you don't want the voltage on Fb to be more than 2V. the Fb pin is for setting the output voltage, not for controlling the switcher's operation. if you are trying to use Q1 to signal a current limit, you're going about it the wrong way, current limit circuits appear on page 11 of the data sheet. you also did not mention what type of battery you are trying to charge. different batteries have different requirements for controlling the charging current and charging voltage. if you use this charging circuit with a lithium battery, you could set the battery on fire from overcharging.
it looks like you need to read the data sheet carefully.