Need proofing on this 12v battery protection circuit
This circuit will not work.  There are many problems and it's hard to understand exactly what you mean.

all load current (20a) will return through a1 - Arduino microcontrol pins can't support that current and will break.
there is no resistor on gate of transistor - this will break digital pin.
when/if the relay activates the led will draw massive current and break.

There are more - but I suggest some more reading or following a reference diagram would help.  I know electronics is difficult and don't mean to be discouraging!