Prius C forum, My wife's 2012 C II with 30,000 professionally maintained miles had sat for several days. When I tried to start it it acted like a non hybrid with an almost dead 12 volt battery. The dash lights came on and it said "powered up" but every time I tried to put it in gear it said "shift into park". The headlights were dim and the windshield wipers ran very slow. The radio came on but if I stepped on the brake there was a popping sound and the radio went off. Let off the brake and the radio would come back on. I jumper cabled another 12 volt battery with the one under the back seat but it didn't change anything. I turned the car off and let it stand for about 5 minutes, jumpered battery still connected. When I started it again everything was normal and the traction battery was all charged up. I drove it around for 15 minutes, let is sit for 4 hours and it started as normal. Any ideas as to what the heck happened? Jerryd
Then the 12-volt battery is going on 6 years of age now. That is generally as long a life as can be expected from them. When they start to fail all sorts of strange electronic behavior starts to happen, and generally continues until the battery is replaced. Just a heads up, the 12 volt is probably on the way out.
OK I'll take the battery out and have it checked. What is going to have to be "reset" after I replace the battery?
I can't think of anything that needs to be reset when changing the 12 volt. You do lose some information like the radio presets and some data from the accumulated mileage calculations gets wiped out. So for a bit it will seem like you are getting poorer mileage then before the battery was changed. Which is not true, it is just a computer thing. Nothing wrong with just continuing driving until some more strange electrical things start happening then change it out. I changed a battery some 6 months ago and replaced it with an OEM (Toyota) battery, which I am happy with. Warning, these are small batteries but are not cheap to replace.
I've disconnected the 12v in ours a few times when installing and adjusting the aftermarket stereo. I don't recall having to reset anything beyond the radio presets. Some cars need to re-learn their idle or power windows, but the c seems to get away with a smaller blackout penalty, more like an early 90s car. I don't know how to explain why the connection to the second battery didn't immediately help, but possibly the electrical contact was poor at first and slight vibrations improved the grip of the spring jaws after a minute or two. Either way, a gentle cross-charge is by far the safest way to do an electrical rescue on one of these cars. So even if it was by accident, it was for the best. I agree with the base diagnosis, your factory 12v battery is pining for a fjord. The only direct-fit replacement I'm aware of is exclusively sold across the dealer parts counter for about $200 and would take a careful DIYer about 30 minutes to install. You can get similar generics but they all require one adaptive compromise or another and they don't really save much vs. the real deal.
When I replaced my Liftback's battery last week, I left the ignition turned on and Ready during the swap so that nothing would need to be reset. This is an advantage of the Prius electrical system, once the car is started and Ready, it can run without the 12V battery in place. Checking a voltmeter, it seemed a bit high, so I turned on the headlights to make sure the HV->12V inverter had a reasonable mimimum load to stay in regulation. And that did bring the voltage down to normal running level. The swap then went quickly and smoothly, no problems. Do be sure to disconnect the negative cable first, reconnect it last, and keep the positive cable from touching any nearby metal parts.
I just found out that my starting problem was due to a low key battery....I replaced it and since then no problems....previously I had taken car in to check 12 volt battery and had to jump start it several times.
