Sluggish battery is measuring 13.25V while engine is running.
 

My 911 has been doing a lot of sitting recently
Recently, I trickled charged my battery, yet it does not feel 100%.
Slight sluggish hesitation on the first few cranks, before firing up.
I drove it 30 minutes a few days ago, yet there is that sluggish crank.
Today, I drove it for 10 minutes, pulled into the driveway, and it would not restart.

Engine off, battery measures 12.5 volts (80% charged, see link below)

Engine running at idle, battery measures 13.25 volts
This is lower than the last time I measured.
When the battery was new, voltage would read 13.6v (engine running)

Alternator was replaced in 2001 (20k miles ago)

I bought the AGM battery about 2.5 years ago.
But, it has been trickle charged from totally dead perhaps a dozen times since.

I am going to trickle charge it overnight, and see if the voltage can get up to 12.7v (100%)
I will also measure voltage with the RPM's higher than idle.

I'll also check the ground strap and the connections at the starter and alternator (battery is clean)

https://www.emarineinc.com/Marine-Batteries-Maintenance-101



===================================

Thanks for all the advice.

I brought it to the store.
They verified the battery was bad.
Turns out, the AGM had a 4 year warranty.
So, I got a replacement for free.

New battery is obviously stronger
Cranks like a bumblebee on Adderall now

Car off 12.8v
Running 13.7v

Back on the road!
I will try to change that strap, either way.

Thanks again for all the advice.

sugar wood, why not take it to a FLAPS and have them do a free load test on the battery? That'll rule out a failing cell

ground strap from transmission to chassis. Had same symptoms on mine years ago and that was the root cause. It was a braided metal strap and was quite dirty and oily. Even clean it had too much resistance and had to be replaced.

Battery
 

With a charged battery at 12.5 volts, you should be seeing the voltage that the alternator is charging the battery at. My 82 SC alternator was charging at 13.2 - I replaced ground strap and it went up to 13.7

I've got it on a charger, and will measure the voltages again tomorrow.

I actually have that tranny ground strap sitting on my shelf for years.
When I tried to replace it a few years ago, it was harder than it looked.
I could not even see the stud so I bailed out.
I will get under the car again this week and take another look.

Try this....

http://forums.pelicanparts.com/porsche-911-technical-forum/962014-how-r-r-transmission-chassis-ground.html

System voltage supplied to the vehicle while running falls to the alternator. Should be around 14 volts.

If the charging rate is 13.25, raise the idle a little. 13.25 is a bit low Put a load on the system (headlights, motors, window defroster, other electrical accessories ON). See if the charging rate increases. It should. Otherwise, the battery will be operating on a deficit (discharging) and it may discharge to a threshold below cranking voltage.

Sherwood

After a night of charging

Car off 12.5v
Running 13.6v
Lights and heat running 13.2v

If those numbers were three tenths of a volt higher across the set I would say fine. The 12.5v raises an eyebrow.

I would start with a battery check at parts store as suggested.

+1 The ground straps and the main battery ground cable should be checked for resistance, not just cleaned. They get internal corrosion, so need to be replaced from time to time.

CLEAN battery connections AND the ground strap first. you don't have to replace anything.

a fully charged battery is 12.6 volts. some could be around 13v. depending on if it is 2.1v per cell or 2.2v per cell or look up the data sheet for that particular battery to see what the charging voltage SHOULD be and what the voltage is fully charged.

your issue is not really anything you have measured.
measure the cranking voltage. if it is less than 10v the battery is bad.
OR, take it to some place like advance and let the load test the battery to remove all doubt.

if the battery is bad clean every thing and check you voltages again with a good battery.

if you suspect a charging issue, again, take it to an advance and let THEM test it for free. don't guess at what or if anything is wrong.

+1

The machine they have locally can tell if there are oddball things with the alternator as well. Not just charging capability.

With respect to ground, I have used jumper cables from engine to ground on BMW's with corroded ground straps just for a test. I could then get the car to start. The braided cables were essentially green dust though.

+2 -- getting battery capacity/health tested makes sense as a first(ish) step whenever it's been completely discharged in the past AND one is having charging issues. It's low-hanging fruit -- rule out the battery before tackling other (more difficult to address/rectify) possibilities.

Your FLAPS may have the latest state-of-the-art battery tester. It tests battery conductance, a very accurate method that can test 12v lead acid batteries in various states of charge. Testing doesn’t require charging or potentially hazardous load testing and is more accurate. More details here:

CONDUCTANCE TESTING QUESTIONS AND ANSWERS

I should have added; the parts store may use this type tester to test customer batteries. Typically no charge and lots more accurate than inexpensive testers.

Sherwood

Battery is failing/dead. You cannot let a lead acid battery go dead, everytime you do it dies a little bit more. The reason the alternator running voltage is so low it is trying its best to charge a dead battery, and is instead is producing heat. Buy a flooded lead acid battery and keep a CTEK smart charger on it whenever not in use.

Thanks guys.
I suspect it's been dead too many times over the last year.

I will buy this for $20 and load test it myself.
https://www.harborfreight.com/100-amp-612v-battery-load-tester-61747.html


<iframe width="560" height="315" src="https://www.youtube.com/embed/qP5r6NB0ypo" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Yep. This is a nice tool to have on the shelf for the battery test. You will use it plenty across time.

A toaster with cables. Not sophisticated but functional.

What is a load test?
It measures how much voltage is supplied during load?
A damaged battery can measure 12.5v, but will measure less under load?

sugar wood -- yep -- all covered in the video posted above -- meter shows voltage with no load and then shows voltage drop when load is applied (toaster with cables comment by bob k is spot on)

I do like the info that Sherwood (911pcars) provided above about how the FLAPS (friendly local auto part store) uses a more sophisticated conductive tester to determine battery health/condition -- plus, having the FLAPS run the test for you has the added advantage of saving you the cost of a load toaster :)

Yes to second sentence.

The load test tool routes the battery juice through heavy coils (toaster coils) that put a "load" on it similar to starting.

As one member noted, check voltage while cranking. The toaster tool does the same thing. When you flip the toggle switch at the bottom of the tool it subjects the battery to a big draw of power.

If your battery is robust, the needle will drop some when load tested but pretty much hang near 12v, maybe a touch less but not much. It will continue providing power across time (many seconds). Especially, if fully charged.

If the battery is crippled, the needle will sink to the left. Sometimes slow, sometimes fast. If it sinks slow, repeated load tests will erode voltage if the battery is compromised.