You might think about spending some money on a quality battery like an Optima, or you might add another battery dedicated to the stereo by using a battery isolator. An isolator will allow the alternator to charge the extra battery when the car is running, but it will not drain the main car battery while the stereo is playing.
'00 416ex -- 11:1 Wiseco, stage 1 HotCams, LEXX slip-on w/ modded header, ASR +2 arms, 450r shocks, G-Force axle, FST style cooler, Trail Tech bars and clamp
Temporarily retired from my extremely short (but successful) racing career.
Maybe some IXCR in the near future.
A government big enough to give you everything you want is strong enough to take everything you have. - Thomas Jefferson
None are more hopelessly enslaved than those who falsely believe they are free.
-- Johann Wolfgang von Goethe