Always read the owner’s manual first for specific battery maintenance information
When it comes to the main EV battery though, things are a bit more complicated. The first thing you should do is check your owner’s manual as each EV manufacturer offers different advice for laying up your car for an extended period of time. For example, the manuals of the Tesla Model 3 and BMW i3 recommend keeping the car plugged in. In contrast, instructions for the Nissan Leaf (2015) and Hyundai Kona suggest leaving the car nearly fully charged and only recharging it once every three months. The Chevrolet Bolt EV’s manual states it’s best to plug in your EV for any shorter times (up to four weeks) but to disconnect it for any longer periods. Finally, the manual of the Renault Zoe offers no specific advice on this at all.
By the way, don’t worry too much about your 12V battery (which supplies energy needed to operate lights, wipers, audio systems and so on). Most EVs, such as a Tesla Model 3, will automatically use energy from the main battery to keep it charged and make sure it doesn’t die.
Keep your EV plugged in unless your car manufacturer recommends otherwise
In general, as highlighted by EV expert Saul Lopez, it's best to leave your car connected to your home charger if you won’t use it for several days. Typically, both plug-in and electric hybrids have programs to automatically manage the battery charge based on the residual consumption, time parked and outdoor temperature. As a result, they self-manage the battery charge to maintain the health of the accumulator and store as much energy as possible.
What is the best charge level setting for your EV battery?
If you have a smart charger, set your EV charger to the recommended charge level stated by your EV manufacturer. If nothing is mentioned, maintaining a charge level of 65-75% to optimize battery life even further as lithium-ion batteries perform best when operating in this charge range. In contrast, complete charges or discharges are best avoided for several reasons. For instance, research shows that letting your charge level drop to close to zero will have a negative impact on battery life. Hyundai's video on how to maximize your EV battery’s life comes to the same conclusion. Renault Zoe’s driver handbook states its EV battery might even be impossible to charge if you leave the car parked for several months at a near-zero charging level. Additionally, Tesla points out that other components may get damaged or require repairment (i.e. the 12V battery) if your main battery level reaches 0%. Elon Musk himself also explains that a 100% state of charge is bad for the energy efficiency of your vehicle. Finally, research from the Technische Universität München shows that high state of charge levels reduce battery life.
Turn off features you don’t need & avoid extreme conditions
If you won’t be using your car in the upcoming weeks, it makes sense to turn off any battery draining functions in your EV. This includes automatic map updates, pre-warming or cooling, and so on. In addition to that, always avoid exposing your EV to extreme weather conditions as this might damage the main battery. Finally, remember that while your battery might (rightfully) get the most attention, you should still follow traditional car maintenance practices for everything else during this time.