Can the aux battery be disconnected from under the hood?

The quick answer to the title question is: pop the hood, take the cover off the fuse box, and unplug the fat white wire. That's the one that comes from the aux battery in back.

The trouble with saying stuff like that is it gets picked up by other readers on PriusChat and repeated willy-nilly, overlooking one crucial fact: it's far more common for a person to see warning/error behavior that makes no sense to him or her than to see behavior that really makes no sense.

 

I used to have the same problem with the 12V aux battery discharging in my wife's Ford C-Max Energi (plug-in hybrid). These cars are notorious for phantom discharging so after being locked out of the car a couple of times, my first solution was to hook up a trickle charger connector to the under-hood 12V terminal and a good ground point. This was then led out through the front grill and zip-tied to prevent it falling back inside but with enough slack to allow it to be pushed in almost out of sight. I was then able to hook up the charger without opening the hood or rear hatch.

However, after a few months, I decided to change tactics and for what it's worth, the simple solution I found was to place a small solar panel in the rear quarter window and connect it directly to the battery. This car was not garaged so being in the Florida sun, it produced around 18V (unloaded) and 11V when hooked up. It attached to the glass using the four plastic suckers which were supplied. Mine came with various connectors which allowed it to be hard-wired to the battery and since then, there have not been any issues with it discharging. I wouldn't hesitate to do the same with my Prius if I have issues with the aux battery. Here's a link to a similar item:

 

I sometimes wonder what "could" be done, by the manufacturers. They must already have some sort of flash memory, to store the odometer number and a few other bits of info. If they could have that memory retain everything that needs remembering, and summarily disconnect the 12 volt battery every time the car's shut down, no more phantom draw.

Even though I've got memory saver capability, more often than not I disconnect the 12 volt completely, for brake work (to avoid a caliper piston possibly popping out). I've noticed subsequent to doing that, the car does a strange rev-up, for the next few starts, recalibrating something.

 

What, no more remote fobs or keyless entry or pushbutton start or "where's my car"? Oh, the humanity!

 

Yeah, and dumb keys, that you can dupe for $5~10 at the shoe repair, lol.

 

Then how would the car time the brake test that automatically happens ~2 minutes after shutdown, or the fuel vapor plumbing test that runs 5 hours after shutdown? That's in addition to the items ChapmanF mentioned.

I don't understand the bit about needing to disconnect the battery to avoid brake pressurization. My car never does that except shortly after parking, or if the driver's door is opened or the brake pedal is pressed---all of which could be easily avoided during brake work.

 

I've at least once caught myself about to open the driver's door for something, then walked around and gone in through the passenger side.

Maybe some painter's tape over the driver's door handle....

 

The door can be locked, which should eliminate any risk of absentmindedly opening it, assuming the key fob is far away.

 

One thing, after having the calipers off, when everything is back together and before reconnecting the 12 volt neg cable, I like to tromp the brake pedal multiple times, to reduce pedal travel, travel which might be detected by the car, causing a code. I suppose I could slip in through passenger door, reach over, to avoid the drivers door opening trigger. Still, I’m belts-and-braces, hate getting so close to something that any slip-up could be disastrous.

 

To be honest, I haven't seen any reason (in Gen 3) to be concerned about any automatic action of the brake system while the car is off, other than the self-test that applies pressure to the wheels, 90 seconds on the nose after the car is powered off. I have not stumbled on any other way of triggering that test.

Mere operation of the pump, to restore pressure in the accumulator, doesn't budge any caliper pistons, unless there is some pre-existing problem where system pressure is getting to the brakes when it isn't wanted, which you would likely notice as drag while driving the car. And if that were an issue, the piston would start extending immediately when you take off the caliper whether the pump runs or not. The only way to avoid that would be to zero down all the stored pressure before beginning work.

 

A good 12v battery with a normal (eg low) parasitic draw should last three weeks or more and still Ready the car.

If you only drive every two weeks and then 15 minutes total, the car may not fully recharge the battery and you are heading for lower capacity and more trouble. A staged maintenance charger then makes sense.

If there is more than 25 ma of parasitic draw, most don’t know it and blame their driving habits or the battery. Measuring parasitic draw is key.

Most likely if there is a problem requiring a disconnect power cycle, the problem needs to be diagnosed and repaired.

Driving the car twice a week is the real answer and reduces other storage induced issues like flat spotting tires, condensation in the oil and corrosion buildup.

 

Yeah we're driving once or twice a week; mostly longer drives, 50 kms round trip. And back on a charger in-between.

 

There's been reports here though, owner's DIY'ing brake maintenance, and having the pistons pop out.

 

Reconciling those reports with knowledge of how the system works is always the tricky part ... easier, the more details are available in the report.

If the brake self-test occurred while a caliper was off, certainly the piston would be ejected.

 

At that stage, you could safely unlock the door, and not have to play contortionist climbing over the console and flying buttress.