Battery drained for no apparent reason

I would take it to the dealership. My understanding from reading the Owners Manual is that acessories such as lights, if left on when you leave the car, will be turned off by the car's computer after approximately 20 minutes, as a safeguard. So accidentally leaving on something like a light should not drain the battery, and the culprit is maybe something else.

 

Mine doesn't beep when i lock it. It beeps if you have left a door open, or walk away with the keys when it's still running.

You must have left it in accessory mode, or a light on or a door open.

 

Your car has 2 batteries. The big one is the hybrid battery. That's what is displayed on the battery meter as half full.

The other one is a small 12V battery. When you open the doors, that's what powers the car until you get it into ready. Once in ready mode, there's a charger that charges the little 12V battery off the big one. So, once in ready, it should be ok. (12V power supplied by the charger).

You either left something on and drained the 12V, or that 12V battery went bad and needs to be replaced.

 

The battery meter you describe has absolutely nothing to do with the 12V battery that went dead. The meter displays the approximate state of charge (SOC) of the HV battery, or traction battery, which is used to power the motors and air conditioner of your Prius.

When you say that the first boost "didn't start the conventional motor", I assume you mean the internal combustion engine (ICE). I assume you realize that the ICE doesn't immediately start when you power up your Prius. Do you mean to say that you couldn't shift to D and move the car?

As for why your 12V battery went dead, the common cause is leaving on a light, or leaving a door or hatch open.

Tom

 

A very common one would be the rear hatch not fully closing, leaving the boot/trunk light on. This might be enough to run the 12v battery flat.

As others have said, the HV battery display bears no relation to the 12v. I would add the 12v isn't as 'strong' as in a normal car as it doesn't need to turn over starter motors - it just powers the computers so the Prius can start the car via the big HV battery.

 

This is a catch 22. The 12 v battery powers the computers that test the rest of the car. If the 12 v battery is dead, we can't power the computer to report it. If the 12v battery can power the computers it is usually healthy enough no errors would show. So, no, there are no computer diagnostics. There is a diagnostic screen you can use to run tests:

Toyota Prius 12 Volt Auxiliary Battery with install kit for 2004 - 10

About 3/4 of the way down this page is the '[FONT=arial, helvetica]Prius battery test procedure for 2004 to 2010 Prius Cars'.[/FONT]

 

I hope that you never experience this again. But if you do try this, it's straight from your owners manual. I'm not certain, but this looks like a way to get the HV traction battery power to start the hybrid system if the 12 volt battery is dead.


When the hybrid system does not start, the following steps can be
used as an interim measure to start the hybrid system if the
“POWER” switch is functioning normally.
Set the parking brake.
Set the “POWER” switch to the ACCESSORY mode.
Push and hold the “POWER” switch for about 15 seconds
while depressing the brake pedal firmly.
Even if the hybrid system can be started using the above steps, the
system may be malfunctioning. Have the vehicle checked by your
Toyota dealer.

 

I just happened to reading the above in my manual, on a crowded commuter bus this afternoon. My take: it's not meant to bypass a depleted 12 volt, it's just to bypass some fault detection system for the hybrid battery system and motor. As far as I know, the car can't think without a functioning 12 volt battery, ie: it needs it to "boot".

 

I wouldn't worry about it too much; maybe you left a dome light on, or turned the headlights or parking lights on after turning the car off, or maybe it was just a fluke. If it doesn't happen again, not much point in worrying about it. My battery died about 6 months ago, but has been fine since. I theorized that the installation of a ScanGauge was the culprit, but it hasn't caused any problems since, so I might have suffered the same fate of leaving a light on or something. If it happens again in the near future, then you can worry about it - and maybe discover a commonality between the two events that caused the drain.

 

I thought "left-on" lights would go off too, but that did not happen with a map light that I left on for about 6 hours. I only discovered it accidently when I went out to my dark garage one night. Fortunately, I did not have a problem starting the car the next morning. I know my headlamps will turn off after 20 seconds, but my map light sure didn't.

Happy Motoring,

Tom

 

system showing me a yellow exclamation with a "Key found in car" (sorry, loose French translation) message for 15 seconds. T



You mention "Key found in car" light......are you sure the other key fob is not within the car....this would also be your problem. !!!!!!!

 

There have been other threads here suggesting some of the production runs of the G3 had a defective 12 volt battery that ran down quickly. There shouldn't be any impact on the 12 volt battery by opening the hood. I assume you got no other error messages on the MFD or lights when you got the "boost".

I had suggested Toyota incorporate another icon in the MFD showing the health of the 12 volt battery. At least you would have had a "heads up" something was going wrong with it.

FUTURE MFD:

 

Glad you got it sorted. Not sure the car can be to blame. If you'd left the back dome light on in a normal car it might have run the battery flat. The Prius 12v is weaker in that it doesn't have to start a car but equally it's not meant to power a lamp for a 24 hour period in cold conditions either.

At least problem is solved and no damage done.

 

Welcome :welcome:
I'm sorry you had such a bad early ownership experience. You will enjoy your Prius !:rockon:

 

This is from the Owners Manual, Section 3-4, Page 366:​

To prevent 12 volt battery discharge
If the interior lights and personal lights on when the door is not fully closed or
the interior light switch in door position, the lights will go off automatically
after 20 minutes.

Customization that can be configured at Toyota dealer
Setting (e.g. The time elapsed before lights turn off) can be changed.

(Customizable features P. 587)

The reference to page 587 seems a red herring: I see no mention of a 20 minute interval that can be changed, in that whole section.​

 

Toyota really needs something like GM battery rundown protection. In my last car the only way you could run down the battery is if you had the keys in the car and had the ignition turned on. If you had the keys out of the car every light inside, the headlights would go off on there own after a set ammount of time. if GM has been doing this for over 12 years why can a prius also do this, this would prevent 99% of the dead battery problems.