New Auxiliary battery is drained while parked for one day.

i have a hybrid Prius T4 VVT-1 Auto with only 35,000 miles. it never gave me any problems until last week when it suddently went dead on the road. i noticed that before it stopped while i was on the road the car was moving with incredible extra power - seemed like a Ferrari (with due respect to ferrari owners) when i pressed the gas. But then after about 10 minutes driving like mad, the car gave up and stopped - it could not be switched off though after about 20 minutes cooling we could switch it off.
We towed the it to a nearby mechanic who replaced the auxiliary battery but could not locate the correct error on the computer - so we towed it again to a toyota approved mechanic - However, now the main battery is almost drained and the new auxiliary battery keeps going dead also.
By not having any reserved power in the main hybrid battery we cannot locate the correct error or the faulty part or even if the main hybrid battery needs to be replaced.
does anyone have any experience on similar occurrences please? i am not sure if it is a small or a critical problem but it was such a good car that it is a pity to bury it in a scrap yard if i can locate the error.

 

thanks.... still trying to revive the old hybrid battery !!