I have noticed lately that my voltage drops pretty significantly while stuck in traffic. It drops so much that it makes my XM radio shut off every now and then. Voltage varied from 14 v while cruising down to 11.2 v while at idle and stuck in traffic. What is the normal voltage while at idle?

I had the Vishnu power pulley on and I just removed that and replaced it with the stock crank pulley. I also had to replace the battery today with a new one... the parts store said the old one had a bad cell. I drove around with the new battery and I am seeing the same damn thing on my volt meter while stuck at a light or parked in my driveway.... 11.8 volts! What's going on here?

Assuming stock pullies and a good battery and alternator, the voltage at idle when at operating temperature should be 13.5-14.5 volts at the battery with minimal or no loads turned on. The alternator does not produce its full output at idle, so if there are enough accessories running it will max out and the voltage will begin to drop. I've seen this happen with AC + headlights + defrost, for example. Once above idle again the voltage comes back up to normal.
If you're seeing battery voltage or less at idle with no accessories on, the alternator may be weak, or the new battery may not be charged.

The only recent mod that I have done was to hardwire my X50 Passport radar detector off of my interior light power source above my rear view mirror. I was suspecting that I may be grounding out there, so I reinspected the wiring and found nothing.

Maybe my alternator has been working overtime, who knows. It may have because I have had the Vishnu power pulley on my car for the last 5,000 miles and it may have taken it's toll on the alternator. I have replaced it with the stock crank pulley but I am still seeing the same voltage drop when I let the car sit in the driveway.

I set my "test conditions" by having the A/C at full crank + radio on + headlights on + brake depressed (turning on the brake lights). This represents conditions that are experienced while stuck in traffic in Florida. My car still drops down below 12 volts with the brand new battery. So, I'm pretty sure that it's the alternator.... but it's only a couple of years old and its a japanese product. :confused: The Vishnu power pulley isn't that underdriven right?

Well under those test conditions I'm not surprised that the voltage was low. As I explained above this is normal even with a fully functional alternator and battery. As long as the voltage rises to the proper level of 13-14V above idle there is probably nothing wrong.
I don't know the underdrive ratio of the pulley you have, but given that the alternator is already marginal at idle with the stock pulley any amount of underdrive is bad and will only make things worse. If you must have an aftermarket pulley you're much better off with one that is lightened but not underdriven.
Aside from doing your own voltage checks, you can have a shop test the alternator more thoroughly to be sure it is producing its rated output.

Yep, like I said- the Vishnu power pulley has been taken off. What a waste of money I guess. As far as how much it's underdriven... it's about 10mm smaller in total diameter than the stock crank pulley. I guess that's enough to do some long term damage. Damn. Thanks for the help though.

there is no long term damage from the UD pulley.

as mulder keeps telling you, it is not surprising that your voltage was low under those conditions.

you can get a smaller alt pulley and put it on to overdrive it if you want to keep voltage higher at idle. the gfb set uses one.

I'm guessing that a battery purchased at the "parts store" is not an Interstate or a Die-Hard? I can't think of another brand off hand that'd I'd run in a car outside of an emergency situation. A good battery is worth HP. People will spend tons on aftermarket ECUs and power upgrades, then run a Delco battery or whatever autozone carries. Ug. If you are running a battery that was less than 80 or 90 bucks (A good Interstate at distributor cost is around $75 I think, so try to find an employee buddy to hook you up) then you are just getting what you pay for...

I don't know if this is your case, I'm just typing this out for the general audience here.

You have XM, is that the only stereo mod? I'm running 7 channels worth of Xtant amps, with a cumulative rating of, lets see, 900 watts (granted most are gained way down and I'm not boomin' with a trunk-o-funk like a teenager.... much ;) ) And I have a PC almost constantly charging (because I'm a forgetful idiot) So I'm taxing my Red-top (bad choice but it sure is pretty) and my alternator.

