So my 2 month old car went completely dead…

I know, right?
They crammed in a larger engine and who knows what else?
So something had to go

 

I think the larger wheels and sleeker profile reduced the engine bay space.

 

Maybe that’s why the tow driver didn’t jump start it? Couldn’t find the battery.

I didn’t realize a Prius can’t roll with ignition off, because it goes into park. Does it stay in park with a dead battery? Then there is neutral which sometimes I go into accidentally. I suppose it’s in the manual what about neutral.

 

Would the electronic parking brake still be applied if the battery is dead? I have it on automatic; every time I put it into PARK.

 

This is nonsense.

Prius Prime does not have a 12-V battery problem. I have owned one for three years. Your last paragraph is also total nonsense.

Dealer couldn't find any issues. So, that is the reason why I suspected an owner error.

 

The Gen 5 is like the Gen 2, battery is in the corner of the hatch. And for both of them, the Jump Start procedure, as documented in the owners manual, takes place under the hood, not at the battery in the hatch.

In the fuse box next to the engine there's a connector with a red cover for the positive jumper cable and negative goes to a ground in the engine bay. For Gen 2 there's a body bolt they want you to clamp onto, and for Gen 5 there's a place on the engine.

So for both, you don't need to open the hatch, just the hood, and that doesn't require power, so the mechanical key would allow you to jump start the car.

And Toyota says this is only to jump start the Prius, and to never attempt to jump start another car from the Prius.

 

How can your floor mat shift?? All Toyota floor mats have had latches since the Toyota floor-mat fiasco of the late 2000s. Those latches must be secured at all times, which will prevent the floor mat from shifting.

Chances are that the 12-V battery suffered damage because of the deep discharge and needs to be replaced.

 

Interesting. If they kept the original 12-volt battery, I would want it tested with a good tester. If you don't have one, any Auto parts store will test it for you. If it's below 80%, I'd take it back to the dealer and ask for it to be replaced under warranty.

 

The dealership is full of something "#$%", as Gokan stated, mats are locked-down. The brake pedal can discharge your battery, but your tail lights would be ON, pretty obvious. Pressing on the accelerator pedal with the car off, isn't going to do anything, it's basically a variable resistor with no power going to it - again the car is OFF.
As ColoradoBoo stated; take it to Autozone, Napa, O'riley's, or Pepboys to get the battery load tested or ask the dealership if they load tested the supposedly good battery they left in your car. What was the documented results - you can compare notes with the free auto parts load test results.

 

I too call BS on this. Years go with my Gen 2, there was a recall with Toyota's fix for reported unexplained sudden acceleration that I don't think they ever found a real cause for, but blamed it on the floor mat and the gas pedal. The recall included cutting off part of the gas pedal, and I think some zip ties to keep the floor mat from moving. Since then I think Toyota has been pretty paranoid about floor mats interfering with the gas pedal, and would improve the design to eliminate the possibility. As mentioned the Gen 5 floor mats are locked in place.

And every time my Gen 5 goes in for service there's a line item on the work order for "Check for proper installation of driver's floor mat, secure if necessary.", sometimes it's even listed twice. Again very paranoid about the driver floor mat.

And my Gen 5 did get a new 12V battery under warranty. Twice in one week it needed to be jumped, and it failed their battery test when I brought it to them. Coincidentally, this all happened less than a week after the dealer had it for the 5,000 mile service.

And when the battery is drained too low to start the car it behaves really weirdly. It seems like every piece of electronics in it has different thresholds for when it can operate, so some things work, some don't and some are just really weird. The headlights were flashing like strobe lights one time when this happened.

 

I'll add I think that is dealer BS and suspect bad 12v battery. I have been on a 2 week trip to Colorado/Utah a 6 weeks ago and only drove my Prius for a few days before going to the Keys for 2 weeks before Thanksgiving. When I got back I put the car on a tender just to see what parasitic loss it had for sitting those last 2 weeks and the 12v was drained significantly.

My motorcycles typically won't take any charge from the trickle even after sitting a month. Most of my other cars will take 20 min to an hour before getting the green light after sitting 2 weeks. The prius took a full 12 hrs on the trickle charger to get the green light, which is more than any other vehicle I have owned in the last 5 years (dozens of vehichles)

I suspect the Prius has some significant parasitic draw or maybe the 12v is just undersized or junk. Or maybe its designed to discharge more like a deep cycle??? I am going to be keeping an eye on it.

 

Don't forget to consider the charging system. Older generation Prii tend to recharge the 12V battery fairly slowly, so in typical usage many of them end up being perpetually half charged. Gen3 improved over the Gen2 by using a multi-step charger, but still doesn't reach full charge in many people's ordinary usage. I haven't heard whether Gen4 or Gen5 include further improvement.

 

Good to know! My commute is only 10 mi each way so maybe just not getting enough longer trips to charge the 12v system. Also about 50% EV on average (assume 12v charges only when engine running?).

 

The 12V battery is charging whenever the car is in ready mode. The problem is the electronic conservation software that has a bias towards placing most generated electricity back into the traction battery. If the 12V battery is OK, all power generated goes to the traction battery pack. this makes sense since the further you can go under battery power, the better mileage you get. Most gen4 & gen5 Prius batteries hover just above 12V, unless they are being plugged into a battery maintainer. If your battery gets below 10.5V, strange electronic things begin to happen; as if your car is possessed. Call a priest!!! - that's if the car will start at all.
Now that we're heading into the winter months, this problem will become more pronounced; if you didn't install a battery maintainer.

Hope this helps.

I'd probably throw a jump pack in the trunk - if it doesn't get too cold to freeze the jump pack.

 

Most new cars, especially HEVs and probably more so PHEVs, have high 12-V drains.

I have owned a 2021 Prius Prime Limited for three years with no 12-V-battery issues whatsoever. I never trickle-charge it. I had the Toyota Connected Services subscription until recently. I left the car undriven for more than six weeks last summer—again, with no issues whatsoever.

These are possibilities why you might experience 12-V-battery problems:

• Car sitting at the dealer for a long time before purchase and the battery having been degraded at the time of purchase
• Mostly doing short trips
• Leaving the car plugged in to PHEV AC charger for long periods (it drains the 12-V battery when plugged in)
• Sitting in the car with the ACC mode or lights on and not in the READY mode
• Inadvertent owner errors, such as accidentally leaving the car in the ACC mode, leaving the lights on, etc.

 

As you already heard, it charges whenever the car is in READY mode, regardless of the ICE spinning or not.

Agreed.

That is not at all true of Gen3 and older, and I'd be surprised if it were true of the later generations.

At least in the older non-plug-in models, all generation and regeneration is directed initially to the traction battery. From there, the 12V bus and charging system takes whatever it needs from the traction battery (whenever it is in READY mode), independent of propulsion activity. It makes no sense to 'starve' the 12V system to leave more energy in the traction battery for better mileage, then have to shut down prematurely due to a drained 12V battery while usable energy remains in the traction battery.

 

The OP had plugged it in, I would assume an adequate traction battery level would keep the 12-volt battery charged.

 

I remember the floor mat and gas pedal fiasco all too well back in the 2000s. The local OC Prius club visited Toyota in Irvine to find the truth and to get some straight answers!

 

No, the 12-V battery is typically charged only when the car is in the READY mode. In fact, leaving the car plugged in drains the 12-V battery and Toyota advises against it.