Dear Car Talk:
We recently had to jump-start our 4-year-old Toyota Prius. It turns out that the Prius uses a 12- volt battery for starting. My question is: When the Prius has a large battery that can move the car, why does it need a separate 12-volt battery to start the engine? Is this just poor engineering? — Malcomb
Good question, Malcomb.
Here’s the answer. For decades, cars have used 12-volt electrical systems. Those 12-volt systems run computers, radios, windshield wipers, lights, seat heaters, butt scratchers and pretty much every other accessory.
So when Toyota’s engineers built the Prius, they could either grab all that 12-volt stuff off the Toyota shelves and put it in the Prius, or they could start from scratch and design a whole new set of electrical components to run on high voltage.
Not surprisingly, they took the easier route. Why spend time designing a new windshield wiper motor when you’re already reinventing the propulsion system?
And that’s why there are two electrical systems in your Prius. One is a 12-volt system that runs all the traditional electronics. The other is a high-voltage system that powers the wheels and the motor-generator that starts the gasoline engine.
OK then, you may ask: If the high-voltage battery runs the motor-generator, which starts the gasoline engine, why can’t I still drive my Prius when the 12-volt battery is dead? It sounds like I don’t need the 12-volt battery to start the gasoline engine.
Another good question, Malcomb. And here’s the answer: All of the car’s computers run on 12-volt power. And in order to turn on the high voltage battery, the 12- volt battery has to first power up the 12-volt computers that control it. That’s why your Prius is useless when its 12-volt battery dies.
But think of the good you’re doing in the world. Those AAA drivers’ kids have to go to college, too, Malcomb.
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(c) 2021 by Ray Magliozzi
and Doug Berman
Distributed by King Features