[GeeTwo]

If the goal is to have a way to charge the tank for practice, it would be much simpler to use a single A/C air compressor than four or more 12V ones.

• Include a tee somewhere in your high-pressure side, just on the tank side of the pressure relief valve so that relief valve will function as a safety.
• For competition, the third branch of the tee will be capped with a solid plug.
• For practice, replace the solid plug with a vent plug, with a quick disconnect on the open end. Keep this vent plug closed except when externally charging.
• To do an external charge:
  • attach the external compressor at the quick disconnect
  • open the extra vent plug
  • turn on the compressor
  • When you reach the desired pressure (or the relief valve vents), turn off the compressor
  • close the extra vent plug
  • disconnect the quick disconnect fitting