The oem battery on small to medium cars are small. Generally only around the 280-380cc.

Since your running an amp, and prob a few LED globes ad possibly a few gadgets.

Go to supercheap or something and buy yourself a 380-480cc maintenance free with indicator battery.

Be prepared to spend around $120+ for a decent size/cc battery.

Just make sure the positive and negative points and location will fit your existing terminals.

Make sure the battery u buy fits within the cavity location of your existing battery.