okay Les. i measured it and its measuring 4,75 v under that load there is something horribly wrong here my power supply is old computer atx do you think when i conencting my battery because of that voltage drop that is why cant give 690 mA to the battery ?With your 1 ohm sense resistor the current should be about 700 mA. So the 690 mA when the output is shorted is about right. What is the MEASURED voltage input to your circuit when the output is shorted. In other words what is the voltage from your supply that you claim is 5.1 volts when it is providing 690 mA of current ?
Les.