As i am not able to figure it out by myself, here we go

I have a 2009 Bravo T-Jet 150, and a really demanding car audio system (About 10kW), and have added 5 62Ah batteries. Now the alternator seems to be the weak link in all of this, since it takes forever to charge the batteries, and it can't hold up with the instant demand while playing music. I can't figure out what is the amperage of the currently installed alternator (there was a 70A fuse on the wire from the alternator to the battery clamp, so I assume that it is a 70A alt.?). Is there a way to check this?

Also i am curious what is the strongest alternator available that will fit that engine. All I found was 120A.

Thanks.

the original alternator is probably rated at 80 amps MAX.
At that required power it would need more than one alternator.
I would suggest removing the AC compressor and add a second alternator there.

Forget about direct replacement alternator, go to a good garage and ask them to add a 200-250 AMP alternator maybe from GM as i remember they have a few factory 200 AMP alternator versions.
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Even with two 250 AMP alternators you will still be getting only 6000 W MAX ...more realistically ~4500-5000W

(but now you only have 840W MAX)

An other possible way is getting marine grade deep cycle batteries.
Those can be drained more, more times without getting damaged.
Then charge them at home with 5 different battery chargers.( this is not the best way)

check sticker on alternator for amps

Checked the alternator, after doing some gymnastics with the phone to take the picture of the sticker. It is a 120A alt installed on my Bravo.

The batteries I have are deep cycle AGM, so no worries with that, I just need a solution how to get a stronger alternator installed.

There are some aftermarket alternators available in the US, like mechman, which put out 370A, but the problem would be the fitment, as the space behind the engine looks limited, as well, there will be a battery error on the dash, since these alternators are self regulated.

Also, removing the AC is not an option.

I was hoping there is a bolt on solution...

how is 120amp alternator not enough? you sure its charging good?

It is charging 14.2V, but it is struggling with altogether 6 batteries, and it takes a 2 hour drive to get all of them to that voltage.

there is one guy in punto forum, he got a lot of batteries too, you should ask him what he did with charging

https://www.fiatforum.com/members-motors/345885-fiat-punto-ferrari-demo-build.html