Car amp draining battery fault?

I don't think so... It sounds to me like your battery may be going bad and it cannot support all your devices.

(not positive however)

If it goes flat in 3 days WITHOUT DRIVING IT, then it's nothing to do with the alternator. Either the battery is dead (regardless of how old it is) or there's some current draw while the car is parked up. If you have a multi meter you can check this. There will always be a few milliamps for radio, clock, alarm etc, but it shouldn't be any more than that. I'd suspect some after market stereo, alarm or lighting equipment if it's fitted but tbh, I think it's the battery.

Sounds like your amps are too big for your alternator, so the battery is never being charged properly. Many people even with one amp will look at a bigger alternator or second battery, let alone two amps!

if you have no lights on the amplifiers they have no power in them.try getting a new yellow top deep cycle battery and install a capacitor closer to the amplifier it stores energy for the amps when there calling for it.and have the alternator tested for output to make sure its ok.with amplifiers running it is know to take its tole on your charging system.and look into the( big 3 upgrade) on u tube or google it. this may help you out.there are many how to videos on u tube for car audio sound systems take a look.i think your battery is in need of replacement,and have the alternator tested,any auto parts store will do this test for free.if you find out your alternator is not working right,ask about a higher output alternator than the stock one is.this will give your ride the power it needs.hope this helps.

It could just be that your batteries on its last legs. At this time of year they're under their most strain, driving with lights, heater, stereo etc on your alternator has to work double hard to keep charge... Unlike in the summer when you'd just have your stereo. How old's the battery?

Some mechanics understand electrics, but not many. I suggest you get an auto electrician to check it out, it seems to me more like an alternator fault, or a dying battery.