Will the hybrid battery charge the 12V battery when car is turned off?

"Design flaw" is a subjective opinion,
and it not up to anyone to declare it incorrect.

 

A damaged box is not the same as tapping some power to boost a battery.

 

Had jumper cables in "every" car I've ever had since my first in 1976...including my current hybrids and ZR1. If I need to jump it, I either f-cked up, didn't drive it for to long or there was/is a problem that I address/ed. YMMV.

 

Jumper cables or a portable jumper box work, but they are not ideal. It would be better if the car could utilize a little bit of power from the hybrid battery when you press the start button. Instead of disabling the car and requiring a boost or a service call.

 

Paul,

I have owned Prii for 21 years and have never had an issue with the 12v battery. I keep track of battery health and only replaced one when it was 8 years old and appeared to be weakening. If you are concerned, get a battery maintainer and hook it up when your Prius will be idle for more than a few days. I have kept a portable jumper in my Prius, but have only used it to help other people.

Prii are not perfect, but Gen2, Gen4 and perhaps Gen5 Prii (I left out Gen3 due to their engine issues) are very good, well designed cars.

JeffD

 

Some EVs do do this. Well, actually, they monitor 12V system voltage while off, and if it drops too low reconnect the traction battery to top it off. The question is if it is worth the cost with the much smaller traction battery? Besides the cost of the feature, is it worth the extra strain on the traction battery to keep a potentially failing 12V charged up?

There are at least two hybrids that do have a self jumping feature for the starter; the Ioniq and gen1 Niro. They don't have a lead acid 12 though, but is a portion of the traction pack. So relay tripping required, just a switch in the software.

Likely originated with an outside standards organization, such as the SAE. My guess is that it started among the EV1 era EVs.

 

Of course the risk in that might be a continual drain off the hybrid battery with a constant parasitic drain over time, which is probably why it was designed to keep the hybrid battery isolated. But a little more thought into the design might do the sensing before the 12V battery goes completely dead, and it should keep topping up the 12V battery with a sufficient charge to operate the sensing circuit. If this continues over a long period of time, it could run the hybrid battery flat as well, so the sensing circuit should then isolate the hybrid battery from further discharge.

Owing to the enormous capacity of the hybrid battery, this occurrence would be relatively rare, and would require a boost. But this would be much more favorable than the overnight drain that many of us experience with the current design, requiring either a boost or a service call.

 

Regulation and standards are always way behind innovation. That's the reason we can't have those European headlamp control systems; they run afoul of US standards regulations - even though they're safer and offer a larger field of view at night.

YMMV

 

Yet European regulation managed to allow those lights on the roads. Our lack of access from lagging regs isn't the regulations' fault.

 

One can only hope that the feedback and ideas in this forum will somehow make it back to Toyota.

 

I wonder how many Prius owners would have been saved a boost or a tow if they had such a feature?

 

Hyundai hybrids can do this because they don't have a separate 12V battery for a starter. They use a notedly larger traction battery that Toyota, and a module of it is for 12V duties. Being all within the same battery case, no physical switches need to flip for the main portion of the pack to dump some juice into the 12V part. Pushing the boost button just turns on a bit of software.

As long as the 12V is a separate battery, it needs to have enough charge to flip the relays to the traction pack for any boost button to work. Toyota could go to a self contained 12V battery like Hyundai, but lead-acid costs them less for that job.

 

I still think the main reason that Toyota designed it that way is to drive their customers to the dealers.

 

i dont know bud... ive got 140ah in my gen 2 that doesnt take 8 hours to fully charge via the car in ready mode... are you sure? any data to back this up?

have you any quirks? i went through three isolators and it wasn't until battery combiner did all function without hiccup.


https://black.jmyntrn.com/2023/06/03/prius-dual-battery-upgrade-victron/

i put a high powered relay back there on the negative line so I can turn off using the smart car system or IR remote. Id love to have a pcb board made with an esp32 so i could connect the negative and positive from the vehicle at the press of a button then another to reconnect it all up properly through a relay and the pcb board.

 

A conventional isolator is just a bank of diodes, which lets power into the battery when it's charging, but stops it from draining. but the "smart isolator" I use cuts out at 12.8V. I use the aux battery to run a 12 V cooler in the summer, but it's also a reserve battery for boosting the system. The way I do this is to use a heavy duty toggle (40A) switch. I have a combination volt/amp meter at each end of the connection, and the current has never been excessive, probably because the connecting wires are 3m of 12 gauge wires. The resistance of that length apparently prevents the current exceeding the capacity of the switch. I also have wired in a 40 amp fuse.
But, I discovered that the 12 V system is getting a charge while the hybrid battery is being charged. Once that is done, I disconnect the plug, and then I can use my NOCO 5 battery maintainer. I upgraded from a NOCO 2, because it wasn't keeping up with the 2.7A draw from the 12V cooler.

 

A battery maintainer is a better investment. I tried a trickle charger and destroyed my battery. Topping it up is fine if you remember to do it regularly, but a maintainer will test it periodically and deliver a top-up charge when needed.

I never disconnect the terminals. I can't see any benefit to that.

 

i have read that replacing the hyundai 12v segment is an expensive nightmare

 

This is 2nd hand info, and comes from the Volt forums. So grain of salt. I just traded my 2018 Volt for a 2025 Prius PHEV because the PHEV finally got a range close to the Volt (and they don't make the Volt anymore).

Anywho, there used to be one of the lead engineers who designed the Volt at GM who was a member of the Volt forum I was on. The Volt has the same problems as the Prius when it comes to the 12v battery. What he said is that they are required to isolate the HV battery except if the car is running. And also if a crash is detected. The first is to protect DIYers and mechanics from accidentally getting into a live HV circuit when doing work on the car. The other is to protect rescue workers responding to a crash from both HV injuries and fires as much as possible. Even 12v can cause fires (ever welded with a 12v?), but 400+ volts? HV Shorts can cause one heck of a wicked fire before fuses/breakers kick in. I had an old boss who was a Volunteer fire Chief (country life FTW) and he can tell stories. Nothing scared them more than working EV crashes, cause you never knew what you were getting into and what could happen.

I don't remember if he said it was just a safety standard or regulations. But he presented it as a non-negotiable truth that if you wanted to sell an EV in the US with HV systems, you had to isolate it except when "running". And cut it if crashed. And Volts won't just not start, a 12v strong enough to start the car but not strong enough to run all the systems at full voltage will cause the car to go crazy in wild and wacky ways. Windows rolling up and down on their own, locks going nuts, lights being weird. Crazy dash lights and error messages.

Bottom line was the same complaints and requests were talked about over there, and his response was "we aren't allowed to do that". So at the least isn't not just a Toyota thing. He also said it was the same pretty much across the world. My understanding is you can't have any HV power going outside the battery at ALL till AFTER the car is started (READY for both cars in question here). But I don't see why you couldn't have a DC HV>12v converter either inside the battery housing or attached to it. And run the starting system at least from that. Seems to be close to what Hyundai is doing. My guess is some rule/regulation is preventing it. Some safety thing we don't understand. Some 1 in a billion scenario a government lackey looking to not get laid off made up to justify his existence.