I've got an '02 WRX with a 4-month-old Optima Red-Top battery, the right one for this model (along with the slightly longer negative cable) and I'm starting to notice the following problem: once the car is warmed up and the idle is down to 750rpm, the headlights are noticeably dimmer and my battery voltage (as read from the ECU via a live ODBII scanner) drops to 11.4 or 11.2, and sometimes as low as 10.8. I can see my headlights visibly dim and grow bright as I step on the accelerator. I've only got 42k on the engine, but is it new alternator time? Or new accessory belt time?

I did a search on this and many called it "normal" behavior, but the car sucking voltage (instead of making it) at idle with the A/C turned off doesn't seem ideal to me. I can't wait to see how bad it gets with the A/C, radio and headlights turned on simultaneously.

 

experiencing same thing on mine. (02 as well with 58k miles)

recently changed belts, no change in the dimming of headlight issue. yesterday I put in some grounding cables and have noticed a slightly better voltage reading (on blitz TT) at idle. Nothing spectacular, but 0.3 to 0.5 volts better than before. Lights still dim as before (hard to tell if any improvement in that department)

 

And this is only happening when swapping to an Optima Red Top battery?

 

This is not normal behavior. You shouldn't be seeing this with a properly working alternator and fully charged battery. With only a minimal electrical load the alternator should have no problem maintaining the correct voltage at idle (13.5-14.5V). The only time you should see it drop below that is with a heavier load, such as headlights, A/C, and defrost on simultaneously. The alternator is not able to produce its full output at idle, so with enough accessories running it can be maxed out enough to bring the voltage down to battery voltage or even lower, but as soon as the engine is raised above idle the voltage should go back up. This is how my car behaves with almost 80K and a 3-1/2 year old Red Top.
I'd say have someone with the right equipment fully test your alternator and battery, and also take a good look at all the connections- battery terminals, grounds, etc. You could have a voltage drop someplace due to a loose or corroded connection. Battery terminals can corrode on the inside where it isn't obvious without actually removing them and looking.
And, exactly which Red-Top do you have? If it was really the "right" one for the WRX it would be the Group 35, Optima model # 35-910, which requires no modifications and is a direct drop-in. I'm assuming you have the "universal" 35/75 which has the relocated posts and requires a longer positive starter cable, not a negative cable. I have this same battery which was the only one available when I got it 3+ years ago, too bad or I would have gotten the 35 instead. Fortunately they are physically and electrically identical other than the different posts and the side terminals.

 

Cool........thanks for the very informative message

 

Yeah, I thought that was odd. When I swapped out the battery I checked both for corrosion; my previous two batteries were swapped out long before they went dead (I'm careful that way, AZ summers cook batteries to death) and so no problems with battery contact. I am using the #35 battery; the + cable (not the negative, my mistake) was just a fraction of an inch too short with the plastic battery tray in, so I swapped it out for one slightly longer.

It is going in to the shop next week for a look at the rear differential, so I'll have them test the alternator then. All the belts were swapped out at 30k, so those should be fine; I've followed all of the "extreme/heavy driving" guidelines for engine service due to the climate here.

I may add a grounding kit if I can find one in black so no one can see it; I personally think this is a voodoo solution, but looking at how poorly the 02 engine block is grounded (two of the grounds to the engine bay are on areas that have paint on them) it seems like a cheap enough solution. I'll be pissed if it is the battery at fault, these Optimas are extremely expensive.

I got the factory subwoofer kit for the car last week on eBay for $80, I'm glad I haven't installed it yet! My engine would probably die every time the bass kicked in.

 

Also check for loose connections, or something that might be draining the battery when you turn the car off for the night!

 

i just bought a red top now i am having the same problems with my 04 wrx...did anyone find a solution?