There are third-party replacements for the “12 volt” battery that use lithium cells (ca. $500 US!) and I believe I read that Tesla is now using lithium batteries for the 12 volt system from the factory.
1 Like
Good to know: thank you!
Sounds like the right choice to me. For that electrical system, you don’t need massive capacity. A normal 12v battery is enough to meet product requirements.
And lead-acid batteries are cheaper, available at every auto parts store, safer (they can’t explode or catch fire) and they are 100% recyclable.
1 Like
Our 2015 Nissan Leaf has suffered from this as well. In the early days of the pandemic, when Tonya was first suffering from her herniated disc, I took her to several appointments where I couldn’t go in, so I sat in the car with the heat on working on the laptop for quite a while. Although heat was powered by the big battery, the stereo system and internal lights and other things like that came from the 12V battery. These three appointments were only a few miles away, so I was draining the 12V battery without driving enough to recharge it.
Things came to head on the fourth trip, when I had to take her to the emergency room and again could not go in with her. I was working in the car when all of it electronic systems crashed and couldn’t be brought back online. The big battery had plenty of power, but without the 12V battery working, the car was dead in the water. (A gas car would simply have refused to start, something that happened frequently with our Subaru Outback when the dome light would be left on accidentally.)
Happily, my parents were ecstatic to have something to do where they could be useful, so they drove into town with jumper cables and an extra battery they had lying around (I grew up on a farm), and just connecting their spare battery to the Leaf’s 12V battery be a jumper cables while we were waiting for Tonya charged it enough for the Leaf to boot and be drivable again. It may have been the most amusing story to come out of the entire pandemic for us.
3 Likes
I was surprised to learn that the 12V battery in a Prius can die just like in any ‘regular’ gas car even if the traction battery remains fully charged throughout.
I am very much not expert enough in electrical topics to know why this isn’t done, but doesn’t it seem super obvious to charge the 12V battery from the big battery when necessary?
2 Likes
May your tribe increase.
2 Likes
The12-volt battery in my 2023 Prius died after about 2 years and was replaced under warranty. Most of my driving consists of fairly short errands, so I’m not surprised. The general recommendation is that you take the car out at least once a week for a drive of at least 20 minutes.
I do think Toyota needs to give drivers better feedback on the condition of their 12-volt battery, and add an option to prioritize maintaining the 12-volt over fuel economy for folks who do mostly short drives.
Another benefit of staying with the car I bought on 16 Jul 89, my Mazda Miata! 
Our 2024 Ioniq 5 had an amber light on the middle top of dashboard near the windshield. When it came on the big battery was charging the 12v battery. There was no explanation of what the light did and it was really random. The ‘25 doesn’t have that light
1 Like
That’s the way my Civic Sport Hybrid does it, using a DC-DC converter to step the voltage down.
1 Like
I assume the battery will be used to start the car by turning over the engine, as in an older car. They die for the simple reason that something chemical goes wrong in the cells and they no longer will supply electricity. They cannot be recharged. Why it will be a separate battery is that it needs to supply very high current, which may not be good for the traction battery, which is also a lot more expensive. My understanding is that when the vehicle is in motion then the electricity is supplied by a generator so there shouldn’t be wear on the battery.
Every iPhone replacement is a form of Russian roulette where you wonder what will or won’t work. I have accounts with 3 banks. 2 of those transferred without problems. The third I didn’t use for several months, and it wouldn’t work. I could do transfer from the old iPhone but it is in storage. The other alternative is to contact them and they will set it up by asking me questions.
True for a Prius, I guess, but my 2026 Civic hybrid starts the gas engine with power from the Li-ion battery under the back seat that, as I said earlier, uses the 12V battery up front to step its voltage down for the lights, infotainment, climate control and the vehicle management and control systems.
And the gas engine, as I noted earlier, charges the Li-ion battery . . . and so it goes . . .
The Civic Hybrid uses a completely different design than the Prius. The rotor of the electric motor is the flywheel of the engine, so the electric motor just turns on – powered by the traction battery – and the internal combustion engine starts more-or-less as a side effect. If the traction battery is dead, the 12v battery will start the car in a more conventional sense by driving a starter motor, though this shouldn’t happen in regular use. (At least, that was the case for my 2011 Civic Hybrid.)
1 Like
Interesting, and I think Honda’s design has changed since too–on or off the gas engine is a generator and contributes nothing to traction until the car’s going fast enough (think fifth gear) for its highly efficient but low-torque Atkinson design to maintain speed–a clutch engages it directly then (there’s no transmission). From there on the electric engine is secondary and is only used to assist during acceleration or going up a slope, for example. I think that’s the way Toyota’s design works all the time, but I’m really not familiar with it.
Otherwise my Honda is fully electric, with the Atkinson serving only to charge the big battery. It actually gets somewhat better mileage in the city (50 mpg EPA) than on the highway (48).
Managing it all really takes a car with a (12V) mind of its own. As for mine, it dates from before synchromesh was standard . . .
1 Like