12V Battery Drain/Power Switch Issue - Toyota Prius Forum : Prius Online Toyota Forums
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post #1 of 3 (permalink) Old 02-23-2018, 02:40 PM Thread Starter
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Thumbs down 12V Battery Drain/Power Switch Issue

A couple of mornings ago, my 2010 Prius experienced a battery drain and would not start. It was completely dead. After figuring out that I could get the battery out, I did and charged it to 12.5 V. Once reinstalled, the dash lights came on brightly but the car would not start. (The smart key has a new battery in it.) I was also unable to turn the dash lights off when I tried. Ultimately the dash went dark. I could hear a humming noise after everything was supposedly off and the battery had drained down to 9V when I tested it after another removal. Has anyone here had a similar issue? What needs to be done? I have a code reader. Can I read error codes with the 12V battery out of car?
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post #2 of 3 (permalink) Old 02-23-2018, 08:44 PM Thread Starter
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Well, I'm a little embarrassed. Turns out it was only the 12V battery at fault. Installation of a new battery cured all bad symptoms. I'd delete the original post if I knew how.
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post #3 of 3 (permalink) Old 03-05-2018, 03:43 PM
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The Prius 12 volt lead acid type batteries don't last forever and if ignored one day will be the day when the car won't start. Don't ask how this was irretrievably buried in my psyche. Once your battery is say 5 years old you should consider having the battery load tested at least annually and at 6-7 years test every 6 months.

Harbor Freight has decent battery load testers. They are easy to operate and help predict when you need to change your battery. Do not settle for a voltage reading as that will not predict impending battery failure nearly as well as a load tester.

A load tester simultaneously measures battery voltage and current through a load. This is how the pros do it.

I own a tester and it is really very easy to use and tells the truth about your battery. If you don't like Harbor Freight, no problem, most decent auto parts stores have or can get load testers.

Concerned with cost? One call to a towing service for a jump will exceed the tester cost. Some auto parts places will test your battery for free, expecting to maybe sell you a new one.

Can you "jump start" a Prius with a bad 12 volt battery? Yes via a power point under the hood or by connecting directly to the battery. This will get you going again but it is highly likely when you shut down the car it won't start again without another jump.

The portable battery jump start accessory will almost always provide a means of getting going again. The replacement batteries for Prius are not widely stocked except at Toyota dealers. The group size is limited to Prius. The battery posts are smaller than standard making substitution difficult.

Patrick ('04 Prius)

A blah day in the country beats a whoopee day in the city!
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