Prius Prime 12v dead, what to do after?

OK, I did the experiment. See here 12v Battery Is Discharging, Re-Charge Now ! | Page 10 | PriusChat
The conclusion: EVSE plug does not substantially charge the 12v battery. 0.02v in 3hours.

 

It is not possible to charge the traction battery if the 12V is dead because the computer system doesn't start. A near dead battery should be able to start the computers and start charging the traction battery, but not if the 12V is dead below the threshold.

I jump started the Prime from a dead battery a couple of days ago. I then left it in READY for over an hour, I even started the Charge Mode at some point to give the gas engine a chance to stretch her legs for a few minutes. This brought the traction battery from 49% to about 60% and then I disengaged Charge Mode. After that I turned off the Prime and then measured the 12V battery. It was at 12.43V. Next day, I went out for some errands and forgot to measure the battery prior to starting the car. For the errands I drove the car for about an hour and there was another hour where I was just parked in the vehicle for another hour. Essentially, the car was in Ready Mode for a good 2 hours. When I got home, measured the battery at it was at 12.43V again. Today about 24 hours later, I measured the 12V battery and it was at 11.6V. I didn't know if this was sufficient to start the Prime, but it did just fine. I turned it off and then connected my new smart charger to recondition and recharge the battery to full. I disconnected the Prime from the 12V until it finishes charging with the smart charger. Don't know how long that will take but I will check later tonight. Looks like I have a bad battery, right?

By the way, I also received my Lithium Ion jump starter mini-pack. Right now that is connected to a USB charger until it is fully charged. It shall be in the car for whenever it is needed.

 

Yeah, the fact 2+ hours of READY mode did not bring up your 12V SoC seems to indicate it is dying or not taking charge from on-board charge system anymore. As I experimented in another thread (link in comment #22), charging the traction battery with EVSE does not charge a 12v battery. It only maintains it. Before the experiment, I had 12.4v, after 3+ hours of EVSE charging brought up the 12v SoC only to 12.42v. Overnight the battery was back to 12.40v (60%), but I took the car for a short 10 miles drive today, after the trip and 3 hours later the resting state 12v battery SoC is 12.7v (100%). So, with a healthy 12v battery, a short 10 miles trip is all needed. When the 12v battery drains down to 60% level in my car, I am going to try 5 min on READY mode without driving to see how much of SoC it will raise.

 

I don't think you actually have to move the car for the 12v battery to be recharged; just being in READY and sitting still should be sufficient. This is unlike in a conventional car, where the alternator that charges the battery is driven by a belt to the gasoline engine, so if the engine is not turning, nothing is going into the battery.

 

You are correct that READY mode is the only thing that need to be in order to charge the 12v. I just have not measured the amount of SoC charged with 5 min of READY mode (un-driven) yet. That is my next experiment. But I know 10 miles of drive is sufficient to bring the 12v SoC from 12.4v(60%) to 12.7v(100%).

 

You definitely have a bad battery. Any good 12 V battery will hold at least a 12.4 V or higher charge not only overnight but even a week or two or three. Replace your 12 V battery and you should be good for another four or five years. Unless you have a serious electrical drain somewhere Going on.

 

2020 Prius Prime, parked and traction battery charger plugged in for 5 days and now the vehicle is dead and undrivable.

12v battery shows dead (2.5v)

A 10amp 12v charger attached, but a relay somewhere on the passenger side of the engine compartment cycles with charger ammeter constantly flipping from 1amp to 10amps.

6 days for the 12v battery to be depleted with the traction charger attached???

Completely stranded during covid because I don't drive every day????

I don't think the battery is defective. Because of the relay cycling, it makes more sense that something is seriously wrong in the car's electrical system that drained the battery.

Any wisdom out there?
Beverly Howard

 

Just checked again... 12v battery now showing 5amp charging.

the relay cycling has stopped, but I have to assume that the relay is closed and sucking current... see following.

Pressed the Power button and the traction battery shows full charge.

Switched to "Accessory" mode and brought up radio presets... they were there, but the radio was tuned to a nowhere frequency.

A few seconds later, (in Accessory mode) the lights started to dim and the console began flickering.

Powered off the car.

