Hi

I'm new to sins of solar empire and I have been playing with 8 normal or easy AIs on three solar system random map and trying to beat them but the last AI to beat seems impossible to beat for some reason.

In my last game I had two solar systems entirely for my self and the last alive AI has one. But when the AI attacks me and I try to defend with max out fleet the AI always beats me easily. I don't understand why my fleet is no match for the AI? I keep buying all kinds of ships until my resources run out but I am unable to beat the AI in the battle. It seems like my ships are too weak or something even I have all tech researched. Does anyone have any tips, what am I doing wrong?

thx!