2008 Prius 12 v electrical system question

are you looking for power when the car is ready, aux or off?

 

the 12v battery is in the right side of the cargo area behind the wheel. just hook up to the battery. for switched power tap into the lighter socket.

 

The Prius provides a very nicely regulated 12 V supply (usually right around 13.8 V when READY, unlike traditional cars that would swing around between 12-something and nearly-15 depending on engine speed) and you should look up the specs for your own year but in mine the total capacity from the regulated source is about 100 amps (which has to include everything the car uses, what you get to use is less ... how much less depends on whether you're using the headlights, wipers, heater fan, defog, PTC heaters, etc.).

In my year and I think also the gen 2 (not sure about the gen 3), the converter/regulator also has an output signal called IDH that it can turn on when too much current is being drawn. That signal is wired to the heater controller, which won't turn on the PTC heaters if the signal is on. If you're thinking of an accessory that might draw a lot of juice, it might be a good idea to have it also watch that signal.

The 12 V battery has relatively small capacity, so people mostly wire their accessories to be powered only when the car is READY, and not just in ACC mode. Because the 12 V battery and its connections are right inside the car, it is easier to attach new circuits than in most traditional cars. If they haven't changed the design from Gen 1, the battery positive terminal contains a fuse, then a cable running the length of the car. There's another fuse at the other end before it attaches to the DC/DC converter. The best place to tap off is essentially where the cable connects to the battery terminal assembly, or to the cable-side of that terminal fuse. That way your tap is fuse-protected both with respect to the battery and to the DC/DC converter. Those are high-amp, high-interrupt-rating fuses; of course you'll add a fuse on your own circuit appropriate to its wire size.

Building a circuit to turn on only when the car is READY is quite easy and doesn't require digging out any particular switched circuit wiring already in the car. Because the bus is at regulated 13.8 or so at all times when the car is READY, and the battery on its own won't be over 13, you can just tap in right there at the battery with a voltage-sensitive switch set somewhere around 13.5, done and done.

A fully worked out example (for Gen 1 though) is here.

In that one, I rolled my own voltage-sensitive switch, but there's a better off the shelf option I found later, the VCM-06. It does exactly the right things, plus has extra features my homebrew doesn't. You can set it to keep the power on for a set time after you shut the car off, so if you're running something you want to keep going while you stop for gas or a bite to eat or whatever, the 12 V battery can run it that long. Also, the VCM-06 has a second, lower, safety voltage threshold, where it always will shut off if the voltage falls below, so you can't come back to a dead battery.

-Chap