Take a look at
this post for more details on the 50A Anderson Connector.
A 48V 8Ah bottle battery is unlikely to provide enough current to adequately power your MP4 under full load.
Check the specifications of the battery to make sure it can deliver at least 20 Amps continuous and 30-35 Amps Max
If the current output is too low you should be able to run it in parallel with your existing battery if the voltage and chemistry are the same to give an extended range, rather than using it as a standalone reserve battery.
Even if the chemistry and nominal voltages are the same, you still cannot connect it in parallel with your existing battery if the voltages are not matched
(i.e. one fully charged and the other almost empty), they must be the same actual voltage.
Alan