2007CalSpec

Hey guys, yesterday I noticed my battery light came on for a brief moment during cruising. I got home and did some tests on the battery and alternator. The readings are...

Battery: 11.9v
Alternator output at idle: 11.7v - 12v
Alternator output at 2k RPM: 13.6v - 14.1v

I disconnected the battery and charged it with my portable charger to 12.5v. The next day I drove the car for about 45 minutes doing some errands. Came home and checked the battery voltage and it was about 11.9 volts again. Seems like the battery isn't holding a charge but could that be due to the fact that the alternator output at idle is so low? Hmmm

A few notes...I have a Steeda underdrive pulley that came with the Steeda belt drive system for my Whipple. Car has been running fine ever since the Steeda parts were installed 5 yrs ago. I just can't remember if the alternator output was this low at idle before. Optima Redtop battery is 2 years and 4 months old.

Guys with underdrive pulleys...what kind of alternator output are you seeing at idle?

Maybe it's time for me to upgrade to a 200 amp alternator.

08'MustangDude

Your idle alternator output should still be 13.7 or more... So, I would say Alternator.

Underdrive pulley; probably should have tested the voltage after it was installed for
reference. However, if you have had the pulley on, and this just started, then
that's not the issue. You should not see voltages below 13.5 with an underdrive
pulley with a good alternator.

AMPS do not charge the battery, the voltages does. Amps determines how fast it
charges. If you're putting out 12 charge volts, it won't matter how many amps you
have, the battery will not charge past 11.x volts. If you look at any AA/AAA/C/D
rechargable batteries, notice, they are NOT 1.5 volts. They are 1.2, and charge
voltages is 1.5 volts.

So, A 12V battery needs 13.5+ to charge, and the amps determines how fast it
charges. My '15 Jetta TSI, my idle charge voltage is ~14.3 volts.

The underdrive pulley may just now be showing the alternator is getting weak,
due to the low voltage output at idle... The alternator can be tested at Auto Zone,
or Advance Auto.

Dino Dino Bambino

The voltage regulator inside your alternator is failing. Replace the alternator with a 2010 OEM unit.

Goldenpony

I went with a PA Performance 130 amp. Alternator, power wire kit and an Interstate battery. Problems solved.

2007CalSpec

Thanks guys! I went ahead and ordered a high output alternator and upgraded battery cables.

JJHoover

most parts stores check it for free

08'MustangDude

Too bad someone didn't say that already; oh, wait...