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For background, I bought my Prius used in the early 2015 with about 160,000 on it.

After a few months I noticed that the hybrid battery would be systematically drained or nearly so upon starting the car, even if at the moment of turn it off it had been full or nearly so. The car struggled going uphill. I found the 12v battery drained one day (yes, will all the flashy lights on the dashboard, then nothing, etc). I replaced it and I found the new one drained a few days later. (Note that I do not drive my car that much and sometimes do not use it for more than a week. I have a motorcycle.) I still have that same one, though. I just charged it up and made sure I drove the car at least once a week. But the other issues remained. I though the hybrid battery was going bad and was pulling on the 12v battery somehow.

What do you think was happening?

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    This is great information that may be helpful to many people. Can you rework it a little to make it fit into the Question and Answer format? – RockPaperLizard Aug 22 '16 at 0:42
  • Agreed. You can post a question and post your own answer to your own question, that's perfectly acceptable. So if you could frame this post as a question that others may have, and then work the information as an answer, it can fit the format of the site. For now this post is on hold; it can be reopened once it's edited to a question (or you can delete it and start over). – Jason C Aug 22 '16 at 3:01
  • Thank for the feedback. It's feels a bit odd, but I reworded it. Feel free to edit if you like, but obviously keep the essentials intact. I think this might be valuable information and thought I would post it here. If you know of a better place where I can share that information, please let me know. – Eric Anonymous Aug 23 '16 at 4:18
  • How do I answer my question? Should I wait for it to be posted? (It shows "on hold".) – Eric Anonymous Aug 23 '16 at 4:19
  • @EricAnonymous It is reopened and you can post an answer now. It just takes some time for enough people to review questions in the reopen queue for it to actually happen, so sometimes you have to be patient. – Jason C Aug 23 '16 at 21:49
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This is what happened: the left brake caliper was seized, although not completely. (I was asked to post the question first, and answer it later. This all happened several months ago.)

I did not realize that for quite a while. The gas mileage was not great in town but the car was getting 40+ mpg on the freeway. The problem became obvious when the car started making a screechy noise, which became loud quite quickly, at which point I brought it to the shop. They found that the left caliper was almost stuck. After replacement, all the other issues above resolved.

It makes sense, retrospectively, that a brake issue could impact the hybrid battery (which in turn would impact the 12v battery), since the battery recovers some of the kinetic energy that the brakes dissipate. But I did not foresee this, to say the least.

If anyone knows more about this, please comment.

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