Problems with the 12V battery of hybrids are well-known - latest, see this long-term test of a Kia Niro in Autocar last week:
www.autocar.co.uk/car-news/our-cars/kia-niro-long-...s
This quote from a Toyota forum gives an explanation:
""...the 12V battery is a slight weakness in Toyota Hybrids. Since it doesn't have to start the car (the big HV battery does that), Toyota put a low capacity (and lighter) battery in. However, it does mean it's sensitive to leaving lights on, or sitting with ignition on but not 'READY'. (When in 'READY' state, the HV battery charges the 12V battery, even when the engine is not running). Also, when you open a door, the 12V battery runs an electric motor to pressurise the brakes, which drains it a bit more, as does the alarm and remote door lock sensors.
If you don't start a Hybrid for a few weeks, on a long holiday for example, (4-5 weeks on a Gen 1 or 2 Prius) the battery can flatten even if nothing is left on. If the battery is weak, it can flatten more quickly."
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