I was wondering, is it possible to get the Prius into Ready mode with just a 12V battery connected to the battery charging posts under the hood, and no battery in the hatchback area? Obviously you couldn't expect to drive the car like that. But I just want to get it to Ready mode while I don't have a proper correct 12V battery in the back.
I don't see why not. The front 12V post is electrically equivalent to the 12V post on the battery in the trunk (provided the fuse isn't blown). But, please play it safe - don't remove the 12V battery when the Prius is in Ready Mode.
Yes you can start the car with a connection to the 12V jump post in the front. Once in Ready mode you can actually disconnect both 12V batteries and the car will continue to run until stopped. The 12V will be supplied by the DC/DC converter through the HV battery. If you disconnect the battery in the rear, just be sure to be careful and tape up the positive terminal so it does not come in contact with chassis ground since it is being energized by the DC/DC converter circuit that charges the battery. But why would you want it in Ready mode while changing the battery? With the backup battery connected to the 12V jump post all of your preset information will be maintained.
Glad to see that Toyota engineered the DC/DC converter to be well behaved even without a 12V battery attached to the output. It's quite possible to have a 12V source (DC/DC converter as in the Prius, or a standard alternator as in other cars) that is very well behaved with a battery attached (even a really crummy one) but will become ill behaved if there is no battery load on it. That's why I recommended playing it safe and not disconnecting the 12V battery when the Prius is in Ready mode.
