“If you have a home charger, keep the vehicle plugged in at all times to ensure the battery will maintain a full charge.”
I wonder why they included this there as it’s generally recommended to avoid 100% to maximise battery health in the long term.
Teslas can be exceptions as they have vampire drain and so Tesla recommends keeping the car always plugged in. However, Tesla software allows one to set the car to keep the charge at lower than 100%.
It’s generally good because you can pre-condition the car and not loose any range, but it also ensures the 12V battery is correctly maintained at a good charge level, otherwise you get all manner of oddities and worse the car may not even start. Having it plugged in all the time, assuming you have that ability, is best practise for a lease/subscription car, as nobody is worried about maximising long term battery health.
Which cars that Onto offers require being plugged in for pre-conditioning? I know that most don’t. Likewise 12V system is usually designed to be charged from the main battery – not mains.
Most cars will preheat via an app on demand while unplugged, but most timers will only function while plugged in. Doing it while plugged in will pre-heat using mains power, rather than wasting power from the battery (This was the point that @burnt_crisps2 was making)
Off the top of my head, The BMW i3 is one example of a car which will charge the 12v battery while switched off and plugged in to a charge point. I believe Tesla is another.
@Koda beat me to replying and as he stated that was the point I was making. In the winter months plugging in is recommended to ensure you always have max range.
Without getting into too much electrical/electronics jargon, all the energy in a car main battery (traction, high voltage, usually 400v, but newer ones are moving to 800v) provides the energy to drive the car (electric motors) and also effectively provides the energy to operate all the electrical devices which are actually connected to the 12v battery, just like an ICE. Using wipers, heated front/rear screens, lights, fans, indicators, entertainment system etc. are usually all standard 12v connected devices. The traction battery keeps the 12v battery charged, so the traction battery needs to be kept charged up. If the traction battery is low on charge then the 12v battery might not be charged enough and even lead-acid 12v batteries don’t like being cold. Connecting to the mains is the only way to charge both the main traction battery and as a consequence the 12v battery. All of the 12v charging is managed by various technology systems which are probably using the 12v battery supply to achieve this.
At this time of year everything is a high drain on the main traction battery which does not like being cold and performs poorly compared to the summer months. Pre conditioning the car in the morning whilst it is connected to the mains means you still have a fully charged main traction battery and the car is warm and in most cases the main traction battery has also been warmed up to so you get maximum performance from the car and range also.
In general most people charge their phone/laptops as often as they can. EV’s should be treated the same.
The key is the temperature is lowest overnight, so battery performance is always at its lowest overnight too.
And since it’s a winter theme. Be prepared, get those thermal blankets, snow shovels, thermos/drinks etc. Stored in the car ready for the worst the season can throw at you.
Many EV drivers will also carry de-icer, scrapers etc. and the reason is they can plug in at home, but at work they don’t have that capability. So when they get to return home from work, it is dark and cold and using the main traction battery to provide all the power to warm and clear the vehicle screens might compromise them having enough range to safely get home.
Nicely put… 
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