Year by year, lithium-ion battery technology improves, and EV range and performance leap forward. We’re now seeing electric pickups trucks with what would’ve been supercar acceleration not too long ago, a sedan with 520 miles of range, and Hyundais and Kias that use 800-volt charging. And yet, most of the electric cars and PHEVs on the road right now, whatever their range or 0-to-60-mph time, depend on a relic to get moving: a 12-volt battery, usually of the lead-acid variety. Your Tesla Model 3 Performance might have dual motors and the ability to drift, but its lithium traction battery is worthless without the help of a battery you might see lining the shelves at your local O’Reilly’s. And if you kill it, you’ll be bricked, regardless of how much charge is left in the high-voltage battery. In that photo above, the Bronco is jump-starting the Leaf, not the other way around. Water, water everywhere, but not a drop to drink. Why is that?
There are a couple of reasons. First of all, an electric vehicle has two distinct needs when it comes to dispatching electrons: moving the car and everything else. Propulsion is handled by the big, expensive, latest-and-greatest high-voltage battery, because you need lightning in a bottle if you want to do the quarter-mile in 9.4 seconds. For charging, the more voltage, the better. But powering up the stereo does not require 800 volts. Nor would you want that coursing through every circuit in the car, for a variety of reasons. Safety, for one.
We asked Hyundai’s EV engineers why the 12-volt battery persists, and Ryan Miller, manager of electrified powertrain development, responded. “All the ECUs in the vehicle are powered from the low voltage, as well as the power relays that separate power from the high-voltage battery pack and the rest of the high-voltage network in the car,” he said. “That separation allows us to safely disconnect the high voltage from the low voltage when the vehicle is not being driven or in the event of a crash.” You don’t want first responders to contend with door locks powered by Doc Brown’s Mr. Fusion.
Read more: Why Do Electric Cars Still Use 12-Volt Batteries?