Digital capacitor

[jtatexc]

Please don't kill me if this is a stupid question. I caught a bunch of hell from a friend who saw I had 4 amps (1 to my sub, 1 in cab, and 2 for the 2 sets of tower speakers) and not running one. He said I was killing my batteries and risking a lot but couldn't tell me much more. He said I would be smart to wire one in between the battery and my rack of positive terminals but the diagram on the package shows that each amp should have its own and be between the in-line fuse and amp.
I rarely to never listen to music with the boat not running. We are pretty much always on the gas pedal and when not I kill all power and we just chill. Am I in need of 1...or 4...or these? Fine without? Risking anything? Anyone else running them and if so how is it wired? He said this one is what I should put....

The amps I have are.
Stock kicker from moomba for in cab
Zx450.2 (150w rmsx2 for tower set #1)
Kx400.4 (bridged 200w rmsx2 for tower set#2)
Jl audio 500/1 for sub
I have the dual batteries in the boat and use my trickle charger in between each ride.

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