Asking on another’s behalf. I don’t want to give too many details including the car make and model.
This person’s new car has the ability for you to ‘start’ it and also ‘turn off’ when you finish the journey. It’s confusing what the turn off really means. It keeps lights on for some time after you disembark the vehicle whether you want it to or not, and if you open a door it turns on the instrument cluster screen to display a diagram of the car with a door being opened. You can also turn on the infotainment screen and access some but not all options. The manual has some warning about not plugging things in when the car is off as it could drain the battery.
Is there some physical state the high voltage battery is in when the car is ‘started’ that’s different to the state it’s in when it’s ‘off’? Does it have some effect on wear when the battery cycles between those states too often?
This issue came up when they were thinking of buying a dash cam. The dashcam was designed mainly for ICE vehicles and has a feature called ‘park mode’ where the camera can be in a kind of standby off state while a vehicle is parked and the car engine is off, but can switch itself on if it detects some kind of movement or impact like if someone drives into your parked car. The dashcam website has some warning saying that for EVs, you should buy a separate battery pack for it because this ‘parked mode’ doesn’t work if the dashcam is installed hard wired in to an EV. This is confusing because the 12v battery should always be accessible regardless of the car’s “on/off” state and I would have thought worked just like it does in an ICE car, whereby the camera continues to draw some small amount of power to power the standby mode and allows the maximum power draw the camera could need if the camera is triggered by an impact. In ICE cars, this usually only works when something is hardwired because somehow the cigarette lighter outlet doesn’t work without the engine running, (I guess by design so you don’t drain the battery with accessories and can’t start?) but it sounds like from the manual in this EV it continues to work whether the battery is considered “on” or “off” but conversely somehow if you hardwire an accessory to it doesn’t?? It’s unclear as well whether that means the dashcam’s park mode would work if you plug it in to the cigarette lighter outlet of the EV rather than hardwiring, or if it just doesn’t work in EVs no matter what you do and requires its own battery, which seems unlikely but is not spelled out anywhere.
In most EVs, the 12V battery is much smaller than in an ICE vehicle. That 12V battery is responsible for powering nearly everything while the vehicle is “off” unless it needs to reconnect the HV system for something like maintaining pack temperatures or timed charging. Most EVs will physically disconnect the High Voltage battery pack when not in use, so the 12V battery will not be recharged while “off”, similar to an ICE since the alternator can’t recharge that when the engine isn’t running either. Unlike an ICE, you can’t jump start an EV so a dead 12V battery is a much larger headache.
For a power usage comparison, my Tesla Model 3 Long Range (75 kWh) with it’s built-in Sentry mode, recording from all the cameras around the vehicle, and the sensors for motion, etc. can use up to 10% of the HV battery capacity per day. A regular separate dashcam won’t use nearly that amount of power, but it also won’t be able to pull from the HV battery either, just the small 12V one. IIRC the smaller 12V batteries are only like 50 Ah, so that would only be 0.6 kWh, or about 2% the size of the HV battery. I think I did all that math right, but don’t take it as 100% accurate.
Point is, the 12V system in an EV is VERY small, and not meant to power anything other than the basics while the vehicle is not in use. If it dies, you can’t just jump the vehicle to get it working again either, and in some vehicles may not even be able to unlock or open the doors, so you REALLY don’t want it to die.
doesn’t it engage the HV battery to top up its own charge once it dips below a certain level?
Depends on the vehicle. There’s not really a standard for this stuff. Heck, we barely have a national charging standard in North America now.
Either way, ideally you should have a way to charge at home where it doesn’t really matter. Plugging it in whenever it’s parked maintains not only charge level, but allows the Battery Management System to optimize and level individual cell usage.
Incorrect about not being able to jump a dead EV. You still very much can and there are terminals for it (seen it used on an Audi and MB), the issue is you have to let the other system charge the battery, rather than “cranking” your own to get power. As such, there’s an increased chance for damage and manufacturers dont want normal people to do it.
Source: I had an EV with a dead 12V and the tow truck driver sent by the dealer just charged it for a bit so I could drive it to the dealer. (Turns out the HV to 12V charging system, which acts like an alternator, had failed)
I was simplifying so someone doesn’t destroy something because they tried to treat it like jumping a normal ICE. If there’s anything we have seen from people when it comes to EVs, it’s that they’ll do really stupid shit because they don’t do even the tiniest amount of research on things that should be treated differently. Easier to just tell the average person not to do it, and for those that actually research, they’ll know what to ignore.