wont start starts from front jump but not back

I even replaced the back battery (with a lawn tractor size (160 cca) -- had a larger one in there that was about 6 months old so - just eliminating that possibility --- have to put power at the front hood fuseblock to get it to start and will stay running but will not boot up from back battery area (aux) only ..-- requires the under hood jump to start ... what could be wrong ?

 

will chekck the fuse .. sorry about title... pulled back muscle and on pain killer - lead acid or sealed doesn't much matter 12-14v source ... will report back on under front fuse 125 AMPERES thx for suggestion !

 

i unscrewed the 10mm nut holding down the terminal that is under RED post - there is a 100A dc-dc convertor there--- but I don't see a fuse under -- an aluminum plate is there is that the fuse ?

 

it is an 05 prius with key phobe package ...

 

The 120A fuse is intact -- no breaks or burns visible through the looking glass of that square fuse on the + terminal for the battery ...

 

Will do Patrick, I noticed that there is significant draw when the front is jumpered -- when the glass pack battery died ( i presume the original) last year .. i didnt want to dump $200 for a new bat so i went for the miata alternative .. of course i didnt have the kit and tried whittling down the terminals to make the origanals fit ) -- this made for intermittents every now and aagain and alot of crystal growth white podwer secretions -- so i took the battery out of the back compartment and into the hatch in a cardboard box so i could make sure the connections are good ---
i may have jumped the car wiht inverse polarity from the front .. but if an ecu is fried would it even start at all ?

 

I get about 12.6 on frt and back before ready .. in READY i get about 14.1 in front and still only 12.6 in hte back battery. I checked the battery cables and the connections seemed to be tight from the terminals to the terminal adapter cables to the battery posts - the NEG. seemed to be tight against the frame and the battery - like I said, it was working with the intermittent connection and now it has better , tighter connections --- but I'm not sure which ecu is susceptable when u have a brief inverted jump in the front .. if that's the aux. computer then ... that could explain it ... but i can still start it by jumping the front and removing the battery and driving and repeating everytime i turn it off ... (not much fun ) ...

 

I will have to go over the connections in day light as it was getting dark/rainy - will have to go over it very thoroughly and try and restore things as much as possible to original (and reliably working) conditions ... Thx for your expertise Patrick - I do like the gen2 much more than the gen1 -- huge improvements in so many areas -- and a bit of learning curve like that jumping from the front just to be able to open the dang hatch.. but still a great car -- i have been experimenting with some water injection for further gas reduction -- i have seen the gallons per hour go down with some water injection - likely the 500 gph boat bilge pump rig was and variable PWM for control was not helping the 12v system as i didnt have it on a switch to turn off when engine not running....

 

in my restoration .. when i got to the back battery terminals .. and the jumper from the ground , I had rapped in electric tape, which fealt strong , but upon unraveling saw the intermittent cable shoved into the term. holder with a bolt and that had to be redone /replaced .. so that was the fix .. the NEG. was flaked out - white stuff and not tiight ... -- !