Output will vary with load. The alternator will only put out enough to match the load on the system, and maintain system voltage within a certain range. The battery will place a higher load on the system initially to recover what it discharged during starting. That higher load from the battery may cause the voltage to run higher than what it will maintain during normal operation. Could also be an effect of the diode pack heating up and changing the charging characteristics. Either way, 72A seems high for a "no load" situation. I think you may have a parasitic draw somewhere that is slowly discharging your batteries beyond the alternator's ability to recover them. The dual battery setup masked the problem, since it takes a very long time to discharge both. It is as I said: one is completely dead and the other marginal.
If you have an inductive current meter you can clamp it around tbe battery cable and monitor current flow. Do it first with the key off, doors shut, etc. Give the truck a few minutes for any timed circuits (interior lights, ECM, etc) to go idle. I don't know minimum specs for current draw on our trucks, but it won't be more than 10 milliamps. If you find you have a draw, you can pull fuses individually to isolate the circuit. A common place for our trucks (and GM vehicles in general) are the vanity mirror lights. The bulb can burn out and short internally, and when the switch fails it will cause a constant draw while not putting out any light. Aftermarket radios, alarms, CB's, etc are also common sources.
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If you have an inductive current meter you can clamp it around tbe battery cable and monitor current flow. Do it first with the key off, doors shut, etc. Give the truck a few minutes for any timed circuits (interior lights, ECM, etc) to go idle. I don't know minimum specs for current draw on our trucks, but it won't be more than 10 milliamps. If you find you have a draw, you can pull fuses individually to isolate the circuit. A common place for our trucks (and GM vehicles in general) are the vanity mirror lights. The bulb can burn out and short internally, and when the switch fails it will cause a constant draw while not putting out any light. Aftermarket radios, alarms, CB's, etc are also common sources.
Sent from my XT1585 using Tapatalk