Your vehicle doesn't truly sleep when you lock it.
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It's already costly enough to leave your EV at the airport for a week without it quietly discharging electricity while you're away. You could end up with a reduced battery percentage, even if the odometer hasn't budged. The truth is that an EV will lose a bit of charge while it's parked, but the amount lost will vary depending on the EV and what it has been doing while you were away.
There are a lot of factors involved when it comes to EV energy loss. Lithium-ion cells can lose charge naturally through electrolyte oxidation and reduction via a process called self-discharge. However, that is negligible compared with the electricity used while driving. The larger loss is from systems that continue to operate after you've locked the doors. It is not usually a problem over a weekend, but if you leave an EV unplugged for a couple of weeks, those little losses can begin to add up.
A parked EV still has work to do
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A locked vehicle doesn't automatically shut down every electrical system. The battery management system periodically checks battery conditions, and the high-voltage battery is sometimes called upon to recharge smaller low-voltage batteries used for the locks, computers and other electronics. Tesla says that its Model 3, when parked, routinely performs these system checks and any necessary battery maintenance. Rivian also confirms its R1T also uses the high-voltage battery to maintain its 12V battery.
Then there's the convenience tech. Connected services include the ability to check the status of your vehicle, start the climate control or prepare the cabin before getting behind the wheel. Tesla explicitly mentions app requests, climate functions and infotainment as sources of power consumption while parked.
Third-party apps that repeatedly wake a sleeping vehicle can add to the drain. If you've left cabin cooling enabled during hot weather, it's not cooling for free. That's one area where Slate's barebones electric pickup might have an advantage, with fewer connected features to keep running while parked, though its actual battery drain remains untested.
Cold weather complicates the situation further. The range estimate can decrease with battery temperature fluctuations, even if the vehicle has not used an equivalent amount of electricity. Model 3 owners can use the Energy app's Park tab to see how much energy the car consumed while parked. This is more informative than watching the range estimate move around. What drains an EV battery while driving is another story.
When parked battery drain becomes a problem
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Optional features can make the biggest difference. Tesla's Sentry Mode employs cameras and sensors to keep an eye on suspicious activity, and Rivian's Gear Guard Motion Cam can capture activity around a parked vehicle. That's a fair compromise in an unknown parking lot, but an electronic lookout does require power.
How much charge can you expect to lose? Tesla's Model 3 manual says the battery can lose approximately 1 percent per day, depending on the settings and conditions. Recurrent reports that sleeping Teslas can lose just 0.1 to 0.3 percent a day, while those running energy-intensive features can lose several percent daily. These figures reflect different conditions, so neither is a guarantee.
The size of the battery is also important. A 0.5kWh loss, for example, is roughly 1.2 percent of a 42kWh battery and 0.6 percent of an 80kWh battery. That makes the same drain more noticeable in smaller city EVs.
If you are going to leave your car for a long time, turn off unneeded surveillance and climate controls, limit apps that repeatedly wake the vehicle and make sure you have enough juice to get back. Tesla actually recommends keeping the Model 3 plugged in when it's not in use, especially for several weeks, and maintaining around 50 percent charge during long-term storage.