how do i save the cars memory when replacing the 12v battery?

When I took my 12V batter out I placed an old scrap battery in parallel via jumper leads to the front jump terminal. You can do the same thing with a 12v power supply or a battery charger, as long as it's got a regulated output. Some old battery chargers just have an unregulated rectifier output, so I wouldn't recommend leaving them connected to the 12V system with the battery removed.

 

BTW. If you power the front jumper terminal while removing the battery, then be careful to remember that the 12V +ive terminal assembly in the rear will remain live even after you disconnect it from the battery. So have a couple of solid plastic bags on hand when you do the job, and stuff the +ive terminal assembly inside a plastic bag as soon as you remove it from the battery. This will save you from shorting anything out.

 

I've used a low amperage battery charger when swapping batteries; the car's settings were retained. I believe it's 0.5 amperage. You basically need a modest voltage source, not a lot, but I think a bit more than (for example) a rectangular 9 volt battery.

With the Prius I would connect the charger's (or battery's) positive lead at the usual charging location: the red capped copper bar in the under-hood fuse box. Then connect the charger's negative to bare metal on the car body or engine. And finally, plug the charger into 110 volt.

After the 12 volt battery has been replaced, reverse the the above steps to disconnect the charger.

 

When you replace the 12 volt battery how do you restore the memory? Does the car prompt you and guide you in the process?

 

^ There's nothing to "restore". The car remembers some settings, as long as it's supplied with sufficient voltage during the battery swap. It doesn't know anything's happened with the battery, as long as you keep it hooked up to a voltage source during the transition. But:

If that voltage is interupted, the settings are gone. No big deal. In my experience the biggest hassle was the drivers door window losing the auto up/down function (the passenger window, with same function, was ok, go figure). I think you retrain it by running it up manually and then holding the up switch for a few more seconds. But I'm hazy on that.

Maybe in a few years some manufacturer will start putting in a jump drive, something that can retain memory without power. And the other's will follow suit.

 

That would be useful. One thing that does happen with the loss of 12V is that some data that controls the engine/hybrid drive is also lost, so after reestablishing 12V the MPG may be briefly lower until the ECU relearns the conditions and usage pattern. Not a big concern, but it is worthwhile keeping the memory alive, IMO.

 

I think most people would like to replace their own 12v battery - it's a no brainer on conventional cars, but taking the car to a dealer and paying for disconnecting 2 terminals and reconnecting to a new battery just seems like utter stupidity and a waste of $$$, and especially if you need a tow. But I will probably do that to eliminate the risk of blowing the computer. Very poor Toyota engineering for the do-it-yourselfer; very good for the dealer profit bottom line.
In the past, on a conventional car, I have gotten a jump; driven to Costco along with my wrenches, bought a new battery & changed it in the parking lot. Same deal with Sears several times.
If you take some of the other advice about using trickle chargers, heed the warning about accidentaly sparking against the chassis, or using a trickle charger that doesn't have a clean filtered DC output. Dont take the risk - you may blow the whole computer board or screw something up. Either way, it's gonna cost you. My car: Prius 2010 III -White, 44.1-44.3 MPG - hilly terrain in California.