Electrical for 7.5k at .5 ohm

[Sergeant_Skyrim]

Just because equipment is new doesn't mean that it can't be faulty. Best to take apart and test piece by piece as others have said dipping that low with the electrical you have shouldn't be happening

[Smoove]

12 hours ago, snafu said:

7500W / 75% amplifier efficiency = 10,000W Input

10,000W / 13.8 VDC = 725A Required to reproduce sine waves

725A x 50% Duty Cycle = 362.5A Required to reproduce music at clipping

A single run of 2/0 AWG or two runs of 1/0 AWG would be more than sufficient here - you've got double that.  You've also got more than enough alternator to supply your amplifier and (4) XS Power batteries, recognizing that idle output (800 engine rpm) will be less than cruise output (2,000 rpm).  Even still, you should be OK at idle with as much alternator output capability as you have.

Alternators should each be grounded to the frame rail via 1/0 AWG.  Primary battery should be grounded to the frame rail via 1/0 AWG.  Batteries in the rear should be grounded to the frame rail via a minimum of (2) runs of 1/0 AWG - you have double that.

You have several thousand Amps of battery at 12.6V yet you report readings of 11V when pulling a fraction of that.  Something is wrong here.  What are you measuring the 11V with?  Where?  Have you compared that with a reading with a quality DMM?  Let's make sure we have good data before diving in.  I can't tell you how many times I've seen dash mounted volt meters that displayed bad data due to a poorly chosen source of power.  Once you've ruled that out, proceed.

Before doing any troubleshooting, you'll want to charge each battery fully with an AGM battery charger (like the one from XS Power).  That may require that you separate them to do so.  That's OK - in the unlikely event you have a bad cell in a battery in a battery bank, you'll find it this way - this can wreak havoc.  Personally, I've never seen that with an XS Power battery, but let's rule it out.  Assuming you don't find anything like that, and STILL have the same problem with five freshly charged batteries, it's time to do some troubleshooting.

Diagnosis will require two quality DMMs and a carbon pile load capable of around 500A (Snap-On MT3750, MT3760, etc).  I'd connect the carbon pile load to the rear batteries - same as where your amp connects and connect its current probe around its positive lead.  Connect one DMM to the front battery, the other to the rear batteries.  With the engine at an idle, note voltage readings on each DMM - they should be identical.  If not, what kind of a voltage difference do you see?  Even a very small difference here could be a tell tale sign of a poor connection, termination, etc.  Assuming voltage readings are identical, set each DMM to record voltage over time and to display MINimum voltage readings.  Add load from the carbon pile until the DMM at the front battery reads 12.6V.  [Normally, we would shoot for 12.0V, but you have so much battery in the rear a 500A carbon pile won't be enough to draw them down much below 12.6V.]  Note the current reading on the carbon pile and the voltage reading on the DMM in the rear - look at the MIN voltage over that time period.  How much of a difference in voltage between the front and rear DMMs?  How many Amps of load did it require from the carbon pile to achieve this?  [If it took all 500A from the carbon pile and you still couldn't get voltage to drop below 12.6V you may have solved your problem in previous steps.]  How does that compare with what I outlined above?  Differences in voltage readings between the two DMMs indicate voltage drop between the charging system and the load (your amplifier).  These should always be factored as a percentage, keeping in mind that 3% differences are acceptable.

If during this testing, your voltage is dropping into the 11s, then something is not right.  Then, you would disconnect all loads from the primary battery up front (rear batteries, etc).  Move the carbon pile to the front battery.  Use a current clamp around the charge lead of one alternator (current probe of the carbon pile is fine).  Load with the carbon pile until voltage at the front battery measures 12.0V.  The current reading on the carbon pile will be the current output from the alternator.  Move on the alternator #2, etc.  What you're looking for here is an alternator that isn't functioning, not pulling its weight, etc.  For example, maybe you find that you can only load 200A with the carbon pile before voltage measures 12.0V at the primary battery but each alternator checks OK.  Then, you may have an issue that affects the voltage regulator of all three alternators.

Somewhere along the way, you'll find the source of the problem.  This is how you quickly and easily get to the bottom of such a problem.

Because math .
Thanks for the info Tony