Obvious + Slowly discovered consequences of 12 V battery disconnect

5. Driver's side window auto function must be reset. Lower the window halfway, then close the window and hold the switch for a couple of seconds after the window is closed.
6. Trip odometers are reset

(Depending upon model year...)
7. MFD average MPG and odometer may (or may not) be reset
8. MFD MPG calculation takes into account accumulated history; may be upset by power loss

(Classic only)
9. On my 2001, the hybrid vehicle ECU occasionally got confused where the engine wanted to run but MG1 would not spin, so the engine lugged and felt like it was going to stall. Think of a manual transmission vehicle where the engine is idling and you let the clutch out with your left foot while holding the car stationary by pressing on the brake pedal with your right foot. By disconnecting 12V power then reconnecting, that weirdness seemed to go away.

 

I have disconnected the 12V battery twice. Each time, it took a couple of tankfuls at lower mpg before the mileage returned to previous average. Similar to the improvement I saw with the first 3 or 4 tanks when I first bought the car. Apparently, the ECU has to relearn some parameters that affect the FE.

 

Hi Jason,

Did you notice a drop in displayed mpg per MFD, actual mpg (based upon odometer readings & # of gallons purchased), or both?

I understand the displayed mpg algorithm is complex and requires learning. However that doesn't necessarily impact actual mpg.

Some have said the hybrid vehicle ECU goes through a learning process, but if true this is an undocumented feature...

 

Hi Patrick,

Although I didn't go back through my gas receipts to verify, it appeared to be both - the MFD mpg was lower and I drove less miles between fillups for about 3 tankfuls. The MFD mpg increased with each fillup after reconnecting.

 

I recently had the 12v battery on my 2008 Prius disconnected.

I did not loose nav info, radio presets, or the driver's window "sticky-key" feature.
(I don't remember loosing the clock. Its so easy to reset, it's no big deal.)

This is contrary to much that I've read here. Perhaps this is a 2008 model year feature.

I did loose a passenger-door-opened-first-opens-all-doors SKS reprogram.

And I noticed that it took a while to get back up to my typical MPG, but it was only one tank.

 

Does this mean that a dealer mechanic did the battery disconnection for you? If so, the mechanic is supposed to note your radio settings etc. and restore them. Or, the mechanic may have had access to a 12VDC power source and kept power applied while the battery was removed.

If you disconnected the battery yourself and found that all your settings were retained, please let us know. That would be a new feature not found in older 2G or Classic.

 

I disconnected the 12v battery myself with no alternate source of power.

As to a Toyota tech resetting the nav and radio info, where would the input-out port be? This is an intriguing idea. It implies the possibility of editing the info with a text editor and bulk loading new nav data.

 

Wow, this is very interesting.

There's no port. The mechanic is supposed to manually make note of the station settings on the radio; then enter them after working on the car.

 

The ECU goes through a learning process. This is true with most modern automobiles. They adapt to your driving style.

Tom

 

My experience varies. I have had the battery go empty several times. Mileage dropped more than 5 mpg after I had the first service. I noticed that just recently mileage comes back up from a long time average of 46 to 51. The timing is kind of weird, after 3 weeks back on my 50 mile commute the mileage started like shooting back up this week. Seriously it looks that the computer learns the driving style and looses its memory when battery is low. Funny though, the milage from the 1st mile was up 52 until I had my first service and/or battery outage, couldn't tell now which. My Prius had anticipated my driving style?
Maybe I'm gonna reset the average Monday and it'll go back up to 52 after a new start.
Note, that at the pump the mileage always is 2 to 2.5 mpg lower than on the computer. I do compare that most times.
Yes, I've lost radio, beeper and other settings, too.
Not the garage door opener setting, though

frosh