A Mystery

Four years ago I had just graduated college and didn’t own a car. My mom was visiting and I wanted her to be able to get around, so I rented a really old Mercury on one of those car sharing sites because it was cheap.

Well, its battery died and I needed a jump. Today I have one of those jump-starting packs because they’ve become ridiculously affordable and because I actually own a car, but back then all I had was my friend’s Prius C – a hybrid car.

The first surprise was that the battery was underneath the back seat. Not in the trunk, not under the hood, but underneath the freaking back seat. OK, we found it after digging through the manual.

The second surprise was that it was incapable of a jump. It seemed to trickle charge my rental’s battery, but even after hours of trickle charging there was never enough to start the car. I believe at the time I went out to Wal-Mart to get myself one of those jumpstart packs, and it was sort of expensive.

Now, if the car is a hybrid, shouldn’t it have a more substantial battery?! After all, it’s capable of going miles on battery power alone at times.

LV Battery

Today I learned why this was the case: that battery under the seat was one of TWO batteries in the Prius. I’m guessing the actual large battery is not very accessible. The under-seat battery is also known as the Low-Voltage (12v) battery, or LV battery.

The LV battery is fed via a step-down DC-DC converter from the high voltage, high capacity battery. Its main purpose is to turn on the computers of the prius and control some subsystems including lights, wipers, brake pump, radio, and electric power steering. Components like the A/C compressor is obviously linked directly to the high voltage (HV) battery.

Hacking Hybrids

With the SDC craze you see lots of people cough hacking their cars to do various things. And some of these cars are hybrids. You can imagine: if your computer systems are partially fed by the LV battery and you turn off the HV battery first, you might experience rapid discharge of the LV battery which subsequently means your car won’t start until the LV battery has been brought back to life!