Alternator issue

[Big_daddy]

hello to all
well i jus fitted my sub and amp back into car its a seat leon 1.8 T.
the amp is kenwood 1800 watts and 2 pioneer running at 3000 watts each. these were in my audi a3 before and every thing was fine

The problem is that my lights are dimming when the amp is running at higher power. so i asum the battery is weak. I got battery and alternator tester that plugs into cigratte lighter.
The battery indicators stay full but the alternator drops down and hits the zero mark.

Any suggestion how i could over this

 

[Davcupra]

Check the battery terminal connections - maybe a bit of hidden corrosion limiting the amount of current. Is the starter motor also a bit sluggish ?

 

[Big_daddy]

the starts straight away on the 1st click, the battery terminals are very good with grease stuff (lol). the battery is wrapped so dont sum dats d case.