In mira e:s there are two of the following functions.
1) Eco-idle - (Which means engine stops to save fuel, this also includes A/C compressor)
2) Eco - (Also called regenerative brakes, works as a generator for when you press brakes it uses that kinetic energy to charge the battery)
If im not mistaken, Both these fuctions do not (or completely) utilize the car battery, but instead there is also a small lithium battery pack under the front left passenger seat.
Hope someone with a Mira e:s can clear things up more.
P.S: I did a 2 week research for the car including 3 test drives, but decided not to buy it and save up for swift.
Peace.