The Stovebolt Site Forums The Shop Area The Electrical Bay Alternator over kill!

I bought an 80 amp alternator a while back from Patricks.I haven't installed it yet,because I'am still lining up all the electrical ducks,Harness,switches,etc.But now I'am worried that I'll have a overcharging problem or my original amp gauge won't take it.I have a 51' GMC 3/4 ton,that I'am only adding fog lights,electric wipers and a modern stereo.According to Skip Readio in his book "How to do Electirical Systems"2nd edition page 17,he says my up graded alternator could turn my battery in to a bomb if the internal voltage regulator goes bad. Thanks jimmy

I don't think the 80 amper is overkill as long as you have all the electrical stuff sorted out correctly. Afterall the alternator only outputs the amps your system is requesting. The ampmeter is another issue. As you have changed the electrical system my guess is your desire is not to keep the truck orginal. I would change the ampmeter to a voltmeter for two reasons. The voltmeter will give you a better idea on what's going on (I sure some will disagree) and you lose that big gauge wire runing into your dash.

Any charging system, even a generator, can overcharge a battery if the voltage regulator malfunctions. Even a 200-amp alternator cannot supply more current than the battery can accept, unless the voltage control goes bad. The alternator will simply supply all the accessories' demand, and then use any leftover capacity to recharge the battery.

When I was running a truck shop, a local trucker brought me a "Skipper 600" (600 watt) linear amplifier to install for his CB radio. The FCC power limit is 5 watts, BTW! I used #1 welder cable to wire the power to the linear, and he took off from central California to New York on a turnaround trip. Two weeks later, he's back, saying "The radio talks fine, but my headlights go dim every time I key the mike!" Of course- - - -because the linear was drawing 75 amps, and he had a 60-amp alternator on it! I put a 200-amp brushless alternator on, and solved his problems!
Jerry

I want to keep my original amp gauge if possible.I just don't want to blow anything up! thanks jimmy

We discussed the problem a few days ago of an original ammeter pegging out when used with a high-output alternator, and more or less agreed that an extra shunt around the ammeter is a good idea. Since the original generator could barely deliver 25 amps to a 6 volt battery at full output, the ammeter will go off-scale for a short time after starting if a high-output alternator is installed. To avoid this situation, and possible damage to the ammeter, part of the charging current needs to bypass the ammeter. NAPA and several of the online parts stores offer such a shunt.
Jerry

Ignorance may be able to be fixed Hotrod, but at my age I'am afraid stupid may be forever when it comes to wiring. Sorry Jerry you probably get sick of answering the same questions all the time.I didn't read the thread covering this,missed it!I've been reading so much lately between books,threads and hear say from arm chair experts I think my head is going to catch fire!At this point I don't know a grunt from a shunt to a runtz.Off to the auto parts again! Thanks jimmy

Found it! The discussion got pretty technical, and everybody got a turn at expressing his opinion. Bottom line- - - -to protect the ammeter, it's best to bypass at least part of the charging current around the meter, and just use it as sort of a charge/no charge indicator.

https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=606818#Post606818

Jerry

Thanks Jerry! It was a bit over my head, but helpful.Is a shunt just a wire or is it a resistor? And before I get anything,what would I ask for at the auto parts? Thanks jimmy

I believe an 80 amp. alt. is going to slip the belt on a 6 cyl. engine. There is not enough contact distance on the pulley. I think a 55-65 would work better.
Like HRL says any alt. will cook the bat among other things if the regulator goes into infinity. It is very rare though. Usually when they fail they don't charge.

A shunt connects in parallel to the ammeter, and bypasses part of the current around it. Here's one that routes 50 amps past the meter, to prevent pegging it out at high current flow.

http://cgi.ebay.com/ebaymotors/Sail...rts_Accessories_Gear?hash=item4cec14e383

The main charge leads from the alternator to the battery would attach to the shunt, with medium-size wires attached to the small terminals, going to both sides of the ammeter in the dash. Now, the ammeter no longer reflects the total current flow, which isn't important anyway, but it does indicate whether or not the alternator is charging.
Jerry

Jerry,
That shunt in the ebay listing looks to me like it may have too little resistance to allow a Chevy's Ammeter needle to move. I think that the 50mV mentioned in the listing refers to the Full Scale Defection (FSD)of the moving coil meter it's meant to work with. So with this shunt in a moving coil meter's circuit, 50 amps current flow through the shunt would push a 50mV moving coil meter to it's full reading.

Because the Chevy truck uses a moving iron style ammeter, it has a much higher "raw" FSD (I measured 40 amps on mine with the needle hard over but not pegged), so a shunt with around .33 ohm total resistance still allows the meter to kick over without in pegging with a 63 amp alternator. I think I would go for a .25 ohm shunt if i was using an 80 amp alternator though.

Stuart.

Wrenchbender;
I've been running an 85 amp alternator on my '57 GMC 270 since I put the engine in,and it works fine. I used a new belt from NAPA when I installed,and set it to about 1/4 inch free play between the alternator and water pump pulleys. So far,the biggest load I've put it to is jump starting a Ford F-700 truck (391 V-8) that had been parked for several years for a friend. It still had it's stock (totally dead) battery,and took quite a lot of cranking to get fuel to the carb.
I've gone with the idea that you can never put too big of an alternator on-as was said earlier,no matter what size you install,the system only uses what it needs. I say it's better to have power you don't use than to need power you don't have.
Speed

And that's why my TV reception went flatline and my garage door opened every time that truck came down the freeway through Northern Nevada-it was YOUR FAULT!!
Speed

"Because the Chevy truck uses a moving iron style ammeter, it has a much higher "raw" FSD (I measured 40 amps on mine with the needle hard over but not pegged), so a shunt with around .33 ohm total resistance still allows the meter to kick over without in pegging with a 63 amp alternator. I think I would go for a .25 ohm shunt if i was using an 80 amp alternator though."

Hi there,

Can anyone share a specific formula for determining the shunt requirement of an ammeter? I'll be installing a 100A alternator in a '55.1 Chevy 3100, as well as a 75A in a '58 Buick shortly, and I'd like to keep the original gauges and not risk burning anything to the ground. Thanks.
Ed K.
Ithaca, NY