They should all be able to transfer current from the HV battery to the auxiliary battery…
Thanks, my mind didn’t comprehend the original post for whatever reason…
The separation is primarily a safety consideration. The hybrid battery is disconnected when the car is off. The separate 12 volt “logic” controls whether or not the hybrid system is activated. You’d have to keep the hybrid system active at all times if it was the primary means to energize and start the car. That would be hazardous and present opportunity for the hybrid battery to be completely discharged, potentially damaging it. The hybrid battery voltage level is also a risk to standard electronics. It could fry everything in the car if something went wrong with the regulation. So to be cost effective and use standard electronic parts, they kept the old 12 volt systems.
Now, could it be done? Most certainly. But the alternative method would be a lot more expensive, less efficient and a lot more complex. The current method is a balance of all considerations. It may not seem like it makes sense but all things considered, it is the best choice currently available.
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Some of them do, under the right conditions. Like when the HV battery is being charged in warm ambient temperatures and HV battery cooling is needed. The DC/DC converter will maintain 12V battery.
But there are other considerations. The 12V system needs to know it needs to be charged. So if a 12V battery is dead, the car has no way of knowing it’s dead, because there’s no 12V to operate the vehicle ECU. Also, I don’t want to have to disable the HV system on a car every time I disconnect the 12V battery. Imagine disconnecting the battery to replace it or just clean the terminals and the HV system tries to charge it.
Probably the lead acid battery lobby or something. The amount of abuse 12V lead acid chargers do batteries to seems to be deliberate. All chargers either over charge or under charge. Also as someone else mentioned, it can be for safety since the HV system is disconnected when the car is off.
If there is enough in the 12V to get the car on, then it will.
Someone said the 12V battery doesn’t do this, the hybrid battery powers the starter. That’s why I was wondering.
Where have you been for the last thirty years?
Tester
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Like I’ve said, most hybrid engines are started by the hybrid motor and hybrid battery, and do not have a 12V starter. They do have to have a charged 12V battery to power the electronics and computer(s) that govern the hybrid system. So dead 12V battery = no start for most hybrids. It looks like Kia/Hyundai has a setup to address some of this issue.
Funny, my newer battery charger has brought a few battery’s back to life, the oldest right now still in service is the battery out of the old van that was bought back in March 2009 (battery not van), that is an almost 17yo battery, it was pulled out of the van after the transmission failed and put in the 2009 Cobalt after the OEM battery failed, then ran up until 2018 when the Cobalt died, after sitting outside in the weather for 5 years, I decided to bring it back to life in 2023, the battery was 100% dead, 0.00 volts, I had to jump charge it off one of the cars enough to get the smart charger to kick in and start charging it, I spent about a week and a half of charging.. The charger would turn off, I’d wait a few hours to a day depending on how I felt and then start charging it again, at some point in the begging the charger went into desulfate mode, well the Cobalt and the battery has since been through a couple of very hot and humid summers and some below freezing winters, so far the almost 17yo battery is doing better than the Cobalt is… lol
I have done this to a few battery’s to bring them bad to life after they had tested bad… Back in the day, before smart chargers, we would just run the battery 100% dead and then hook up a 1 amp charger to it and leave it for a week , brought back many dead battery’s that way… Heck when I was at U-Haul, the service manager would hook up 10 to 20 battery’s at a time to one charger and bring most of them back to life…
The hybrid battery does NOT power the starter
A hybrid vehicle doesn’t even HAVE a starter, afaik
Might be time to move on from the 1990s and read up on how things are nowadays
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What brand is it? My experience with Interstate marine deep cycle is that they develop shorted cells after 8 years, and part of the fault was me not fully charging it for a long time. Your battery may be restored and able to start the vehicle, but is it even 10% of its rated capacity? If you leave the headlights on for 5 or 10 minutes will it still start?
What is the end of cycle float Voltage on that Schumancher charger that you linked, does it stay in float forever, and does it have temperature compensation?
So what do you do if a NiMH battery Prius has been sitting for 18 months and the NiMH traction motor battery is dead? Take it in for service if it jump starts and turns on but cannot start the hybrid system?
How Does Battery Float Charging Work?
Battery float charging operates by delivering a constant voltage to the battery, compensating for self-discharge without causing overcharging. This is achieved using a regulated power source that keeps the battery at its full charge level while minimizing stress on its internal components.
The Float Charging Process
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Voltage Regulation – The charger maintains a steady voltage, typically between 2.25V and 2.30V per cell for lead-acid batteries.
