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I have a 2017 Toyota Yaris Hatchback. The car had some cranking issues. I did a cranking voltage drop test (dropping to min 7v when cracking), it seemed like the battery was done. I replaced the battery with a SuperStart battery from O'Reilly. Car was working fine and the issue seemed to be fixed.

Shortly after, I went to travel for 3 months but forgot to unplug the battery. After I came back, the car was cranking fine for 3 weeks but the (new) battery eventually started losing more and more charge. Now, the battery goes from full to half in a night.

  • Checked and cleaned battery terminals
  • I did a test for parasitic draw, but didn't see a lot of draw (just about 15mA).
  • Alternator puts out about 13.3v-13.7v in idle, 13.8v at about 2000rpm. Is that too little? I keep reading conflicting information online (some say not less than 14v, some say not less than 13v, ...)
  • When I drive around for a little bit, the battery charges up to 12.6v. It does crank fine after, so I assume the starter is ok. But if the car sits for the night, the battery drains down to at least half and the engine cranks slowly.

Questions:

  • At this point, I'm 99% sure it is the battery, but since the battery is new I'm still a bit skeptical that the car sitting around for 3 months would destroy the battery (I had other cars sitting around for longer without issues).
    • If you think so as well, what else could I check? I'm really trying to avoid buying another battery.
  • If it is the battery, which battery brand can you recommend?
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Leaving a new battery unused for 3 months has probably killed it.

Most "new" batteries come with sufficient charge to start the car straight away and you are expected to effectively fully charge them with subsequent use. As you left it for 3 months then the car ran all its normal things - clock, alarm etc and this continued to discharge the battery.

I left my car for 4 months, but I removed the battery and kept it warm as it was December and winter when I would need it. Fitted and worked fine.

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If you came back to the car and it cranked and started without having to recharge the battery first, then the battery clearly hadn’t discharged significantly and therefore shouldn’t have been damaged. It would have been much the same as if it had been disconnected for the 3 months.

If you had to recharge the battery first because it had become discharged, then the battery could have been damaged.

Maybe the battery is faulty due to other reasons nothing to do with the 3 months of no use.

There are some well know top quality brands of battery that are worth paying the extra for.

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If your battery is under warranty, have it tested at O'Reilly including an alternator test if possible. Let O'Reilly determine if the battery is damaged, partially discharged or damaged. The battery may be fine and just needs a full charge, preferably overnight using a slow charging mode to ensure it will be fully charged and remain good for the life of its warranty or its replaced. Free if its a three year free replacement type and falls under three years of use. If O'Reilly can't test your alternator in the car then try Autozone or Advance Auto for free alternator testing in the car. You should try Yaris forums for Yaris members sharing similar issues and may have better info to help you. You don't mention how long a drive occurs in daily use as a partially discharged battery may require an hour or more of driving to recharge it as opposed to using a battery charger between 2-15 amps to recharge it overnight. 15ma is parasitic current used to power keep alive memory like radio presets, clock, ecm live memory, etc. This doesn't kill a battery but will discharge it over a long period of non use. It would have been better to disconnect battery negative when anticipating non use beyond a week to halt parasitic current from slowly discharging a battery. Using a small trickle charger of less than 3 amps connected to a battery will not overcharge it but will keep a battery fully charged for long periods of non use.

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