Problem with battery/charger/or user? Newbie

[stecroy]

Hi there i've got a RTR ansmann mad rat, only upgrade so far is alloy suspension. I've got a problem regarding the battery life. i have a ansmann 2000mah pack, which is charged using an ansmann powerpack 6 delta pro. The battery lasts aprox 4-5mins, so i upgraded to a dyna-fun 4500mah pack, i charged and took it for a test, but i'm still getting 4-5 mins run time? am i doing something wrong?? Cheers in advance newbie.