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Compensation for Self-Discharge – Batteries naturally lose charge over time. Float chargers supply just enough current to counteract this loss.
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Prevention of Overcharging – Unlike trickle chargers, which can continuously send current and cause overcharging, float chargers adjust output based on battery needs.
Why Is Float Charging Important?
-
Extends Battery Life – By preventing deep discharges and overcharging, float charging enhances longevity.
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Ensures Readiness – Ideal for standby applications like UPS systems and emergency backups.
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Minimizes Maintenance – Reduces the need for frequent battery replacements and manual charging.
Tester
It is a regular run of the mill MT34 Interstate (green top) battery, not even the better MTP version, it is not a marine or any kind of deep cycle, yes it is fully charged, just the other night, we were messing with it trying to get it to start, shifter bushing(s) worn out keeping it from going into park, the head lights were on for 15-20 minutes outside in the cold, once I got the transmission shift lever to fully go into Park, it started right up, like normal… I was watching the gear command in live data to see what was wrong and I thought for sure it would have ran the old battery down, but it didn’t… So YES, I brought the (dead for years) battery back to life, but like I said, it is a slow process that most people don’t have time to do…
Will ruin a battery in 2 to 4 years. That information has the format of being generated by AI.
That’s impressive from a 16 year old battery!
Edit: It may be the case that if a lead acid battery is discharged very slowly, it responds much better to desulfating.
.I think you were generated by AI by your comments.
Tester
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The Kia Niro has a built-in override for a dead 12V battery. There’s a dash button that lets the car start using power from the EV battery if the 12V won’t start it. Once started, the 12V battery recharges normally. Ours had this issue once after sitting for months before purchase, and we haven’t had any starting problems since.
Interesting discussion on hybrid starting systems. The Kia Niro’s override button using the traction battery when the 12V dies is a neat feature, and it’s cool how different brands handle it—Toyotas mostly rely on the big HV battery and motors, while some like Chrysler stick with a regular 12V starter. Good to know the 12V is still crucial for waking up the electronics even in many PHEVs. Thanks for the breakdown!
This exact thing happened to our Lexus RX350h, and I wish it had the Kia/Hyundai system. Instead, I had to go through several steps to mechanically open the rear hatch (not simple) to get to the 12V battery under the rear floor to test it and charge it. A real pain.
I looked in to it myself. If a NiMH Toyota Prius sits for several months, the HV battery will be dead and it needs a special 200+ Volt charger. The lithium versions would stay charged longer, but repeated starting attempts when out of gas can cause the same situation.
It would be nice if they let you put the car in drive and pushed it along to use the regenerative braking to charge the HV battery or get it started, but I doubt they allow that. A $15 DC-DC converter connected to another 12V jump starter terminal could have been added to let customers slow charge the HV battery, but that would take money away from the dealer. You can make your own DC charger for the NiMH versions. An isolation transformer, two diodes, two capacitors, a fuse, and a high wattage resistor would make 120VAC to Prius slow charger. Otherwise it takes a special charger or a tow to the dealer. Not sure about the lithium version as they may need over charge protection.
I have worked on several hybrid vehicles that were stored for 12 months or more; recharged or replaced the 12-volt battery was all that was needed.
Toyota/Lexus shipped HV battery chargers to dealers around 2015. I have never seen a technician need to use the high voltage charger to revive a hybrid vehicle.
Are going to push a hybrid vehicle 10 miles to charger the HV battery? How much current can you generate at 20 mph?
Discharged high voltage batteries are few and far between, a non-issue. If vehicles were being towed to dealers to have batteries recharged, it would be the towing services that profit. Dealers don’t get paid much if anything to recharge batteries.
More than several, there are 4 steps.
- Raise the hood.
- Connect battery jumper.
- Push power back door button.
- Lift cargo floor panel to access battery.
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5 Amps, at 200 Volts. It depends a lot on how steep the hill is. Less than 50 feet at 20 MPH will start a manual transmission vehicle.
Lithium can stay charged for years. I’m talking about the NiMH older Prii. Most people don’t let their cars sit for 6+ months, but those that do may need a charger. Back then the batteries could go dead in the lot before the car sold, so the dealers had a a high Voltage charger for them.
All you have to do on most manual transmission cars is have them moving, like 3-5mph, and dump the clutch and they will start, only a few feet can do it sometimes…
Where in the world did you get 50 feet at 20 mph from?? I have push started many vehicles in a small parking lot in a short distance…
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