Basttion

Recap:

Battery was 5 yrs old next month (oem)
After picking it up from dealer, car wouldn't start and the voltage reading on my passport read 13.5-13.9v fluctuating while driving.
Replaced the battery and car now starts, but the voltage is virtually the same.

Is this suspect,or am I just delusional since I have the flu?

Many thanks.

MCPaudiTT - DEFCON 1

Running voltage is supplied by your alternator, not your battery.

Basttion

Not what I wanted to hear... Oh well, maybe it will be CPO'd next week with the brake booster...

TTschwing

A fresh battery will have a resting voltage of around 12.7V. (measured at 65 degrees after sitting with no draw for 12 hours). The only way to measure this accurrately is with a digital voltmeter AT the battery terminals. Just switching on your car will skew the numbers as you're throwing a bunch of power draws into the equation which will lower the V. readout. When an alternator is running it will put out between 13.5 and 14.1 volts into the battery depending on the batteries state of charge. This voltage is regulated by (duh!) a regulator. A batteries ability to hold and accept this charge has nothing really to do with how much you're trying to stuff into it. A fully charged batter will generally show an alternator supplied voltage of about 13.7 V depending on the voltage draw of the cars electrical systems. A battery with a low charge will typicallly show 14V or so as the battery capacity is replenished by the alternator output. A totally bad battery may not accept a charge at all. A higher charge rate will show at the first start of the day as the battery recovers lost voltage from sitting and parasitic draws as well as needing to recover from a high load engine start. Batteries naturally age and over a period of time their ability to hold a charge and full voltage diminishes. At 4 years or so this usually drops to about 50% of the original capacity. A bad or shorted cell can also cause failure as all cells need to be fairly consistent... Any full drain of a battery will do incremental and irreversible harm even if it "seems" to come back up after a charge.

Test a battery by either:
1)Taking a resting voltage reading (key off, measure with a digital meter at the posts after sitting overnight. Should be 12.5 to 12.7V)
2)doing a load test

Test an alternator by:
1)measuring voltage while running. Anything over the batteries resting voltage means it's putting out. Voltages less than 13 or so means it's not putting out enough..Sometimes a bad or shorted battery will not accept any voltage even when the alternator is good.

Precise Voltage measurements (% of charge) are a good indicator of a batteries health:
<img src="http://images16.fotki.com/v271/photos/7/7305/776256/DSC04647-vi.jpg">

TTschwing

Your car is fine. See thread....\/

Basttion

Thanks, Steve. It's been a bad weekend and I need some good news...

contenTT

Does the TT's alternator create enough current to run the engine with a totally flat battery?

TTschwing

I wouldn't try it...That alternator rarely has enough output at the low RPM's used in a bump start to power up the ignition. If he battery has juice in it, just not enough to turn the car over, then yes, a bump start will generally work..

contenTT

Thanks, Steve

coasTTal

I had not driven the TT for 5 weeks. The yellow top Optima was dead as it could be. Autozone guy charged it and all seemed pretty well. While just slowly cruising around I see the battery dropping from near 13V to the mid to low 11's. Out on the road driving pretty well gets it up to almost 12. I let the car sit for 2 hours and the voltage at cranking is over 13 and settles to 12.9. A bunch of driving today and it runs around 12.8. Then after a bunch of crawling around it drops to 11.8. On the highway again and it pulls up to 12.6.

Now, not sure if it is the battery or alternator.

What do you think?