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I have a Prius 2004 bought used 2 years ago. The 12v battery keeps dying. I changed the battery twice, with the last one installed in a few weeks ago, and since then the most I have left the car parked without driving it has been around a week. Yet this last battery is already weak and failed me three times. At this point there is no redeeming chance for it I suppose.

Any tips appreciated on what could be happening. My mechanic skills are limited but I should be able to follow instructions.

Thank you.

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    Apr 4 '17 at 16:05
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Either something is draining the battery while the vehicle is parked, or it's not being charged back up properly while being driven.

To check the charging, use a multimeter set to 20 volts. Check the voltage between the battery posts while the vehicle is running. You should be seeing something between about 13.5 and 14.5. You may need to rev the engine up a few thousand RPMs to get the full charging voltage.

http://www.aa1car.com/library/charging_checks.htm

To check for a parasitic drain, you can turn a multimeter to amperage (start high, work your way down) and connect it between the positive post on the battery and the positive cable. Anything more than about fifty milliamps is likely a problem.

http://www.popularmechanics.com/cars/technology/how-to/a5859/how-to-stop-car-battery-drains/

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    (Sorry for the long silence.) Thank you very much @raydowe. I followed your advice and discovered a drain of about 0.8 amps, which is a lot. After removing fuses one by one, the culprit seems to be one of the devices on the DOME fuse circuit. These include the dome lights themselves and also the audio system (which I believe is stock on my car), and possibly a few others.
    – Eric Anonymous
    May 17 '17 at 17:48
  • How exactly are you going to rev up the engine on a Prius that is a hybrid electric vehicle and for what benefit, as the DC-DC converter in a Prius has anyway lots of energy in the HV battery that can be fed to the 12V battery regardless of the engine RPM? (For nitpickers: yes, I know Prius has a service mode which allows you to rev up the engine at standstill, but I don't think you want to use that.)
    – juhist
    May 19 '17 at 13:47

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