well im pretty sure with any battery at 1400rms with the vehicle off your gonna get dimming, and with the vehicle on it should be less, assuming the alts good
but just turning the key to accesory and cranking up the volume to notice that your getting dimming isnt a reliable way to tell if you battery is good or not... i have 360 rms and on accesory i get dimming
i would just save yourself the hastle get a good battery now, and save from upgrading in the future
also 60 bucks seems a little cheap for a battery, what type are you looking at getting?