During day VFR, the avionics and engine instruments in my C182 draw ~12A from the alternator after the battery is recharged. If I turn on nav lights, the beacon, the pitot/stall switch heaters, the load increases to ~35A.
I'm thinking that the 12V output from an ATX supply is too whimpy. I use a MeanWell SE600-15 (similar to **broken link removed**), set to 14.25V. I bought mine on EBay for ~$35. It had a bad fan and I changed it. Works great, and can charge the battery, too.