I don't think it's the batteries at all. I think the problem is in either the car charger or the way you are charging the batteries. Try draining the batteries. I mean really draining them. Put the plane at half throttle till the plane cuts out. Let the battery sit for about half hour and then put the plane at quarter throttle till it cuts out again. After all this charge the battery full. If this improves the battery life, repeat the process one more time.

Try it, it worked for me.

Hope this helps