As others have said, problem is likely related to the 12V battery under the hood. The battery under the back seat is the drive battery for your hybrid.
I still have my 2006 hybrid Lexus with just over 100K miles. Have replaced the 12V battery 2 or 3 times so far. I asked my Lexus dealer how many drive batteries they had replaced since 2006 - one, due to damage caused by an accident.
We also had a 2008 Prius which we sold a few months ago before moving here. Same thing, had to replace the 12V battery 2 times.
Similar drive battery under the rear seat that was still doing well.
For those years, the drive batteries in the hybrid cars were NiMh, not Lithium. The transition to Lithium for hybrids has been slow, as NiMh has been quite reliable.
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Mike
Village of Marsh Bend
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We live in interesting times
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