MOSFET Fails After Few Uses
Your LDR is a very slow device that responds to daylight that also varies slowly .Sunrise and sunset can take an hour or more depending on season and lattitude .This means that your power mosfet will spend lots of time half on or half off .Under these conditions the power wasted and hence device heating will be bad . Cheap modern powermos devices are specified for switching .They can current crowd in analog mode .This internal current crowding can cause early failure due to internal hotspots.The gate should be close to 5VDC   for charger on or close to 0VDC for charger off .This is also better for the charger because it gets clean 42V for on and zero for off .You can employ schmitt action by whatever means to switch clean and stop blowing fets.