Built me a simple battery cooling box with a radio shack 5" 12v fan, small 12v motorcycle battery, switch and some ply. Put in couple rails to raise the batteries off the floor to get airflow completely around them. Haven't tried it at the field as I just built it, but it pulls lots of air through the tunnel. I think it will help get my batteries cooled off a lot quicker then just letting them sit in the shade. Just slide the packs in and flip the switch. About $40 without the ply.