hi,I wonder y u didn't connect that 820R before current sendor R4.
When the battery is being charged the current causes a voltage drop across the sense resistor also the current for the LED [as its ON] causes a small voltage drop across the sense resistor, which drives the transistor harder on.
When the charge current falls to a point where the transistor start to switch off, the LED will go off and the current in the sense resistor will fall even further, giving a faster LED switch off.