skunks

help help help, my battery seems to be screwed but im not sure it could be causing me all this problems. one cell is totally dead but the battery is still putting out 12.5 volts when the engine is off and 13.5 when the engine is on. the thing is that i can start the car. the problem is that if i hit my brakes, turn on the lights or use any kind of accessory, the car shuts it self off, help! is this some kind of security system or something? has anyone else had this problem?

2-Rotor

Your battery itself could be shorted inside. I have seen new batteries even do this. Best and cheapest advice is to swap in another battery from another car and see if thats it. Start from there.