The battery, dispite the abuse, will take a very healthy trickle charge off my smart charger and come back to 100% pretty well. It's about a year old. Sometimes it has to trickle a looooong time, more than just overnight. But the results tell me it's still chemicaly stable. Now the alternator, which is still stock, is probably looking at a limited lifetime now. I'll see just under 12v on my autometer gauge (take the grain of salt with that data) at idle w/o AC, with or without lights and with the stereo playing, but not cranked. It'll slowly drop and hold at about 11.5 if I make the music obnoxiously loud.

So after all this yammering... what do I have to say. That my adderall is really kicking in and that you might want to research a: better batteries and b: a new and/or imporved alternator. It's on my short list. After a nice long trickle, my car feels soooo mcuh peppier. I try to remember to do that before each auto-x.

I purchased the Red Top Optima, tried to put it in, but the positive cable wouldn't reach. So, I reluctantly purchased a cheaper autozone battery that would fit. I'm thinking about returning it today (it said "made in mexico") :( No offense to mexicans though. :D

I have no "system" and I never will. My sound system is my exhaust. I didn't know that the alternator would not be able to charge enough when the car is experiencing typical traffic jam situations. That's interesting... almost a design flaw. Thanks for the input.

What is it about Optima batteries that you don't like? What would you recommend other than Interstate?

I purchased the Red Top Optima, tried to put it in, but the positive cable wouldn't reach

Should have done some research here before you returned it. You got the "universal" 35/75 which is what the stores sell, because of the terminal locations the positive starter cable is about 4" too short. Ideally you'd want the standard Group 35 which is an exact fit but is only available online in most places, however to make the 35/75 work all you need is a $5 cable from the parts store. I did this when I bought mine, at the time the 35 wasn't available yet.

Excuse me for not doing enough "research". I went to the only place that sells Optima batteries and they only had one in stock.... the one that didn't fit. By oem battery wouldn't provide enough juice to start my car after they tested it, therefore I had to go with what would work at the time. I've tried interstate but they are not open on the weekends here. We all would appreciate it if we could cut all the arrogant bs and limit our input to information that is constructive. If you didn't mean to come of that way... well you do.

I'm not being arrogant, not at all. The reason I said that is that there is a wealth of information here on exactly this issue, dating back several years. Some of us had to figure out how to make the 35/75 work, and afterward posted detailed threads explaining how to do it so others would be able to use this battery. At the time the "drop-in" Group 35 Red Top was not available so this was the closest thing to an OEM replacement if you wanted a Red Top.
I do understand, however, that in your situation you did not have the luxury of time to look into the matter beforehand. If you'd still like to get rid of your "el cheapo" Autozone battery and get a Red Top that will fit with no modifications, there are a number of online vendors who sell the Group 35 for around $100 shipped. I'm not sure about the return policies for batteries though, usually electrical items aren't returnable.
Here is one site that has them-
http://www.remybattery.com/350/shopexd.asp?id=5437&catid=171&cat=Red+Top%3Cbr%3EStarting+Batteries&subcat=166&L2=&L3=

I used to live in New York for a small while and some of my friends are from Jersey so I know how it is with youse guys :lol: We just communicate differently. But seriously, thanks for the info. I toured around today trying to get rid of my cheap $75 battery from Advance Auto and find the right Optima red top.... no luck. I am just very limited as far as time goes because I am about to fininsh up with flight school here in the Pensacola area this week.

I don't think you'll find the 35 in a store anyplace. They keep the 35/75 in stock because it fits more cars and is therefore more worthwhile for them.

What is it about Optima batteries that you don't like? What would you recommend other than Interstate?


Just that once they start getting a bit drained, they don't seem to want to come back to a full charge with most stock alternators. I don't know if bigger alternators would really help charge them better, but they might not get overwhelmed as easilly with a little more amperage available. Without a tax on the electrical system though, the redtop is fine.

In your case though, I'd lean towards the interstate Megatron. Just the sandard Megatron and not the Megatron Plus for similar reasons. You just don't need the Plus. It's overkill. The standard Megatron takes a charge very easilly and holds it very well. It's an easy, reliable battery to get along with.

The other battery I'd recommend, although it's been a decade since I've used one, is the Die-Hard. They always seemed to hold up very well, especially in over advanced, high compression V8s that could be a bear to start once the engine was warm.

Good info, thanks.