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I would like to hear from those who have had experience with the little battery back in the trunk of the Prius. I have gone through the service book and cannot see anytime when it is tested for replacement. Is it necessary to just finally have it poop out before anything can be done to check its condition? I know that Pb-acid batteries eventually self poison and cannot come to full charge after many recharges. If this battery fades out what are all of the consequences? I'd hate to have it happen when I'm out in the middle of the New Mexico desert in one of those areas where the cell phone doesn't work.
If you've had experience with this (motorcycle size) battery, I would be grateful if you would share it with me. I've had my Prius for 3 years. :?:
 

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The 12 volt battery mainly supplies power to the computer for starting the car. There are also some other components that rely on that battery when the car is off. However, there is very little drain on the battery. The most common problem is an interior or exterior light left on after powering the car off. In that case, the battery will drain in a short period of time. With normal use, the battery should last 4 to 5 years. Many of us driving the 04 model are still on the original battery.

If you do experience a problem you can "jump start" the car as described in the Owner's Manual. Once the car is started (READY mode) the 12 volt battery is charged from the traction battery.
 

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Edward said:
I would like to hear from those who have had experience with the little battery back in the trunk of the Prius. I have gone through the service book and cannot see anytime when it is tested for replacement. Is it necessary to just finally have it poop out before anything can be done to check its condition? I know that Pb-acid batteries eventually self poison and cannot come to full charge after many recharges. If this battery fades out what are all of the consequences? I'd hate to have it happen when I'm out in the middle of the New Mexico desert in one of those areas where the cell phone doesn't work.
If you've had experience with this (motorcycle size) battery, I would be grateful if you would share it with me. I've had my Prius for 3 years. :?:
Ed,

You can do a quick check on the battery yourself at any time.
  • Put your Prius in ACC mode (just turn it on without pressing on the brake pedal)
    Put the MFD in Mainenance mode:
    Press the "Info" button while flashing the headlights three times
    Press the on-screen menu field
    Press the on-screen signal check
    One of the items will be the 12v battery voltage which should be a little above 12 volts
    Now press the power button (no brake pedal) to add a current load and the voltage should drop slightly, but stay above 12v
    Now press the brake pedal and then the power button to go into "ready" mode and the voltage should jump up to about 14 volts (charging)
    Turning off the car resets the MFD to its normal operation.

I am still using the original 12v battery in my 2004 Prius (4+ years and 127,000 miles). If you never fully discharge it, it will last a long time. A few discharges and it will be dead.

JeffD
 

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I've had lots of trouble with my 12v battery. It started when I used a 12v compressor to adjust the tire pressure. I didn't realize the car was in ACC rather than ON, so it drained the battery. After that, it would die every time I don't use it for a few days.

I finally got the dealer to replace the battery, although he said the problem was that I don't drive enough. The new battery worked fine for more than a year until a few weeks ago when I was away for a week and came back to find it dead. I jump started it and it worked for 2 weeks, and after I didn't use the car for one day it was dead again.

Once the battery drains once, it will continue to have problems.
 

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It's a darn shame that this has been a complaint since "day one" with these cars and it could have been eliminated years ago with just the installation of a standard and much cheaper 12 volt battery such as the one used in the Honda Civic hybrid. My son has been driving one for four years and has never heard of a single problem concerning the 12 volt.
 

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Mine finally failed for the first time in August on my 2001 Prius. After startup the "death warning screen" would come on at random as well as a bunch warning lights but the car would still run fine.
I measured the little battery voltage at its terminals with a dvm. It measured 12.4 untill I turned on the ignition and it dropped to 11.2V. I replaced it and so far no other problems/warnings.
 

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I jump started it and it worked for 2 weeks, and after I didn't use the car for one day it was dead again.

Once the battery drains once, it will continue to have problems.
Jump starting does not magically charge a battery. Running a battery flat then recharging is not the best way to treat the battery, but will only kill it if you leave it flat for ages. Pull it out and recharge it, or drive for miles and miles for the same effect.

We had a couple of bobbles with interior lights when our machine was new, and completely drained the battery 3 or 4 times. A jump start and a long drive and the same battery is giving fine service with no further problems 3.5 years and 170,000 km later.
 

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hyperion said:
It's a darn shame that this has been a complaint since "day one" with these cars and it could have been eliminated years ago with just the installation of a standard and much cheaper 12 volt battery such as the one used in the Honda Civic hybrid. My son has been driving one for four years and has never heard of a single problem concerning the 12 volt.
The high cost of the 12v battery is due to it living in the passenger area of the car. This requires it to be "spill proof" and as an additional safety feature, vented outside the car. The Prius battery is the same capacity and size as that in the Mazda Miata. It is small since it is not required to put out high current (the Traction Battery starts the ICE), but it does have to run some pumps during shutdown and start up cycles (short duration 30 Amp loads).

The Civic Hybrid uses the 12v Battery to start the ICE and therefore MUST be a larger battery.

JeffD
 

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But the Civic battery is has a much larger capacity and would rarely fail if a dome light was left on for a week due to a cracked open door or parking light left on. It is much cheaper, there is a lot of excess room in the trunk of the Prius, and there are many made for several cars like the Corvette which have vented batteries located in the car proper. Vented batteries are not one dime more expensive than a standard 12 volt.
I've had one for years installed in my Cadillac Allante.
 

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The smallest battery that reliably starts a Prius for me is a 7 amp hour gel cell; about the size of a brick.

*the usual advice* when connecting any external 12 v source to Prius be extremely careful to get the polarity correct. Otherwise, expensive repairs are nearly certain.

DAS
 

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The little batt in the back can be load tested using (wait for it...) a battery load tester. Any shop should have one or you can buy one pretty inexpensively at Harbor Freight or such. You don't need the heavy-duty one, just the least expensive one is heavy-duty enough for the little battery in the back. IF your little batt is over 3-4 years old it would be good to load test it. The older the batt the more often you might want to load test it as the likelihood of failure goes up with age. A test just takes a minute and is very easy to do. It is insurance that you won't be stranded out somewhere inconvenient or dangerous.
 
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