12v Battery Is Discharging, Re-Charge Now !

If you don't have a load tester there's still a way to get a good idea of battery health: turn on the brights and the rear windows defrost and test the voltage. You're looking for a big voltage drop. Let them run for a minute or two and shut them off. Test the voltage again. If the battery is still 12.5V+ your battery is fine.

 

I have a Solar digital tester. I found that if the battery is not fully charged, the CCA it shows for the battery is significantly less than if fully charged. Honestly, I don't trust those things. I bought another brand and it gave me varying results so I returned it. The Solar one is better, but I think the carbon pile is the best way to test. I basically went on the age of the battery to make the decision to replace and not the test results, which said the battery was still good (95% SOH fully charged).

 

How long are you charging? what's the Schumachers amp rating? what voltage are you getting after charging?

I'd suggest if you like the results better when disconnecting the batteries, than do it that way,
I've not hooked up a charger to the Primes 12 volt so far, if memory serves, but I have seen the 12 volt below 12 volts (like 11,8) once on a 30 * F fall morning a couple years ago. But I had to unlock and open the door (to pop the hood) which woke the car up that morning.

 

Look at the link I posted:
Preemptive 12V Battery Replacement | Page 2 | PriusChat

It was not a Solar tester, but a very similar built Ancel BA101 load tester showed the opposite of what you have described. That is the battery that had only a Charge level of 79% @12.48v in-vehicle load test resulted in being a GOOD BATTERY with 48% 335CCA registered. Then when this battery was removed from the car and charged fully using an external charger, then retested by the same Ancel BA101 digital load tester, it said REPLACE and showed lower 36% 275 CCA even though the Charge was showing 98% @12.90v.

 

Your tester looks similar (and may use the same firmware) to the KONNWEI KW208 I bought off of Amazon. It kept giving me different/inconsistent results so I returned it. With the same battery, I got every result except Bad Cell.

 

Well, in my case, Ancel BA101 digital load tester and KONNWEI KW600 Digital Battery Analyzer, and Solar 1874 Carbon Pile Battery Load Tester all three testers gave REPLACE result on the same battery tested out of the vehicle. The point was the in-vehicle load test may not be accurate.

 

Brute force carbon pile resistors don't lie. You can buy them at walmart or harbor freight for $20. They're just a bit bulkier than your Ancel or Konnwei and way cheaper than the Solar 1874.
Yes, it's a bad idea to load test with the battery still attached to the car. If the battery is bad; the low voltage caused by the test will mess with your sensors and CPUs.
You probably could've got thru summer and fall on 275CC, but you would definately need to replace it by winter time. Just throw a jump pack in the car, just in case.

 

If you read the link I posted, you notice the test results were on the 8 years old OEM battery in our Pathfinder Hybrid. Yeah, it would not start the car when it was -26F. So, I replaced it with a brand new AGM battery. All the tests were done after removing it from the PathHy.

 

2021 PP 9kmiles with ~1-year-old OEM 12v GS Yuasa battery. The car is being driven once or twice a week on average. I haven't driven for an entire week, and the last time I charged the 12v battery was a few months ago. The resting voltage of the 12v battery had dropped to 12.36v. Charged the battery overnight. Now it should be good for a couple months.