I have a Toyota Tacoma 2001 and my car won't start off the battery under the hood. The battery is brand new and has a full charge. It's been working perfectly for 3 months since I installed the new battery. Today it's like the car is not getting a connection to the battery. When I put the key in I get lights as normal, but when I try to turn the key everything shuts off and only 2 dash lights work. I would think it's just a starter issue, except when I hook up a portable jumper it starts up just fine. The battery voltage is at 12.8 and I was wondering if this seems like a solvable issue.
Take the cables off of the battery and clean the terminals. You have a bad connection on one side or the other which is not allowing full power to get past. When you place the jumper onto the system, it is directly fed to the cables and allows the vehicle to start. Along the same lines, it could also be that one of the two battery connections is not completely tight ... same result as being dirty or having corrosion.
The fact that battery has a good voltage would suggest that there's an open circuit in the battery. The battery is full of current it just can't come out. If your battery polls are clean and terminals are tight, then that's definitely the problem. The battery has to be changed