Our 2011 Prius front end was destroyed by a deer, great body shop mechanic has put it back together, looks great but the new 12 V battery in the back won't keep a charge. He thinks there may be a "parasite" draining it somewhere. It doesn't throw a code, and when it's running on the road, it's great. Hybrid battery charges to full as it should. Talked to a Toyota mechanic who said "this problem doesn't come up very often". Where do we even start to look?
Start with a complete health check of the battery. If it’s okay, measure the draw with the car off to see how many milliamperes it is drawing. If it’s much over 20, you’ll need to pull the fuses one at a time to see which circuit is the problem. But start by making sure the dome light is off with the doors closed
