I have a 2013 stx with 5.0. First off I have a few questions regarding the way the truck controls the alternator voltage and load. I have come here for more knowledge on how the charging system works and less on power requirement for the system, or I would go to car audio forum. I have installed a system (4500w) with 0 gauge running from alt to batt + ,and engine to batt - and also 0 guage (+ and -) to back seat where I have a secondary battery. Also, I have disconnected the bms ampere clamp. I have a voltage meter hooked to secondary battery terminals and also have forscan running with alternator current, desired generator voltage, and voltage at battery terminals. As I turn it up, I drop to about 12.7v sustained at the secondary while the generator current never gets above 120 amps even at cruising speeds. I have a pdf from ford showing the alternator output curves at rpm and temperature. My alternator is a 225amp. At cruising speeds, I should be able to get around 190 amps from it. Also, the alternator never charges above 14.1v and I would really love to see 14.4v. Is there anything in the truck preventing the alternator from being fully utilized, and is there a way to change the desired output voltage to always be at 14.4v.

Sorry for the long post, but I needed to clarify the wiring configuration and how I recorded my measurements.

 

With the bms disabled as you have noted (mine is as well) the alternator is going to output as much as the system requires, this includes the amount of charge already in the batteries. With fully charged batteries and normal system load it should read just what yours is reading. Disconnect one of the batteries and watch the output with the system cranked. You should notice a spike in output.

By the way 3 tenths of a volt is not going to make any difference one way or the other. The critical factor here is amperage.

Disconnecting the bms sensor takes away pcm control of the alternator effectively allowing it to work like a normal charging system. Is there some problem you're encountering that prompted your post?
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There are not any problems that I know of, I just wanted clarification that there was no other systems acting on the alternator. These truck are pretty complex and I know so little about the control systems in it. I was just making sure because I was pulling a lot of power out and I thought the alternator would be at full load given the voltage drop, but my knowledge on alternators is very little. Also, are the measurements in forscan very accurate as I would assume them to be pretty good.

 

With bms bypassed it's just a regular charging system pretty much. Can't say how accurate Forscans pids are as it is after all a beta program. It's pointless to worry about a problem that does not exist.
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Thanks for the help. This is why I came here, you guys know your stuff.