I've been struggling with battery life. I realized that it might be time to replace my alternator when I left my car running with the lights on and radio on in the cold when the battery went dead. Typically, I replace batteries every other year, and I probably put less than 200 miles a year on the car, leaving it in storage with the battery in my garage connected to a battery tender. My volt meter reads below 12 all the time. I had a replacement oem alternator installed with the thought that the original alternator was shot, but there was no difference.

At this point, I'd like to get a high output alternator. I've read about the Greg Brown and 928 Motorsports alternators, but haven't seen any comparison. Is it a toss up?

 

 

If a brand new modern design (made specifically for a Porsche engine bay) Bosch alternator with two internal fans (decreases "heat soak"), designed and built from the ground up to deliver 80 amps at idle and 150 amps by 1,500 rpms, fits perfectly in the stock location with the stock brackets, has a "free wheeling" pulley in the reverse direction (much easier on the alternator belt), is the same as a rebuilt, rewound (higher output than originally designed for) Chrysler alternator that has "adaptor brackets"....it's totally a toss up!

 

Actually, our current alternator is nothing like the one that Greg describes.

Our alternators are purpose-built for the 928 and feature billet housings and direct-bolt up installation. No adjustments, brackets or adapters needed. They produce 175 AMPS, charge at idle, and come with a one year warranty.

 

eugkim, because of the very few miles per year that your car is driven, be sure to replace your battery with a AGM type, not a traditional wet-cell lead-acid battery. I favor the Optima Red, but there are others. An AGM battery lasts much, much longer in storage than the old lead-acid batteries which sulfate their plates and drive alternators nuts.

 

I too cycle a battery tender between my cars since none of them really see many miles and I have a couple of batteries over 10 years old still in use. I tend to only buy Interstate brand.

All 5 of my 928's have an original alternator. Some could be the original but difficult to say for sure.

Is that below 12V reading at the battery or the jumper post under the hood or both?

 

First solve your alternator/charging issues if any, as the guys have described, as well as any parasitic draw while resting. If you don't solve those, your car is simply a battery killer. A battery tender is just masking unnecessary and potentially dangerous voltage draw while resting.

Then buy this: Optima Yellowtop H6. The H6 was added to the line and is a perfect fit for 928, and has more cranking and reserve capacity than any other battery I could find. Very pleased with mine.
(Note: Optima Yellowtops used to be Deep-Cycle only, but now are "cranking" batteries as well, like Redtops. Also, I looked and looked and could not find any price better locally than directly from Optima, and delivery was very fast and free.)
https://www.optimabatteries.com/en-u...cle-battery/h6

Finally, (and now I'm sounding like an Optima salesman, which I'm not) this Optima 400 recharger is incredible. It's small, "smart", can be used as a charger or maintainer for AGM(Optima) or Wet(lead acid) batteries, and its "recondition" mode has returned to life three wet batteries that I left for dead and now have used for over a year and going. Paid for itself ($89) with that alone. Note, it is not used as a "jump starter," it is slow and deliberate.
https://www.optimabatteries.com/en-us/battery-charger

-Jason

 

+1 It's a very good smart charger. We use one here, like it a lot.

 

Optima Red Tops were the starting battery and the Blue tops the Deep Cycle Battery. Then they came out with the Yellow Top, which is a hybrid between the two and what I buy now. I have Optimas in all my vehicles with a Starter, (except the Cayenne that needs an H9 size, which Optima doesn't make) - 914, 997s, pick-up, 928, boat, etc.

A Deep Cycle battery is designed to recover from being heavily discharged whereas a starting battery is not. When you have high draw loads on a battery like high output stereos, lighting, etc you can kill a starting battery fairly quickly, especially if at idle you're not generating enough amperage from the alternator to overcome the draw from the vehicle and accessories.

The red top optima in my 914 lasted 12 years and I just replaced it last fall. All the cars (except the truck which is driven multiple times per day) are plugged into Battery Tenders all the time when not being driven. As I replace these batteries, I'm using the new Yellow Tops - just did the one in the 914 and the 2 in the boat this fall. Pick-up is probably next as that one is now going on over 6 years.

I have one of Carl's 200 Amp alternators because of the 1,000 Watt stereo and HID lighting in my 928. Charging was a little slow at idle so I ground down the center of the alternator pulley so it spins faster for the same bell housing revolution and all is good now. I imagine the new one is even better.

 

Thanks for all the replies. Just to be clear, I remove the battery and use a CTEK battery tender. When I take the car out of storage in the summer, it fires right up. I've had the battery tested, and it's got a full charge. The longer I have the car out, the harder it is to start - each successive start takes a little longer until it's a few seconds before the engine catches.

I don't have a wild stereo, but I do have H4 lenses with 90/100w bulbs. When driving at night, the lights will dim a bit when I crank the A/C at idle. As I said, I left the car idling for a few minutes at night with the lights and stereo on and the battery totally drained.

The alternator that I had installed was an original Bosch - I bought it from Roger.

Only at the dash.

I feel like I've neglected the car somewhat. Unfortunately, it doesn't get used because I have 3 kids who drive (they were tiny when I bought the 928) so we've got too many cars sitting around. I got rid of my Cayenne and 996 Turbo because they were sitting too much. I just couldn't get myself to sell the 928. I hunted for it and travelled to get it. It's in great shape and only has 48k miles. This spring, I want to start using it, so I'm prepared to dig a little deep. The battery issue is the main thing, the A/C is the second.

If the feeling is that the Optima battery might do the trick, I'm game. From a practical standpoint, seems like charging is more of the issue.

 

Most of the time the issue there is connections in the gauge pod or the gauge itself. If you don't already have a multi-meter, pick one up (the $5 unit at Harbor Freight is fine). With the car running check the voltage at the jumper post under the hood and the battery.

When checking the one under the hood, try a few different ground points (chassis and on the engine) just to make sure you're getting the best reading.

 

I'd suspect you aren't charging at all - does your "(no) charge light" come on with bulb test?

Alan

 

Replacing the OEM alternator with another OEM alternator is:

Like taking your 1985 IBM PC with the 5 1/4" floppy drive and replacing it with another one just like it.
Like looking for a 1985 Zenith Color TV console to replace the one that just died.
Like continuing to send men into space with a slide-rule, after the computer has been invented.
Like replacing incandescent light bulbs with incandescent light bulbs.

Please update to newer technology when it is available. Science is good. The improvements are real.

 

But I would argue that if you are putting the latest and greatest into the old technology, then you are still constrained by the old technology. If the alternator as originally designed works with the vehicle for which - and when - it was designed, adding a new hi-tech alternator will not enhance anything to the underlying platform to which it gets connected. So yeas, if I have an 85 IBM with a 5 1/4" floppy, and that floppy breaks, the only thing that will wortk with the 85 IBM PC will be another 5 1/4" floppy, and so on.
In our case, the car is old, and all of its electronics are old, so what is the use case to have an alternator that has modern tech inside of it? Not trying to be a jerk, just want the thoughts of Carl, Greg, Alan and the other experts.

 

+1
The technology comparison is absurd. Something to be said about leaving well enough alone when it does the job it's expected to do.

Unnecessary upgrades are the biggest cancer in the automotive hobby. Talk to any seasoned mechanic, nothing frustrates them more than "upgrades" on cars brought in. More often then not, first step is put things back to stock and go from there.

Newer isn't necessarily better. In many cases far from it.

There is a place for upgraded alternators, not saying they don't belong at all. But this person is trying to diagnose a charging problem, go back to basics and solve the problem.