[DonRotolo]

The amp is fused with two 25A fuses. The robot battery will happily supply 50 Amps, but not for long, several minutes at most.

A car with the alternator charging is usually at 13.8 volts - 14.4 is a peak maximum (battery damage occurs higher than that). Any lead-acid battery (like the robot battery) can't source more than 12.65 volts*, so you're already at a disadvantage, and under loads like 50 A the voltage sags to 12.2 or less in several seconds (!).

dpeterson3 gives an excellent suggestion: Go to a car store and buy a large marine deep-dischanrge lead-acid battery, and use it power ONLY the sound system, nothing else. Charge it like a robot battery (but it'll take longer), but treat it better, since it contains liquid acid.

Of course, the louder the sound, the more power it needs.

Good luck, let us know what you do.