New Alternator not charging New Battery, Low Votage

[ciamaichelo]

Over the past two months, I have experienced several dead batteries in my 2005 911 Carrera S (3.8L 997 manual). My battery is less than a year old and it passed two separate test at Kraigen and Autozone. After charging the battery, it will discharge after 4-5 days of driving to about 60%. At that time, the car will struggle to start and eventually fail to start. The battery test OK with a voltage of 12.6. With the car running and high beams, fan and radio on, the battery pulls insufficient voltage to charge or pass the alternator test, using a $15 alternator testing tool, my battery charger or the alternator test at Autozone.

I changed the alternator with a new OEM Bosch alternator and charged the battery to 100%. Thinking I solved the problem, I drove for a week and again the car was slow to start and next time failed to start, requiring a boost.

I then installed a new battery, thinking the bad alternator and frequent discharging killed my old battery. I then tested the new alternator and battery at home and at Autozone. Again, with the car running the battery still pulls insufficient voltage to charge the battery and the alternator tools also state low voltage output. I assume the voltage regulator is part of the new alternator and I disconnected and charged the battery prior to installing the new alternator.

What other components could cause insufficient charging voltage from the alternator to the battery? I read on 6Speed that a bad alternator cable could be the cause. Is there a fuse somewhere? or burn out cable? Any ideas or help would be greatly appreciated.

Thanks,
Chris

[James Brown]

There are two fouls on the play, failure to introduce yourself and your car (with pics), failure to post in proper forum. 15 yard penalty, repeat first down.....Tack on another 10 yards for water cooling.....

[brd]

You check the charging dash light bulb that it's ok?
I know some alternators need this bulb in the circuit to excite the alternator before it'll start charging... hence the dash light coming on when ignition on, then going off when engine starts running and system is charging.

PS: It also needs to be a certain wattage depending on the alternator.

[wwest]

You probably have a voltage drop somewhere between the alternator/regulator all the way in the back of the car and the battery all the way at the front. Check the + connection at the starter solenoid. Also be sure the battery posts and connections are cleaned and burnished and check that the battery ground cable at the chaisis end is well grounded/connected.

Check/read the voltage across the battery terminals vs the voltage at the rear of the car while the engine is running.

[scottb]

Originally Posted by wwest

Also be sure the battery posts and connections are cleaned and burnished and check that the battery ground cable at the chaisis end is well grounded/connected.

That's where I'd start looking for problems. Check ALL of the ground connections. Bad grounds seem to create all sorts of electrical gremlins in our cars.