WL, IMHO, dun see why u shld be topping up ur system wif refrigerant every 1/2 yr or so........ unless there is a hairline fracture, sealant leak, etc somewhere in ur system and refrigerant is slowly but surely leaking out. if everything is normal, refrigerant shld not be depleting out of a system at such an alarming rate.
by right, undercharged system will indicate low load/no load on compressor therefore showing little current drawn. overcharged system will indicate high load on comp; showing high current drawn.
the above is made according to my 2 cents knowledge. better double check.