low voltage at idle with 160A alt & optima

I have recently bought a 1990 I6 Auto, and redid a lot of crap before ever getting to really drive it. A few of the things I did was a 160A at 2000rpm alt with 65-80A at "idle", a red top, & new 4ga ground and pos cables both directly to the alt and to the chassis.

Well at a idle with just the vent to on and fan lowest pos, and lights on running the volt gauge drops to the white line between the 14 and the 9. But with out either on or at anything above like 700rpm its at a hair below the 14v line.

Does this sound normal to other renix models? I am worried that when crawling and waiting in line while on a night run with the rock lights on and any heat at all, that the voltage will drop excessivly low? If I hit the rear defrost, lights, vent on low, and rock lights now at a idle it goes a tad below the mid white line between 14 & 9.

I did the wiring upgrade as well. The difference between the two from what i've read is on mine there was a third wire from ground on the Batt. to chassis (as well as the engine ground) run it to the rad header panel.
You may want to change out your 4awg for welding cables and 1/0 or 2 anyway, they have more strands. Hope this helps.

I been using work time to search this issue out.. and came across the fact that I didnt swap out the bat post to the main relay wire. I guess that could cause some issues? I will try that one when I get home. As for 0 ga that is just over kill I think? A good quality 4ga will carry 60A safely and even trans. a max 135 without issue for shorter periods. I know 0 is at 150/245 but I better never carry a 150A load, or even 60A load for that matter. Running and head lights = @ 12A, heater/AC = @ 17A, defrost = @ 15a, stereo = 2A, and Ign = what like 7A adn then misc = 5A? Then on sep wiring - rock lights and amp and misc....

whats funny is stock to relay is like 10ga from what I read this morning. thats rated at 15a/55a max, then take in to account the age and the terminal ends being so old. So that right there is prolly my issue.... the same volt gauge runs from that same wire circuit.

But I am hoping to get some input as to what others are reading from a Renix model with the lights on and vent on low while idling and maybe with the rear def on?

I don't see a problem, the volt meter reading has little to do with ALT amprage.
No matter how big the Alt. is it will only put out whats required to charge the battery.
Personaly I think a High output ALT with a AGM battery is a bad combination.
Optima batteries or any AGM battery will not take a high charge rate without damage.

I could have swonr I hit the post button.
Anyway, Mine reads low when the heater is blowing, but I checked it at the bat and it read 13.8, guage read about 12 (first white line)
I think that the gueage must share a circuit with the blower, and possible some other things, because turning on my blower, even on low, is an instant 2 volt hit to the guage. I've not don e any wiring changes. I'm hoping to get 5-90s wires sometime soon, but I'm witing until I get my 94 PDC grafted into the renix wiring before I order.

langer1 said:

I don't see a problem, the volt meter reading has little to do with ALT amprage.
No matter how big the Alt. is it will only put out whats required to charge the battery.
Personaly I think a High output ALT with a AGM battery is a bad combination.
Optima batteries or any AGM battery will not take a high charge rate without damage.

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well am I correct in thinking (usually not I find here).. That the battery will never see a "high charge rate" unless I use alot of power while the rig is off? I mean while running the alt is said to output 65A min and thats more then I will ever use. So the alt is running all the load Amperage and battery is just trying to keep the voltage up?

That brings up voltage though. Is it true that Alt no matter what amperage, will not help Voltage? And the only way to keep voltage up would be to add a 2nd battery?

I will check the voltage today when I get home at the battery when the gauge shows low. See what I get.

Have you checked the system voltage direclty at the battery or alternator output with a decent multimeter? The panel voltmeter has a funny habit of "reading low," since it's not tapped into the system very close to the sources.

Just a through - I'd takea voltmeter and test right at the source (engine running, of course,) and make sure there's actually a "low voltage" condition before I worried about it. It may help to take a metallic Sharpie pen, or similar, and make a new and distinctive mark where the needle rests, at battery voltage, provided that the Key ON engine OFF voltage is at least 12.6VDC...

5-90

I love Optima batteries for Boats, Motorcycles, ATV's, Standby generators, portable welders and personal watercraft. None of these have high output charging systems, often set unused for long periods of times, have a high amount of heat and vibration.
But your JX has none of these issues, the only reason you see them on all the jeep stores is because they are the only type battery that can be shipped.

langer1 said:

But your JX has none of these issues, the only reason you see them on all the jeep stores is because they are the only type battery that can be shipped.

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You are sooo wrong in so many ways! I got a Optima for the durability and most importantly, the all the time I plan on spending on the sides and lid of my XJ warranted a battery that will not leak and will survive! I could care less about shipping since costco sells em, and autozone for a Great price. Not to mention costco has a satisfaction warranty, so in 3-5 yrs if I have any issues with it I get a full refund to buy another from them or just get a diff type. THATS THE REASONS I BOUGHT ONE!

ROBERTK said:

You are sooo wrong in so many ways! I got a Optima for the durability and most importantly, the all the time I plan on spending on the sides and lid of my XJ warranted a battery that will not leak and will survive! I could care less about shipping since costco sells em, and autozone for a Great price. Not to mention costco has a satisfaction warranty, so in 3-5 yrs if I have any issues with it I get a full refund to buy another from them or just get a diff type. THATS THE REASONS I BOUGHT ONE!

