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Prius electrical systems each require a battery
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Prius electrical systems each require a battery

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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, 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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