second battery for audio

Battery life has NOT been an issues yet, my alternator is 220amp.

I just want to be able to run it for 15 minutes or so at a time with the engine off. and also maybe get some more out of it while running.

I have never had an issue with dimming even with just the stock battery, but since I am adding another amp, I want to cover my bases.

2004 Hyundai Sonata

Kenwood KDC-X599 HU

2-Sundown X-18D4

2-SQ Q4500.1

2-SQ Q90.4

1-Massive DBX4

8-Niche 5.25 mids

8-Niche Tweeters

4-Skar 8" mid-bass

OhioGen 220a alt

OhioGen 350a alt

156.0

The Kinetik Power Cell Calculator reckon you need an HC2400 (which is equivalent to SIX HC600s), not that I can find any info on how or what the calculator calculates (engine running; reserve time; high surge damage minimisation, etc). My way: HC600 is 20AH; assume at C10 or C20 rate - ie, 2A for 10h or 1A for 20h. Your load is 2360W ~2400W => 240A (a perfect application for the div-10 rule where the 2360W is output power). Using a simple yet invalid calculation, 1A for 20h means 240A for 20x60mins/240A = 5 minutes. But it will be less than 5mins because the capacity of a battery decreases with increasing discharge current. And since 240A is MUCH higher than its C10 rating of 2A, it wouldn't surprise me if it only lasted 1 minute or less instead of 5. (That's to its spec'd end voltage. I'd suspect the HC600 is a cranking type (ie, NOT deep cycle) and hence limited to a 20% discharge. Adding the 2nd HC600 will double that reserve time. (Actually because of capacity-discharge-rate non-linearly, a bit more than double.)    In summary, using 2 HC600s on a 2360W output load will last a MAX of 10 minutes based solely on its C10 or C20 capacity. In practice they should last far less - maybe 2 minutes? The HC600's discharge capacity curves are needed for accurate calcs. As for dimming I can't say. Sometimes that has nothing to do with battery capacity and everything to do with wiring.

Now I am really confused, I can play what I have (1360 watts rms at 14.4v, so probably 1100 at 12v)) for at least 15-20 minutes on my cheap AutoZone stock battery, when it gets down to just under 12volts I start the car and recharge while playing.

I have no idea where to start now!

I figured adding some kinda extra battery would at least help, based on the numbers it won't, and based on the numbers, my stock AutoZone battery probably shouldn't run it for as long as it does,,,,,,,,BUT it does.

Maybe I will just buy another AutoZone battery and take my chances with the fumes.......

2004 Hyundai Sonata

Kenwood KDC-X599 HU

2-Sundown X-18D4

2-SQ Q4500.1

2-SQ Q90.4

1-Massive DBX4

8-Niche 5.25 mids

8-Niche Tweeters

4-Skar 8" mid-bass

OhioGen 220a alt

OhioGen 350a alt

156.0

They are merely calcs.... So you run your existing set up at full output volume for 15-20 minutes? And note that 1360W @ 14.4V should still be 1360W @ 12V (94A increasing to 113A) because of a big amp's SMPS supply (tho I know many reduce output with reducing voltage). And tho your alt shouldn't have any problem with your existing ~100A amp unless the RPM is too low, the newer ~240A might if it's used at high output etc. But assuming you are adding healthy batteries, obviously the total reserve time will increase. But use an isolator. In your case if you want total reserve and don't care about cranking battery independence, control the isolator(s) from your amp. And when you say 'fumes', you mean the gases that corrode lungs and vehicles, and can explode? BTW - the reserve time of a given AH does not vary with battery type - ie, AGM or wet cell.

If handled by the amp, any relay of suitable capacity. If a typical (automated) 'battery isolator' I suggest searching 'oldspark UIBI' where the UIBI is merely a charge-light actuated relay (from the alternator's D+ or L aka charge-light circuit). Otherwise the inferior voltage sensing type (unless you don't have a charge-light type output) aka 'smart' battery isolators etc. Capacity is not important as you merely add whatever relay is required. Note that the principle is that batteries should not be left connected in parallel for too long when NOT being used - ie, when not being charged, or not required by the load. Many use an auto isolator between the main battery/alternator and secondary batteries to ensure cranking battery independence. They might use the same to isolate paralleled secondary/aux batteries, or manual activation if the secondaries are to be paralleled when NOT charging (eg, if you use 2 or more HC600s etc as secondary audio batteries - maybe off the amp remote or other amp signal). And don't forget to fuse EACH end of the inter-battery link unless you can ensure physical security. And check the temperature or open circuit voltage of your secondaries as reasonable intervals. My UIBI etc scribblings should provide various examples.