In a recent column a reader asked about a plug-in hybrid and the status of the gas. The discussion also mentioned the possibility that the 12v “starter” battery could drain over time. I don’t know about other cars but the Kia Niro has an “override” that can use the EV battery to start the car. That was the situation when we originally bought our Niro. There is a button on the dash that, if the 12v battery will not start the car, enables the starter to pull the necessary power to start the car. The battery then charges as normal. We have had no starting problems since that initial situation where the car had sat for several months before we bought it.
All Toyota hybrids and PHEVs (and many others, like my MKZ hybrid) use the large hybrid battery and one of the hybrid motors to start the car. The problem with a dead 12V battery is that it is used to power all the accessories and much of electronics, so a dead 12V = no start.
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All batteries drain over time. Lead acid more than Li-Ion. And all cars never completely turn off and drain a very small amount when the car is off.
That said, the Niro’s system design is smart. There is no reason a hybrid or plug in hybrid should not be able to do this.
My Chevy Volt (plugin hybrid) does the same. Hybrids do not have a traditional starter motor. The electric motor(s) turn the gas engine to start it. Very quickly and quietly I might add. When my car decides to start the engine I hardly know it’s running!
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Looks like the Niro uses a ‘hybrid starter generator (HSG)’ driven by the serpentine belt. Interesting to have the starting dependent on a belt. But it uses the hybrid battery, not the 12V battery, for power, as far as I can tell, just like other hybrids.
There are as many different starter configurations as there are PHEV designs. Some use a conventional 12V battery and starter, some use an aux 12V battery, some use the high voltage battery and motor to start the engine, some use both depending on what’s happening with the car.
It’s useful to remember that any HV component is managed by a 12V control system, so if the main 12V battery is dead, so is the hybrid system.
Conversely, on some cars, if the hybrid system is inoperative due to a fault or simply design limits (like outside temp too cold), the gas engine is inop as well.
I imagine there are, but I’m only familiar with the ones that use the high voltage systems. Which ones use the 12V system?
Mercedes has some ‘mild hybrids’ with a 48V battery, I’d guess that’s the only battery.
EDIT: My brain read are PHEVs, not and PHEVs… Oopsy
Forgive me, I have a hard time comprehending what I read sometimes…
Not all Toyota’s, the 2024/2025 Tacoma Hybrids are not PHEV’s and you can still get at least the Prius in non PHEV The Corolla Hybrid is not even offered in PHEV…
I think that most of the Toyota Hybrids are not PHEV’s… Well more standard Hybrids than PHEV’s…
I’m referring to both their hybrids and to their PHEVs. I didn’t say they were all PHEVs, they’re not.
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I did an EDIT to the post… My bad… 
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I believe Chrysler hybrid products use the conventional 12V starter to get the engine going.
Mazda PHEV and mild hybrid systems have a 12V starter mounted to the engine, and it’s used to start the engine. However, during certain times like autostop or while transitioning from EV to ICE mode they use the electric drive motor to “kick start” the engine.
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Good to know. I was disappointed to see that Ray didn’t get it quite right, maybe he was trying to keep the answer simple:
Well, I don’t know that you could say he was 100% wrong. I mean the 12V battery doesn’t start the gas engine in, for example, a RAV4. But every HV component is operated or managed by a 12V system. So even if the car uses a HV motor and is not equipped with 12V starter, if the 12V battery is dead, so is the car.
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How does someone make use of the 12 Volt jump starter terminal on a 2012 Prius?
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Exactly. Like I said, he didn’t get it quite right, but he’d have to go into a lot more detail. He was right that dead 12V = no start.
To power up the 12V system to allow the ICE to start. However, I found that with my MKZ hybrid (which used the same hybrid system as many Toyotas), if the 12V battery was dead from age jumping didn’t work, and I had to replace it to get the car moving.
As on moms 2010 Prius there’s a positive term8nal under the hood and a threaded rod running to ground for the negative. Had to use a jump pack once when it wouldn’t start after she’d driven about 8m8 to yhe church. New battery installed the next day. Seem to get 6-7yrs out of the 12v. She has the battery checked regularly now.
Tesla model S and others you’re locked out of the car with an emergency release to get into the Frunk
Top gear host James May put in a trickle charger lead like youd use on a motorcycle to have handy to recharge the 12v battery which the way he explained it the ev battery gets fully charged on his garage charger but the 12v doesn’t get charged sitting in the garage.
Never understood why they designed a Hybrid or maybe an EV that has a large HV battery and yet it doesn’t do anything (for some anyway) for the LV (12V) battery, like a built in battery charger/maintainer… Another wise, why can’t it self jump start itself, as long as the HV system is up, I mean some of them have 120V AC electrical outlets, but can’t jump start themselves, just stupid imho…
From what I read on another forum, Kia and Hyundai systems can transfer current from the HV battery to the auxiliary battery during the power up process. Other manufacture’s hybrid systems only charge the auxiliary battery while in “Ready” mode. This is what the original poster was trying to communicate.
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I can confirm that. My Niro 12v battery was virtually dead when I went for a test drive. The sales person said that wasn’t a problem. There is a button on the dash called “12V Battery Reset”. If the 12v battery is discharged, this will allow the car to start. But you need to run the car for a 30 minutes or more. The battery will charge even if the ICE is not running.
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