12V Battery Died after a surprisngly short time...

Yesterday I had to wait in my 2017 Prime for about 2 hours. In accessory mode with the radio on I got a low battery warning and had to jump it. At the time I had nearly a full charge on the traction battery. I had expected the traction battery to keep the 12V battery charged while in accessory mode.

With so much depending on the 12V battery, I agree that there is a major design flaw. I was parked nose in (I won't do that again) and without 12V the car is stuck in park. Fortunately, the space next to me was open and I was able to get a jump. I agree with the need for an override button or some other way to tap into the traction battery in this situation. I really don't understand the need for the 12V battery at all.

 

Always stay in READY. Never accessory. The 12v battery powers computers that check for open circuits or damage to the HV wiring. If all checks out then the relay allows High Voltage to pass through. If there was a manual override then there's the chance that some dummy would have rodent damage to their HV wiring and would override it and then electrocute themselves. A 50 or so dollar portable jump pack would have gotten you back on in 2 seconds

 

you never got a low 12v warning?

while i like the idea that there must be a better way, i have to think toyota knows better, and it would probably be more expensive

 

I haven't no. Original 4 year old battery still.

 

Your explanation of the 12V battery powering a supervisory controller makes sense. However, it makes no sense to me that I need to be in Ready mode to listen to the radio. What the hell is accessory for? I guess I need to go back and read that section of the manual.

 

Reading about the multiple problems and concerns with the Prime's 12V battery, I did a Google search to see if cars like the Tesla had a better idea. Apparently, they don't. Short-lived 12V lead acid batteries seem to be a common problem. And if you buy the Tesla extended warranty, the 12V battery is considered a consumable and is not covered. Mine is a little over 3 years old and I have had no problems. Prior to the current pandemic we often cruised for 6 weeks or so with the car home in the garage. I have a couple of 750 MA Deltran battery tenders that I hook to the batteries of our two cars while we are gone. They have always done the job. I also have a Type S portable Lithium Ion jump starter that I purchased a year ago and haven't used yet. The problem with that is that if I keep it in the Prime I will need it on the road in the 4Runner or the wife's Lexus or vice versa. Murphy's Law sort of thing.

 

Not until it was too late. After I got the warning, pressing the start button would cause the dash lights to come up briefly, a bunch of clicking confusion and then everything off.

 

I'm sorry to sound critical on your first post, but why would it do that? A regular car won't charge the 12V in accessory mode. Neither will a Prius. It has to be ready to drive just like any car. It's designed to be as much like any car as possible while also getting outrageously good gas mileage.

That's been explained ad nauseam.

 

A gassers battery is usually large enough to run a stereo for days, without damage. No need to run the engine to keep it charged.
At least, that’s how it used to be.
I haven’t had one since 2003

 

The 12V battery in a conventional car has to be able to crank the gasoline engine to start it. The 12V battery in a Prius only has to boot up the hybrid controller and close the relays on the traction battery. Thus the battery on a conventional car is a lot larger and can run accessories for a long time. Do other hybrids/EVs have this issue with dead 12V batteries? The Prius has three power states (other than OFF): Accessory, ON, and READY. Different things are powered in Accessory vs ON. And either Accessory or ON has a timer which turns things off after a time, but I don't recall which it is.

 

With both my Volt and Spark EV, the manual states to keep the car powered up when using any accessories, so it won't wear down the 12V battery,

 

Actually, it hasn't been explained why they couldn't get the 12V from the traction battery, either from the total voltage via a DC to DC converter, or just tap off 12V from a few of its individual batteries. Since it never goes lower than 20% of its charge it should have very little effect on the overall performance of the traction battery. There is not much more damaging to the environment than a lead acid battery. It also is unclear why you wouldn't use accessories like the car radio while in the accessory mode. Why is it called "accessory mode"?

 

Just thought I would mention that many times I have gone near my 2017 Prime while it is sitting in the garage in the off condition, and heard a fan running in the car. I believe this is the fan used to cool the traction battery, and it is doing some kind of traction battery maintenance. To me this fan must be driven by the 12 volt battery as the traction battery always is off when the car is off. This drain on the 12 volt could be substantial.

 

With charge on the traction battery, simply turn the car on to the HV-system ready mode with the headlights off and keep it on for half an hour if you are not driving the car regularly. This will recharge the 12 V battery. You can't do these in a non-plug-in car; so, it's a huge plus of having a plug-in car!

Obviously do not use the accessory mode when not necessary and keep the headlights off while charging the 12 V battery from the traction battery.

 

the hybrid battery is completely isolated when not in ready for safety purposes, making it difficult to use it for 12 volts.
hundai has some sort of 12v battery built into the hybrid battery. looks very awkward (re: expensive) to replace? idk.
haven't heard much talk of whether it has been a good design or not.

 

Wait, I think you can do the same thing with regular Prius. In fact, you can do the same with a regular gas engine car as well. For PRIME the advantage is that you don't have to put the car in HV mode and run the engine to charge 12v provided you have enough SoC left in the traction battery. All you need is to put it on READY.

 

It's true, but you're filling your garage with toxic exhaust fumes and wearing your engine with extensive idling, during which the lubrication isn't efficient. Moreover, charging is not efficient at idle speed because the alternator current is small. Some cars automatically stop the engine to prevent idling as well.

 

It sounds like you need a new 12 V battery. Lead - acid batteries do not survive deep discharges, and even if they do, their capacity is greatly reduced.