Are Electric Vehicle 12 Volt Batteries Maintained by the Drive Battery When Off

Xklatmachsadx

Xklatmachsadx

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I heard that in electric vehicle the 12 volt battery get (limited) maintenance charge from the drive battery

Limited because I know that EVs with fully charged drive batteries that haven't been driven for a long periods of time will need a jump to get going (though I don't know why they don't just have a jump button for emergencies)

Anyone have any actual data on the subject!

I only know that unlike ice vehicles in which the engine must be running in order turn the alternator to charge the battery an EV can use the drive battery when off, but only if it is programmed to do so
 
Sadly, the 12v battery is ONLY charged while the car is ON or when the drive battery charger is pugged into the car. I tested it to confirm this. It is a trickle charge that is intended to top off the battery for during normal use. Usually the battery is depleted very minimally... except when there is drain from a dashcam in parking mode. This is why the hardwire kits have a cut off voltage. Because deep cycles shorten battery life and EV 12v batteries are quite small, it is better to keep the cutoff voltage higher, like 12.4 or 12.2. This will sacrifice parking mode hours but preserve battery life.
 
It seems to depend on which manufacturer, which model, and which version of the model, and possibly on what firmware updates it has had!

There are some EVs that will maintain their 12V battery perfectly well, as long as the traction battery is not nearly empty. I think these days the majority fall into that category, but there are certainly some that don't.
 
Nigel said:
It seems to depend on which manufacturer, which model, and which version of the model, and possibly on what firmware updates it has had!

There are some EVs that will maintain their 12V battery perfectly well, as long as the traction battery is not nearly empty. I think these days the majority fall into that category, but there are certainly some that don't.
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Which cars keep the 12v battery charged even when off and unplugged? Logic would think they all should. A simple BMS would prevent overcharging. Or at the very least, a means to startup the car with the drive battery and not depend on the 12v at all, but the claims are the other systems of the car require the stable 12v system to properly function.
 
robert91 said:
Which cars keep the 12v battery charged even when off and unplugged? Logic would think they all should. A simple BMS would prevent overcharging. Or at the very least, a means to startup the car with the drive battery and not depend on the 12v at all, but the claims are the other systems of the car require the stable 12v system to properly function.
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The Korean EVs seem to be pretty good, but all cars in Korea are fitted with dashcams, so they would have sorted it out!

I'm not an expert on fitting dashcams to EVs, and from what I've read, you need more than just general knowledge to answer the question accurately!
 
It's an option in the US, even for Korea cars. I think only Tesla is standard. They really all should include it, especially an EV that with proper battery management systems in place to not have the battery drain problems. Future feature for sure.
 
I was surprised that when the car is plugged in, the 12v is topped off even if the drive battery is full. I agree that the drive battery should maintain the 12v at all times.
 
However, Kia has a feature called 12V Aux. Battery Saver+, which monitors the 12V battery’s state. If the 12V battery voltage drops too low while the car is off, this system can automatically wake up the high-voltage battery and top up the 12V battery. This is standard on their EVs, even in the US.
 
I stand corrected on my original comment and change it to a "it depends on the car".
 
I suspect that EV 12 volt batteries are not as robust as those for those for gas cars as they don't have to crank an engine
 
Xklatmachsadx said:
I suspect that EV 12 volt batteries are not as robust as those for those for gas cars as they don't have to crank an engine
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True. And a dash cam parking mode for extended time can easily discharge the battery resulting in large charge cycles that shorten the battery life. That is why I noted higher cut-off voltages may better protect your battery. It would be nice if the dashcams had a dedicated Parking button to enable only when needed. Or geofencing.
 
I have a hybrid Honda Jazz and the 12v battery is very small because it's not used to start the engine. It's just there to power limited electronics when stationary

I've read countless reports on a Jazz forum of the 12v battery running flat after standing for as short as a month

I'm a low mileage driver now so I give it a CTEK charge every few weeks to keep it topped up
 
I think the limitation is because the high voltage traction battery needs a computer to monitor it to maintain safety and this requires some power which might be an issue if the vehicle is left parked for a long time without a charger connected.

It all depends on what safety systems they have.

Some deal with it by turning it off completely for maximum safety
 
Kremmen said:
Yes, it needs to be listening for keyfob remotes, alarm system, etc
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I'm not sure why they can't be powered off the traction battery, via a voltage regulator. Only reason I can see for needing a 12V battery is so that you can run the traction battery down to zero, or have fault with the traction battery, where it turns itself off, without losing safety critical power for things like steering. If you are going to have a safety backup battery then you might as well use it for other things that you might want to keep working on an empty traction battery too, like the door locks and alarm system.
 
Weird isn't it

The HV battery lasts 3 months according to the handbook, if you don't use the car, but then you have no way of charging it other than firing up the engine
 
Kremmen said:
I have a hybrid Honda Jazz and the 12v battery is very small because it's not used to start the engine. It's just there to power limited electronics when stationary

I've read countless reports on a Jazz forum of the 12v battery running flat after standing for as short as a month

I'm a low mileage driver now so I give it a CTEK charge every few weeks to keep it topped up
Click to expand...

CTEK is good gear. I use my MUS-3300 monthly for maintenance and conditioning of my AGM. My belief is that if one is using any add-on electronics in a car, hybrid or not, it is a good idea to have a quality external charger and to perform a monthly maintenance and conditioning charge. Alas, most people do not.
 
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