Dead battery

at 17 years? probably, but it might just be normal.

press the brake hard and floor it to fully charge

 

The fact that it still is the original battery tells me that car has has been driven alot probably 250K+ miles on it.

Which is great for the hybrid battery. But then letting it sit for a month is really hard on it. 150 miles should have charged it up unless ac is on full blast and going up and down hills. You should be mentally prepared for this anyway right?

No matter what its pretty corroded in there and letting it sit will excite corrosion. And the 12 volt battery is probably now fubar so be aware of that. Buy a jump pack.

 

Left a 2005 parked for a month and had to jump it.....pretty normal if it's an average age 12v battery. Even a brand new one will only last about 6 weeks before it gets extremely weak, due to the normal small loads.
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It showed two bars after sitting for a month. Pretty normal for an original HV battery. Maybe even pretty good, since it's not showing the RTOD. A bit of self discharge, but fairly even. No "one" module doing a crazy self discharge and coding the battery. Not all bad!

Charged to midway after about 50 miles....perfect. Mid way is the normal operating band. The car tries to keep it between ~42% and ~62% during normal operation.

Still midway after 100 miles....see above statement.

 

Get Dr Prius app, preferably the paid version, and look at the results.