
post #9 of (permalink) Old 01-14-2005, 08:42 PM
 
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Re: The right alternator

RRich,

I'm very interested in hearing more about how to get my alternator to charge at idle, but I'm not sure what kind I have. When I did the rebuild, I bought a new alternator but can't remember what I have. It's got a two-blade style connector that plugs into the side and a battery connection and ground connection (posts) on the back. Is that enough info to tell you what it is?

My symptoms are that it won't charge at all until I get RPM up quite a bit (don't have a tach in it). Once it starts charging it shows 14 volts until it's at idle again, then the voltage drops some, but not all the way down to the 11 volts it was before it started charging. Does that make sense?

If I'm hijacking this thread, I apologize. I'm not sure my question exactly matches the original one!
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post #10 of (permalink) Old 01-14-2005, 11:02 PM
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Re: The right alternator

I checked my jeep with a meter tonight. I think I might have other problems. The voltage at the battery stayed above 12.4 volts at idle with the lights on, heater on high and radiator fan on. The dash voltmeter was showing just under 11 volts and the heater blower motor was seeing 10 volts. Seems like the main wire harness can't handle the load.
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