Is a 9v battery enough to keep the memory alive?

Why not just hook up another battery to the jumper in the fuse box under the hood?

 

Prior comments in other threads suggested that a Prius doesn't even need a 12V battery to keep running, once it is started. The DC-DC inverter should maintain the 12V bus even when the battery is absent. So when I changed my 12V, I started up the car into READY, and changed batteries while the car remained turned on, outdoors. Everything went fine, no sparks, all memories were retained.

Do be damn sure that the 12V positive cable end doesn't touch any grounded metal while detached from a battery. (Put a glove over it, or something like that.)

Just because, (based on career electrical experience), I also turned the headlights on to make sure the inverter had enough load to stay in regulation. Without a sufficient minimum load, some inverters will have their output voltage float too high, possibly damaging some devices. I have no idea if this might happen with the Prius, but turned on a significant load 'just in case'.

There are probably some other caveats and warnings that should be added about swapping batteries on a live car. Would anyone else like to chime in?

(Like use wheel chocks to be make sure the dog can't accidentally put the car in gear and put it in motion while you are in an unsafe place?)

 

It's best to replace the battery without the car running. I know a lot of people do it that way......
Any 12v battery will do. I do not think you will reset the back up beep. That's the main reason I got
techstream! I HATE BACK UP BEEPS!!! WHY do they have INSIDE the car? I you don't know
you are backup up, you shouldn't be in the car!

 

If you have another 12V battery available to keep the voltage up, then I don't see any particular reason why it could not be attached to the jump points at the other end of the car, under the hood. This ought to be low risk, and less effort and more dependable than trying to connect a 9V to the same cables being moved and fussed with at the back.

 

I've purposely disconnected the 12 volt negative cable a couple times in the last month, playing around with my charger quick connect.

I do have a memory saver cable for my jump pack, and that's one kosher way to go. The cables have indicator light , some just one light, and some fancy one have an indicator light for both ends. Even with just a single indicator light, just first connect just to the OBD, then disconnect and connect just to the jump pack. If the indicator light went on both times, you know you have good connections at both ends. Anyways: plug into the jump pack (cig lighter style connector), the other end to the OBD port (has just pins for 12 volt supply), and you're memories will be retained during the battery swap.

That said, there are a lot of times when you don't want a memory saver. Aforementioned quick connect for one, or when you're doing brake work, and want the car really dead.

After those two previous disconnects, I found it very simple to get back to normal. But then, I don't disable the reserve beep, or care much about radio presets. Other than those, the main things:

1. Next time you go start the car it'll give you grief the first time. Shut down and retry, all is good.
2. The trip meters are lost, all display are back to defaults. If you care about the trip meters, take a pic before disconnecting.
3. The car does a weird rev-up during the warm up phase, maybe the next couple of start ups. Think something is being recalibrated.
4. Window auto-up may not work. If that happens, to relearn: run it down, holding it the whole way. Then run it up, again holding it the whole way. Then continue to hold up, for 2 second additional.
5. Clock will need resetting.

It might actually be good to periodically do a full disconnect, cleans out the "cobwebs". @MSantos used to recommend this, once in a while?

 

Not allowed? My salescritter offered up front to do it, handing me a customization sheet to be checked with my preferences. And even questioned me about my not marking it to disable the seatbelt warning.

 

I lasted 2 beeps, and that was it! I ordered Techstream. It was worth it just for that!
One beep is ok. I've bumped it a few time and the beep got my attention.
I also changed it to one beep on my friends 2012 Pruis V.

 

Once more for the original question, don't know about a 9 volt, but a jump pack with a cigarette light style connector built-in, plus a memory saver cable, you'll be fine. I use these guys, just lay the jump pack in the driver's footwell:


(Cig lighter socket under the flagged cap)

JNC660 1700 Peak Amp 12 Volt Jump Starter - Clore Automotive



SOLAR ESA30 OBDII Memory Saver Connector - Clore Blog

And as mentioned by @ASRDogman , another 12 volt battery could be used, say connected with jumper cables at the under-hood jump point, just like you're going to do a jump start.

Hmmm, guess you could also use the jump pack thus. But that cable is cool.

 

I have changed at least 10 of those 12 volt batteries and never used a jumper or any thing else.Why would Toyota not have a system to keep the computer from losing all its memory????

 

WHY spend all that money? If you have a 2nd car, use jumper cables.
Or a friends car, even a UPS backup battery. It's just to keep your setting for the radio, seats, etc.

 

Well, "all that money" for me was about $20 for the memory cable. I had the jump pack already, probably something worthwhile to have, if you're occasionally needing to jump start anyone.

 

Sometimes we WANT to erase various memories, to clear out various problems such as the ignition lockout after too many restart attempts before refilling an empty fuel tank, or put everything back to the factory defaults. This is a very convenient single-point path to doing it.

 

Also if you have a bluetooth OBDII dongle, the Carista app can be used to change the constant beeping while in reverse. I back out of a long driveway in the country so that was one of the first things I changed. It's going on about 6 years since i changed it but fuzzy1's comment about the seat belt chime may cause me to re visit the app.

 

The customizable options, like the reverse beep, will not change. They don't return to a default value, in other words.

What you lose with the battery disconnect are your fuel economy records (but the "best mpg" remains) and your radio station presets. I've not ever had to recalibrate anything, either. There is a note in the Repair Manual that the idle speed control may need to be re-initialized if there is a "loud rattle from the hybrid transaxle."