battery to protect user changes

I haven't tried it with the Prius but with other cars you can connect a 9V battery through a diode. It will hold the radio presets etc. I've seen this device sold in stores as well. Usually they are designed to just plug into the cig. lighter, but that wouldn't work with an unmodified Prius (the aux. power and cig. lighter plug goes off with the power). As Evan suggests, the underhood boost point is a good spot to connect.

 

Good Dr.,

The Prius does NOT have to be on IG On mode for a keep-alive battery to keep the settings in the radio & other electronic gizmos. Otherwise, when you turned your Prius off, the settings would vanish.

When I did my battery replacement, I attached a small 12-Volt adaptor to the jump point under the hood. That kept all my settings.

One thing you DON'T want to do is attach a high-current source of 12V to the jump point when changing the battery. (Something like another 12V car battery, or one of those jump start packs can supply a large amount of current). This is because if you happen to short your (disconnected) battery cable to the ground of the car, your external 12V power source will provide LOTS of current and blow a fuse somewhere (probably the 100 Amp fuse in the battery terminal unit).

If you use a low-current source, then you'll simply lose your presets (and, if it's not a current protected source, it might blow too). But, you won't have huge amounts of current flowing, and you won't blow Priu$ fuses.

 

This has been discussed in previous threads. Any good 12V battery can be temporarily connected to the jump posts while you replace your existing battery, thereby saving your settings.

Tom

 

Good point. I have a couple of lab power supplies available, and can set the maximum current delivered by the power supply for this purpose - 0.1A should be enough when the car is IG-OFF and no lights are on.

We should also talk about why the battery is being disconnected. If this is being done to replace an old battery with a new one, then it makes sense to hook up a low-current auxiliary power source to keep the various user settings alive.

If the battery is being disconnected to make it safe to work on the car (for example, you are working on the SRS system or the hybrid drivetrain), then you would not want to have an auxiliary power source maintaining voltage on the 12V bus.

 

Covering the negative lead of the 12V battery (after you removed the battery) won't help.

It is the positive lead that you need to condom-up real good to protect it from hitting any part of the grounded car body.

And, I missed that the 9V battery would be plugged into the cigarette lighter - if that's the case then you're right - it won't work with a Prius (or pretty much any Toyota).

 

If you attach a large 12V battery (like a car battery or one of those 12V jump packs that has a high-current lead acid battery) to the jump post while replacing your battery, then you have a positive lead (that you took off of the battery) flopping around. If it hits any part of the car body, then you have a direct short that will blow the 100 Amp fuse. IIRC, those suckers aren't cheap (~$65 or so?)

 

A quick heads-up:

I have disconneted the 12V battery twice on Amapola, a 2008 model.
Neither time did I loose the GPS, radio, or driver's window auto-down thingy.
Don't know about the back-up or the seat belt beepers.

From watching many posts on this, in my opinion there seems to be a
difference in how the early Gen II Prii -- 2003,4,5 -- and the later ones --
2006,7,8 -- handle having the battery connected. It seems like Toyota slipped
in some kind of sneaky fix along the way.

FWIW.

 

so this hooking something up to keep the settings thread won't do anything to make the airbags fire? i thought i heard something about waiting 30 seconds or something when disconnecting the battery to make sure the air bags are de-energized. just wondering about it

 

There is absolutely nothing you can do with the battery that would make the airbags go off.

 

The 9 volt battery to the cigarette lighter works for other cars, why wouldn't it work for the Prius? I think it feeds just enough power through the radio to keep the presets, you need to leave the car off so the 9 volt transistor radio battery isn't trying to feed all the systems on the car. I also suspect it will hold the personal settings, who wants to give it a try?

 

A cheap possibility is to run wires from the old battery to the jump points, if you can attach and keep them in place while disconnecting the old battery from the car. The old battery would have to be stone dead to be incapable of preserving the presets. As always use a voltmeter to guarantee correct polarity.

 

This technical article illustrates and describes an interesting 12 volt battery swap and an alternate source to keep from losing settings that rely on continuous 12 volts. Even if the swap does not interest you, the method of maintaining the 12 volt signal is pretty complete. Notice that Hobbit does NOT use a full sized battery as the continuous 12 volt level; bright flashes and flying sparks are very expensive when using a Prius as the source.

You notice I emphasize 12 volts; 9 volts is NOT 12 volts and wont do the job.

[ Prius 12V swap ]