EV Hub article – Will my battery go flat if I leave my EV parked for too long?

Several factors determine how your EV will be affected by phantom drain

The fundamental principles behind the lithium-ion battery technology used in EVs today mean that no battery can hold a constant state of charge for an extended period of time.

Of course, the most obvious factor that affects how long your EV can be left before its battery runs flat is the amount of charge it was parked with. While it is intuitive to think that the more charge an EV has, the longer it will take for its battery to discharge, the level of charge can also affect the actual rate of discharge, or phantom drain. In effect, this could mean that an EV parked with 80% charge may initially discharge faster than one with 50% charge.  

More: How to maximise EV battery longevity?

Additionally, just like combustion engined cars, most electric vehicles also use a 12V (typically lead-acid, although 12V lithium-ion batteries are also becoming more common) battery to power a number of onboard electronics and other functions that must remain active even if your EV is parked and switched off. These include systems such as the central locking, clocks and vehicle alarms. In many EVs, various battery and thermal management systems will also remain active in order to ensure the longevity and durability of the main battery.

Using a device known as a DC/DC convertor, the 12V battery will draw charge from the EV’s main battery to power many of these systems, thereby being an additional source of phantom drain for the main battery.

Other factors that can affect the rate of discharge include various environmental factors such as the ambient temperature. The rate of discharge may increase if the temperature is both too cold (such as freezing or below) or too warm.

In its Owner’s Manual for the Model 3, Tesla, for example, claims that the vehicle’s battery can discharge at a rate of approximately 1% per day, although this may vary depending on the specific vehicle’s settings and configuration. The company recommends that owners leave their car plugged in if they will be away for an extended period of time. 

Connected features may cause your battery to drain more quickly

Many EVs are connected cars, and offer features such as OTA updates, app connectivity and advanced dashcam and vehicle monitoring features such as Tesla’s Sentry Mode. To function, these features all require in-car modems and other vehicle components to remain powered, which may drain the battery more quickly, even if parked.

For example, Tesla claims its Sentry Mode will drain the vehicle battery at a rate of 1.6 km/h. Thankfully, Tesla vehicles will turn off Sentry Mode if the charge level falls below 20%.

For more information on Tesla’s Sentry Mode, please see our earlier story on Tesla’s Sentry Mode and other in-built dashcam features.