03 Lancer Oz Rally Idle Surges Changed My Car Battery Now Running Low Rpm?

Changed my car battery now running low rpm? - 03 lancer oz rally idle surges

Friends of the battery died in 03 Lancer Oz Rally is the original battery, I went and did another. The battery we have received is smaller than the original. Now your car Idles 500k rpm is and the light sometimes goes. In think they just need a larger battery ... Ideas?