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That fine for you but most people don't plan to ever be upside down.
I was referring to the DD most people have.
Extreme off road driving is a whole different thing, if that’s what you’re building for then a Optima battery is what you need.

System modifications..... for less resistance...

Adding to this discussion.....
Several months ago I built new battery cables using 2 ga fine stranded wiring along with fusion soldered battery and lug connnectors.... The positive cable that goes from the battery terminal to the bus terminal on the fuse block and the negative cable that goes from the battery terminal to the chassis ground are both 4 ga fine stranded wiring. The wire that goes from the alternator to the fuse block WAS 10 ga with a fusible link built in.... I replaced this wire with a 4 ga fine stranded wire and fabricated another fuse block terminal so that the alternator and battery wires are connected to the same fuse block terminals.... Then installed a 125A wafer fuse between the battery and alternator fuseblock terminals to act as a fusible link.
Normally run a Optima 1000 CCA Series 34 battery with the factory 117A alternator....
When I first start up the Jeep the voltage is 14.0V at the battery with the engine running and slowly drops to 13.6-13.7V after the engine compartment warms up. At rest.... the OPTIMA battery usually reads 12.5-12.8 depending upon how long the vehicle has been off and what temperature it is outside.
The installation of these upgraded wires have provided for faster starting and the alternator handling load changes quicker than before....
The OPTIMA battery provides the electrical system with a storage of amps while the alternator is spinning. The battery only comes into use when starting and when there is a quick heavy load on the alternator so the system voltage does not dip much.

Charles

Everybody reread 5-90's post. The voltmeter in the instrument cluster is a notorious liar. If you doubt this, try the following: turn on the headlights and note the reading. Now turn off the headlights and turn the heater fan on high. Mediate on which of these actually consumes more power. Take your reading at the battery.

ROBERTK said:

I am worried that when crawling and waiting in line while on a night run with the rock lights on and any heat at all, that the voltage will drop excessivly low? If I hit the rear defrost, lights, vent on low, and rock lights now at a idle it goes a tad below the mid white line between 14 & 9.

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What are the amp draw of all of these accessories? Add those up and you'll have your answer.

Also, pull an amp reading at your alt while at idle with and without accessories on.

You're answers will come shortly.

Your battery has nothing to do with your concern noted in the question, that is alternator related. A battery, as Charles stated, is a storage device for starting and super high amp draws (over and above the alt), but nothing more. If you are running amps over the alt's capacity, reduce some of the draws or get a yet bigger alt; relying on the battery in that situation is asking for trouble.

well not really asking about amps, I know its a by product of voltage to a extend, but more concerned with voltage. Voltage is pressure basically and amperage is the flow rate.

the lights dim with voltage drop Not amperage drop.

Robert,

You are running a new 160A alternator... with an Optima....
When you installed this new alternator.... was the pulley the same size, smaller or larger.... If the pulley was different in size... this could be where you problem lies....
Larger will turn the alternator slower requiring faster idle to bring up the voltage and amp load rating.
Smaller will turn the alternator faster raising the voltage a little and putting you into the higher amp load rating....
You can install a smaller pulley to increase the voltage and put you at a slightly higher amp load rating....

Charles

Hmm... Half marks.

While there is a difference between "voltage" and "current" (and "power" - but that's another discussion,) the two are related. System voltage is maintained by the voltage regulator allowing voltage to flow into the field coils, changing the output of the alternator.

However, the voltage regulator is NOT a "current" regulator - even though that's what it, indirectly, does. By monitoring voltage, it is able to assess the current draw on the system, and effectively make more current output from the alternator (by working to increase system voltage - think "voltage drop," only backwards.)

Pully size should not matter, as long as the armature & field coils are up to scratch, and the regulator is working. While an "underdrive" pully will take some load off the engine and the alternator bearings (and lower the output somewhat,) that should not change the voltage - just the effective current output peak.

Just something to think about...

5-90

Adding to 5-90's above, it should be noted that one of the main reasons that alternators superseded DC generators in cars is the ability of an alternator to be overdriven without damage. A DC generator will overheat and burn out if it is run too fast, and as the speed range of engines increased, it became increasingly necessary to underdrive the generator, with the result that most generator equipped vehicles charge poorly at idle. Because an alternator does not suffer from this problem, it is possible to design it so that it charges well at idle and the voltage regulator cuts it out at higher speeds. Most alternators are already somewhat overdriven, and will put out plenty of juice at high idle, so unless you expect to spend a lot of time running a winch or a snowplow at slow idle speed, there's nothing to be gained from additional overdriving.

Could also be because a alternator is much cheaper to build.

langer1 said:

Could also be because a alternator is much cheaper to build.

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No doubt it is now, but I don't think it was back in the early days of expensive semiconductors and cheap copper. There were various industrial and military alternators with huge selenium rectifiers for a long time, but it wasn't until around 1959 (memory poor, but around there) that Chrysler started putting silicon-diode alternators in cars, and it was quite a selling point for a while. I seem to recall that one of their publicity stunts was to pull the battery out of a Valiant or some such car and drive it coast to coast on just the alternator.

One of things not mentioned yet: Jeep idle is somewhere around 500rpm. The alternator idle rating may be for 800 or 1k rpm, and fall fast from there. It's way sketchy, but there was recently(couple weeks ago) a thread in the "electrical" section of the sounddomain.com forums where someone experienced this exact problem.

Contact the people who sold you the alt and ask them for a graph of the output of the alternator model you bought.