12V battery Good but Car needs jump starting

My 3rd gen Prius (2010) stopped with nothing shown on the display, not even the immobilizer symbol. The battery was perfectly fine and showing 12.8V (this reading was taken from rear battery terminals). When I try to jump start from there, car wouldn't start.

Now the interesting part, when I check the voltage under the bonnet in the fuse box, it was only showing 0.6v. So I connected the jump starter to the front terminals and car started straight away.

The issue seems to be either the 12v power line comes from rear to front or something inside fuse box itself (a fuse, relay Integration etc).

Initially this happened occasionally but now happens most of the time and only way I can start is jump starting from front fuse box.

Another interesting observation is that, even when car is dead like above explained, it sometimes come alive automatically with no input from me (accessory mode only, doesn't start the engine). This is so weird. When this happens I manually switch it off as I'm worried it'll consume all existing battery power (Hybrid battery) causing more issues.

Can anyone shed a light on what could be the problem please.

Thanks heaps.

 

Thanks a lot. Exactly what I was looking for. I won't see the car till next weekend but will try these Saturday and report back here. Really appreciate your time showing all little details, such an awesome human being.

 

First of all, thank you so very much, problem is solved...!!
I started from bottom of your list (stuff in fuse box). All of them returned the reading of just 0.7V. Since the fat white cable was obviously coming from the battery at the back, I'm totally confused at this stage what the problem was. Then I started from top of the list. Top three locations returned the reading 12V so battery was all good. Then the 4th dot point. Bingo...! The fuse is so hidden you cannot see there is a fuse there. I wouldn't have guessed there's one if you didn't tell me so. It was totally rotten so I put a new fuse and the car came back to normal life...!! I'm glad that it was such a simple fix and again can't thank you enough for taking time to explain possible scenarios..

 

Good point I was too excited to find out the problem wasn't as bad as I imagined.

My best guess is that the subwoofer and amp setup hooked in to the battery might have something to do with the fuse blow up (it was literally burnt with black residues). The reason for the intermittent issue is that the wire wasn't completely ditached so it would randomly work (which made the diagnosis a bit hard). Well, I don't want to think burning 12v battery fuse isn't so bad sign? If you could give me possible scenarios and hazards associated with it, that would be awesome.

 

Thanks mate, I'm bringing it to a mate of mine to have a look at the end of my next shift. So far all is back to normal however I'm keeping an eye on the battery fuse for any signs of burning. By the look of the previous fuse (burnt one), it appeared to have occured for a long period of time (perhaps years) rather than a sudden burn. Anyways, thanks again, I'll report here if I found anything concerning.. Thanks.