We have a situation whereby we have a system that is about 100m from an AC source and we can't run AC to it. We would like to use a rugged AC-DC adaptor to act as the "solar" input into the charge controller.

When we do that, the DC imput starts to pulse and drops and cycles.

For example:

120VAC --> AC-DC --> 16VDC (4amp) --> MPPT charge controller --> load and 12v battery each in their respective plugs

No matter what type of AC-DC adaptor I use, all in "in-range" voltage and amp allowed by the charge controller, the input drops and starts to cycle down to 10v... essentially not charging

If I plug a solar panel with the same amp and voltage, all works ok.

What am I missing? I tried several charge controllers and dozens of AC-DC adapters, from consumer electronics to industrial AC-DC adaptors and regulators. Once in a blue moon it will work, and then it stops and does not work again.

 

If I understand right you have the AC adapter one end of a 100m cable and regulator far end.
As soon as you apply any load you are going to get volt drop due to the resistance of the cable. Only way to reduce the voltdrop is to increase cable size. Otherwise have to increase the voltage you are feeding into it.

Rather than a solar regulator you are better off getting a dc-dc convertor. Set its output to 13.6v or whatever your float voltage is. Feed it with a 24v adapter. This assumes you have run a decent cable and not some piece of cat5.
There are a number of voltdrop calculators online you can use.

 

My guess is your MPPT controller is expecting a current source as the input and is testing the current draw from your AC power supply causing the voltage at the supply to drop below it's under-voltage set point while finding the MPP. You need a (higher DC voltage could also help) AC/DC power supply with current limiting. One of your industrial quality supplies might have that adjustment already.