Battery voltage drop - 0.43V in 33 mins

[wwdwgs]

87 924S
Battery was bought in Dec. 2020. Few days ago it died when my car was parked for a week or two. I charged it. The charged battery showed 13.75V (post to post), but the voltage was slowly dropping. I monitored the multimeter for about half and hour and recorded the voltage drop from 13.12V to 12.69 over 32-33 minutes. The only two things that "consume" the energy are the stock clock and radio's clock.
Is the battery kaput?

[944 Ecology]

Disconnect the battery and see if the charge dissipates. If so, you have ruled out the car.

[djnolan]

the voltage will drop after taking it off the charger even if it is disconnected.

A fully charged, standard 12-volt car battery should show a resting voltage of approximately 12.6 to 12.8 volts

[walfreyydo]

So two potential things going on here and not sure what the OP is referring to. I assume its in reference to a potential parasitic draw on the battery when connected to the car. If that is the case you must first measure for a draw (using a multimeter), and then secondly, start disconnecting fuses until you isolate where the draw is coming from (giving you further evidence on what circuit or component is causing the draw). Youtube has many videos demonstrating how to test this.

If the battery just isnt holding charge when completely disconnected from the car, that is a different issue likely related to the battery. Since the battery is almost 5 years old, that starts to reach the end of life of many batteries.

[wwdwgs]

Thank you, guys. I'll check for parasitic draw and update.

[Jfrahm]

Over 13v is a surface charge that will dissipate. Disconnected, it might stick around for a couple days or only a few minutes. When it's gone you should see something like 12.8v for a pretty new battery or 12.6 for one a couple years old.
The speed of the loss of the surface charge depends on the internal resistance of the battery.

You can also use a light bulb or hook up the car and open the door for a few seconds to dissipate the surface charge so you can see the real state of your battery. Which sounds fine.

[Cajundaddy]

12.6-12.8 is 100% charge at rest assuming FLA rather than AGM. The first 30 minutes after charging, a battery may retain a surface charge over 13.0v but that always dissipates and should not be considered state of charge.

https://duckduckgo.com/?q=AGM+battery+SOC+chart&t=osx&ia=images&iax=images&iai=http%3A%2F%2Fwww.lanchbury.id.au%2Fwp-content%2Fuploads%2F2018%2F09%2FBattery-State-Of-Charge.png

[Monkey Wrench]

turn on the headlights, just leave it with the headlights on for 1/2 hr. dont run it completely flat but you can give it a little workout just to see if it dies quickly

If it wont stand up to a little "stress test" headlights dim after 10 minutes, or so, it might not make it through winter, if the car is stored in your garage and used next year maybe its ok for now. charge it back up after..

weak batteries on cold winter mornings can lead into frustrating situations especially if you are depending on it to get to work, but it might be ok,

another simple way to troubeshoot is to switch the batterry between cars to see if the problem follows the battery or follows the car., IF you think you are having issues.

I found battery maintainers for under 20 bucks so I bought a few thinking ok just put one in every car so I can plug in once in a while ,, leave it a day or a week maybe? see what the voltage is,,
the battery leads look a bit thin and cheap, I thouhgt Id just find some big battery clamps, maybe one is enough then just move the whole thing and the extension cord around between cars
I have a big charger but I dont like leaving it outside all the time, if I do I put a big tub inverted over it in case it rains. It has the power to do a faster recharge but fast charging might not be great for batteries, I usually use a lower setting , unless its a got to get to work emergency.
sometimes chargers don't shut off and can basically cook the battery so being inherently forgetful Ill turn on an outdoor light as a reminder to myself that the charger is still on just to stop me from forgetting it for too long a long period. in theory the chargers look after regulation but they can fail and then they don't tell you,

Ive left them on too long before and returned and the battery is hot or "sulphating" if so then it's done. in theory that can cause a battery explosion of the offgassing may be giving off bad fumes.

the little trickle chargers have some handy indicator lights.

often Ill find that when I decide to switch out for a new one , my ability to crank longer and faster improves quite drastically , proving it was time.

ill use older questionable ones for electrolysis, I put the battery on a charger and use it's power to run electrodes for removing rust from car parts in a bath of water and washing soda, with some scrap steel for an electrode, the riust and any paint magically migrates off the part and onto to the expendable electrode, any hunk of steel will do. the process removes rust without damaging the good metal. if you short the battery by the parts accidentally touching it can cause a dangerous battery meltdown, so be careful of that.
it leaves a bt of a blackened coating Il scrub the part with baking soda then give it another run if its not finished to my liking.

im often cleaning up and then painting rusty old car parts, springs callipers, disks and so on, I find the process works well and its cheap and not too messy. I figure its a good use for older batteries that are questinable , then recycle them after they are completely done. I prefer to use older batteries in case I wreck them.

[wwdwgs]

Update:
The headlight test results:
12.4V, lights on, after 10 mins 12.2V
another 10 mins - 12.2V
another 10 mins - 12.1V
lights are off, voltage returned to 12.3V-12.4V
Voltage was measured pos post, neg cable terminal.

[Monkey Wrench]

I guess there are more scientific tests but it sounds pretty normal to me.
also agree withthe others , the initial drop after charging, it might be ok.

wont hurt to follow that ground and make sure its got a good connection to the body and the engine. that ground cable is important for every circuit in the car, they all use that circuit because they all must lead back to battery ground. its easy to ignore and important.
also the alternator probably has a ( blue? ) ground cable to engine. if that is bad it can cause charging issues.

to clean cable ends and battery posts, try a jar with soem baking soda and water itll bubble and clean nicely.

I also like to soak a little dish rag in it, let it soak up the baking soda and water, then let it dry, lay it neatly under the battry, now any acid spill gets instantly neutralized by reacting with the baking soda, rather than rust damaging the battery compartment or making holes in your pants when you next pull the battery out.

do not get baking soda INSIDE the battry but you can use a brush and clean the outside case with it.
lots of cars have rusty battrer boxes so I do this, it helps a lot to preent it.

I know if Im woring on and around a car Il frequenrly pull the cables off or one cable since im not driving it, it is very esy then to get used to the habit of the batery posts being not tightened up properly, Just pushed on,

simple stuff, but loose or dirty ones often cause issues. it is easy to forget to tighten them properly after working on the car.