My 2004 Prius was totaled in a rear end I subsequently replaced it with a seemingly nice 2007 I-Tech with 160k km (Sydney, Australia). Shortly after buying the car I noticed that sometimes in the mornings - after a cold start after sitting the night - the car would hesitate or act like it was under-powered when accelerating. I'm talking about a normal mild acceleration where going from 0 to perhaps 20-30k feels normal and then it just dips a lot in power or may stumble. 2 weeks after buying the car the HV battery failed much to my horror. I had the battery replaced with a rebuilt pack (100,000k on the battery) but was pretty surprised to still have the stumble issue. In other respects the battery appears to function normally. There are no warning lights at all. After a cold start most of the time the car runs completely as expected. About 1/7 starts it will stumble. On one occasion I had the power display on immediately after starting. It showed a fully charged battery. The petrol engine had not started (it usually does on first start). When I tried to accelerate the car almost immediately stumbled and the battery showed 100% discharged. The petrol engine then started and the car accelerated poorly. Once the HV battery came up to charged the car ran fine. On another occasion the battery appeared nearly full, the petrol engine was running, but the car again hesitated upon acceleration. The HV battery dropped very quickly to 1 bar. Also, climbing hills is a varied experience. Sometimes the car feels under powered compared to my old 2004 other times, same hill, it feels totally normal. Climbing the hill (several km long) the HV battery goes down fairly quickly and the petrol engine starts to charge it. If this happens at a lower speed I cannot easily get up to the speed limit of 60km. If I'm at 60km and this happens I can accelerate to 70km or higher.

Any ideas on figuring this out would really be appreciated. I'm quite unsure if this problem possibly caused the original HV battery to fail, or if it is the result of a defective battery (meaning the new rebuilt HV pack is also not good enough). I intend to go back to the shop that sold me the rebuilt pack to see if any fault codes show up. But, there are no warning lights currently.