How good are these vehicles at knowing the amount of juice left in the battery? I'm completely ignorant when it comes to electric car battery technology but I c

How good are these vehicles at knowing the amount of juice left in the battery? I'm completely ignorant when it comes to electric car battery technology but I can't help but think about experiences with mobile phones and laptop computers where I have been using them and suddenly lost a large amount of juice for no apparent reason. Either something drained the battery very quickly or the system was miscalculating the amount of energy left. Anyway, it's one thing for that to happen when I'm browsing reddit, it's another thing when you're cruising down the highway at 70mph.