Alternator Perfomance Question
 

My car has been off the road over a year for clutch work and several other projects. It was on a battery tender the whole time but battery (IIRC) is about 3 years old. I took the car out for a good 45 minute drive around town and when I got back decided to check the battery charge while running/idling. 1st test was 12.93V but then turned off the auxiliary fan and the AC and it measured 13.47V. Does that sound about right?

With no cranking issues to start with, thats still in the good range at +13v running, but +14v is likely with a new battery. Cable connections kept tight and clean.
Before jumping into alternator and regulator, better measure would be to get a cold reading under load cranking/ starting…+9.5v to see if battery is on its way out.

Even using a tender the battery is still going to age out. Some last 5-7 years, some only 2-4 yrs. The minimum to start an engine might be about 12.4v with accessories off.
I believe a battery maintainer will hold the battery in a range of 12.7 to 13v.

Yes, voltage at idle is about that on mine too.

Those voltages sound pretty good to me for idle output.

with the car running it really only tells you that the alternator is functioning, and says nothing about the battery.

if it starts well cold with the battery you have in it, I see no reason for concern.

When I bought my car in 2017 it had a battery that was installed in 2005, just before the PO quit driving it. after completing all the mechanical, fuel and hydraulic work needed to bring it back on line, I charged that 2005 battery and continued to use it till I did a battery replacement in our Cayenne last year and swapped the old one from it into the 911. Never had a starting or running problem with either.

These old 911s seem to be pretty easy on batteries from my experience, unlike the Cayenne which seems to really want a really fresh full capacity battery.

I’m going to check how old it is. I wasn’t sure if being on a tender is better or worse than normal use. I also don’t know WHICH battery to buy. Can’t figure out if I should go with AGM or regular.

I would charge the battery then check everything.

The last thing you want to do is have a battery go completely dead routinely, it may never recover full capacity or even charge, so the battery tender helps relieve that from happening as will disconnecting but some loss of volts occurs anyway.

Take that battery to the auto parts store they’ll load test it. Maybe the battery label will indicate the date.

As for new one, AGM, no old style acid battery is any bit more reliable especially w/their corrosive nature. I’d stay away from the lithium - too risky.

Also an alternator is not designed to run 100% of the time. It has to duty cycle on and off to stay cool. That is why a 75 S uses about 25 amps when running lights, cdi etc but the alternator is 75 amps. It will operate about 33% of the time.

Forget flooded batteries.

At a minimum, step into the '90s ('80s?) and go AGM.

@EC900's reticence notwithstanding, I have Antigravity Li-Ion batteries in my 911s and love 'em. IDK if Li-Ion batteries are any riskier than driving around in modern traffic in 35-55 year old 911s, but... We pays our money and we takes our chances.

Excuse me, I do not want to hijack.

Is it safe to use AGM (or EFB) instead of flooded batteries ?
If I remember well AGM batteries have different (lower) charging current ... don't they need a different (or tweaked) VR to perform well ?