Thanks, great info.
I'll do this in the next few days.

I'll also try this

i would just take it to the parts store, most do it for free.

Yea, but why pass up the opportunity to buy a new tool?

It's weird how certain tools are never purchased by DIY'ers,
yet there are 100's of other tools they'll gladly buy and use once.

After 2 days on the charger, the battery measures 12.46v
I will do the other tests today.

if you have a charging problem the parts store may be able to tell you if it is a diode or regulator or maybe something else.
otherwise its a crap shoot as to if it is the regulator or something in the alt unless you know what you are doing with a MM.

I am an electronics tech, also use to auto electric work along with rebuilding starters and alt's and even "I" will just take it and have it tested. its just too easy.

no its a cool tool.
I question his results on the alt being good when he put 100amps to it though.
it looked like the voltage dropped to around 10v(would have to go watch again) and un less he had a 70 amp or even 100amp alt on there I thought his reading was bad.
again, I would have to go watch to be sure.

Latest reading
12.5v but now at 13.7v while running
Gonna get it checked by the store now

Can DIY a load test with a VOM or DMM. Hook up leads to the battery, then with the ign. disabled, crank the engine and see the voltage drop to some lower voltage. That's a poor man's load test.

If starting with a fully-charged battery at around 12.6-13 volts, during cranking, a good battery should be not less than 9.6V.

S

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.

Connect a battery maintainer when vehicle not used for days. The battery will appreciate that.

With your voltmeter, check battery voltage with all possible electrical loads ON. If the system charging rate is too low for the loads in use (low 12s), the battery will discharge and we will hear from you again. :)

S

Took a long drive today.
Pulled into the driveway, and measured voltage

Car running 13.5v
Car running with lights on 13.2v

Does the voltage go down when the battery is full ?
The battery cranks like it's at 100%.

I still think your ground strap is the root cause.
Remember that I mentioned mine would show good charge at first but after a long drive the voltage would start to drop and drop until it was in the 12's.

battery
 

My 1982 SC with a new ground strap and a new alternator about 5K miles ago, usually has a measures engine running voltage of around 13.5, however it keeps the battery resting charge at 12.5 volts and I never have any issues. In addition, my voltage output with lights on is about like yours. I realize that the measured alternator voltage can be considered a bit low but, again, I have no issues.

Dave

The more power (amps) being used the lower the voltage.

Suggest measuring active voltage charge with the maximum number of anticipated electrical loads. That includes engine ON, fog and headlights (high beam), blower fan(s), window wipers and defrosters, sound equipment, etc. All put a load on the charging system which should support all these and at a level above battery recharging threshold.

Connect a voltmeter onto any convenient 12V source wire and observe the voltage level during vehicle operation.

S

No worries. Batteries are boring but necessary.

For kicks, I tested my 911 today

Engine off = 12.9
Running = 14.1V

I run a battery tracker - it records battery voltage during startup and sends alerts to my phone when I'm within bluetooth range. My SC Targa voltage is 13.3 with the engine off, 12.8 during cranking and 14.3 at idle.

https://antigravitybatteries.com/products/accessories/battery/chargers/bluetooth-tracker-lithium/

My 2019 AGM battery is dead.
Last 4 years, I have not driven the car much and it mostly sits on a tender

I put it on a trickle charger overnight and still dead.

I removed the battery from the car and hooked up a 12V charger
Once charged, I will take it to FLAPS and load test.
If fail, I will get a new 48 battery for $200 regular or $250 AGM

Assuming the connections are clean and tight ...4 yrs on a battery is about the average life span, some last longer, some don’t. Doesn’t matter if the car in running or kept on charge.
For example I had a 4 yr old that read 12.4v and wouldn’t start my Harley which sat 5 months overwinter on a trickle charger.
Trickle charge overnight won’t fully revive a stone cold dead battery, you’ll need a lot more amperage. Bad battery cells might not charge more than 12.5v.
Heat kills batteries faster too and from overcharge. . Check the voltage output on your charger/maintainer too. I once thought my charger was plugged into the quick connect, I found socket pin was bad. Another older unit didn’t have a charging indicator light. Even the charger can go bad and could affect charging rate.

sugarwood -- what make/model tender were you using? How frequently do you drive your car? Asking, as I frequently let my car sit for a week or 2 at a time and don't use a tender (unless its sit for a month or so) -- my last 2 AGM batteries each lasted me over 6 years and wondering whether frequent battery tender use is slowly "cooking" your battery.

Having dealt with the very messy aftermath of an overcharging situation with my last non-agm battery (which boiled out onto the battery tray and components beneath the tray), I will never install another non-agm battery in my 911.