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.

 

I found out about this issue the hard way! I went on a two week vacation and left my 2023 Prius Prime XSE in long-term parking near the airport. When we returned from our trip, the car wouldn't start. It never flashed a warning light or gave a clear indication of what the problem was, and I didn't expect or believe that such a short interval of non-use could lead to a crippling battery drain. We ended up staying in a hotel overnight and had AAA come the next morning to give me a jump. When I called the Toyota dealership, they had me bring the car in to see if there was an excessive drain, but subsequently told me everything was to spec. They also said I should expect such issues any time I leave the car unused for a few weeks. I find that totally unacceptable, but there's not much I can do about it.

 

Jay, FuelMiser,
Do you guys know that discharging a Lead Acid battery to zero and leaving it there for any length of time is very harmful to the battery?
Jump starting it in any matter is just a quick fix.
The life of that battery took a big hit.

Having your dealer dogs tell you, "I should expect such issues any time I leave the car unused for a few weeks." is not an official stance of Toyota.
Hopefully they know this is unusual, and 'unexceptable' for any modern car and they have a SW update coming,,, any day now.....

Maybe in Japan there is no such thing as a '2 week vacation'.....

 

you should repost this in one of the several gen 5 12v battery dying threads. there's a lot of discussion going on.

 

This has been an ongoing issue for me with my 2017 Prius Prime. I just installed my 4th battery in this car. The most recent one lasted a little over a year. I plug the traction battery in using the factory L1 charger every time it is at home. This does not appear to recharge / maintain the charge on the 12V battery. Once the 12V dies it is a decreasing time until it dies again and ultimately becomes unchangeable. I see all the suggestions to use an external trickle charger on the 12V battery but it just seems so stupid that having an electric car plugged into a charger for the traction battery also requires a separate charger for the 12V battery in order to be able to actually start the car.

 

agreed, bad form toyota

 

The 12V battery ONLY receives a charge WHILE the traction battery is charging. While the car may be plugged-in, There is no electricity going to the 12V battery, unless it is also ACTIVELY charging the traction battery.

Do you have any non-Toyota accessories installed in the car? You seem to have a parasitic draw or you keep getting bad batteries. If you only drive the car for very short hops, less than 15-20 minutes - that isn't enough time to recharge your 12V battery, so it's slowly being discharged every short hop you take. My commute is 25 minutes one way, haven't had any issues. I recharge every evening and go through my traction charge everyday. I do a voltage check every quarter and had to put it on a charger on one or two occasions because it was around 11.25V. I've never had the car not start when I hit the start button. To the best of my recollections, they were both in the middle of winter - but we get very mild winters here. Snow doesn't stick to the ground - if we do get some, it's usually gone by late afternoon.

 

Ah - I think I understand you to be saying that although the car is plugged into the charger for the traction battery, the 12V battery is only charged during the time that the traction battery is actually charging. If you drive the car today, depleting the traction battery, and plugin the charger cable, the 12V battery will get charged for the 5 or so hours (L1 charger) that it takes to charge up the traction battery. If you now let the car sit for 2-3 weeks without driving it, the 12V battery will not be charged anymore, despite the computer etc drawing power from the 12V battery. If you only drove the car 10-12 miles and then plugged in the charger, the 12V battery would only get charged for a very short time.

thanks for that clarification.

 

That’s how it works though, and get a battery monitor so you don’t have to guess what is happening. I don’t see why the system can’t charge the 12 volt to 100% all the time while driving, then turn on off as needed. When I leave my car in park to charge up the 12 volt a little, like twenty minutes, it doesn’t do a lot to the traction battery charge, some, but not much. You can also do that if not using a battery maintainer. Not sure about if it harms anything to leave it on in park, or any safety reason. I don’t know of any.
I haven’t seen any 12 volt charging when charging the traction battery.

 

That's correct!!!, I believe the operations manual also states that you should unplug your car when it doesn't need a charge. Your example of leaving the car parked and plugged-in for 2-3 weeks at a time is contrary to what the OEM manual is telling you to do. The car runs system checks and internal diagnostics more often when it knows it's plugged-in. All those checks are running off your 12V battery; NOT the wall outlet.
1. I recommend you recharge up your 12V battery, above 12.75V, that'll get your battery where it should be.
2. Start disconnecting the car from the wall outlet the next day - after a full charge. Plug back in after you burn down the traction battery charge a bit.
That small change in your procedures, should alleviate your issues. I have a DVM (digital voltage meter) and do a quick battery check every quarter or biannually. Anytime I find the battery below 12V, I'll put the battery charger on it @ 6A for about 2 hours. That'll usually get the battery back above 12.75V. I've only had to do that once or twice since I've owned her (knock on wood) - still on the original OEM battery.

Hope this helps....