Good battery for a m89 airsoft gun (ump45)?

i have a m89 ump airsoft gun. i like it and it has decent fps but i would like the rate of fire to go up. i heard a better battery does the job. i got it from this site... http://www.airsoftmegastore.com/DE_Metal_Gearbox_SMG_45_Airsoft_AEG_Rifle_Package_p/de-m89p.htm it has to be a stick battery. so whats a good choice for around 20 dollars.