Beverly Howard

 

These 12v battery issues are a new problem. Could there be a production run of bad batteries from the factory? Could there be a run of defective electronics that draws a good battery down in a few days? The battery in my 2017 Prime is fine, and it has sat for a few days more than once. Few others with the earlier Primes have reported problems. Something has changed.

I'd like to see the amperage draw of a parked Prime. It's easy if one knows how to connect an ammeter to the negative cable while the car is parked, everything (assumed to be ) off.

 

I would just be charging the 12 volt. Well except: having dipped to 2.5 volts, I'd replace it, then hook up the charger, with the replacement. A smart charger, rated 5 amp or less. Most you can just leave them connected constantly.

 

Read this...

12v Battery Is Discharging, Re-Charge Now ! | PriusChat



Rob43

 

If you would’ve left the car in ready mode, the traction battery would be charging the 12 V battery. Once you went into accessory mode, the traction battery is no longer charging the 12 V battery. All your accessories run via the 12 V battery, and since you had a low battery that’s why the lights started dimming.

 

>> If you would’ve left the car in ready mode, the traction battery would be charging the 12 V battery. <<

Thanks for that information... makes sense.

However, the question is what completely discharged the battery when I left the car turned off and unused for only 5 days?

It's obvious that having the traction charger attached didn't help the 12v battery, but, I wonder if had I not left the traction charger attached after that charge was finished if that might have allowed either the traction system to maintain the 12v battery or prevented whatever drain that discharged the 12v battery.

Beverly Howard

 

According to the manual, keeping the charge cord connected will consume 12v battery SoC. But I don't think that alone can drain a healthy 12v all the way down in just 5 days.

 

A couple of related items in case they might be of interest to others.

I tested the battery's specific gravity in all cells when the battery voltage reached 12.75v after several hours on a 10amp charger, then re-tested the next day when the charger was set to low current, and all cells on both tests were lower than the lowest reading on my hydrometer of 1.11

I dug out the Torque OBD2 app, configured it for the Prime, and will run a "log" on the 12v battery voltage and will report back on that if I find anything. (Doesn't look like battery current is a supported parameter.)

Looks like Toyota is the only "official source" for the Prime's 12v battery. No other battery vendors offer any Prius Prime options.

The "Prius" batteries that are available from non Toyota vendors have "non standard" terminal posts, but they are incomparable with my 2020 Prius Prime since the terminal locations are reversed and it appears that the Prime's battery has reverted to standard terminal post sizes.

Hope that this information is valuable.
Beverly Howard

 

Have you tried that with a low initial charge on the 12V? I would hazard a guess that, if the 12V started the charge at something like 12.01V (25%), the traction charger would increase it by much more than 0.02V. 12.4V is around 75%. The car may not consider that to be a condition needing intervention. I don't know, but I wonder.

 

Thanks Sal, more valuable information.

I'll start disconnecting the traction battery charger after it finishes with the main battery and see if I can track it that helps.

I've also just finished installing a 12v trickle charger connection in the grill, but, I will now wait to use that until I observe for a longer period.

Assuming I have success setting up an ongoing voltage logger using Torque, that should provide more info to make decisions on... guess that's one benefit of being confined to home to be able to expend the time to do all this fun stuff ;-)

Thanks again,
Beverly

 

You could test that, quantify what the difference is: hook up a multimeter in series, between the negative battery post and ground, wait about an hour with the fobs well away, and the charger cable connected but done. Set the meter to largest amp scale, then drop it one step at a time, just to play it safe.

Once into milliamps scales, see what it's reading. Then disconnect the charger, and continue to monitor.

 

That's a good point. My almost brand new 12v battery has not gone down that low yet. The lowest it has gotten since the lockdown started a little over month ago is 12.38v. This with only once a week drive. If my monitor ever shows that low, I will try the experiment again, but I doubt it will ever go down that low unless I intentionally drain it low.

 

>> Have you tried that with a low initial charge on the 12V? <<

I'm hopeful that the Torque log will provide some answers to that question. In my experience, 12v batteries have enough charge remaining to start non hybrid cars that are left unused for several months.

Again, I don't think this is a 12v "battery" issue. I think it is a system issue that is making nonstandard use of that battery.

We know, at the very least, that the keyfob system is consuming power when the car is not in use, but, again, it should not be consuming so much power that drains the battery in days rather than months.

Beverly