To specfy my guess, I guess (huh) that the reason is that my NP1 is NiMH, and my charger being a NiCd charger, it doesnt fully charge the battery, though still full enough to run my mixer with all four channels phantom power on for more than 24 consecutive hours. So maybe the voltage is just below 9 V, thus too little to be approved by the R44 as valid power source. Does this make any sense?
It doesn't really make sense, because AFAIK most NiCd chargers will charge NiMHs alright. Except if the charger is too cheap, which probably is not your case. What does the charger specify as its charging voltage? Is it an intelligent charger?
NP1 batteries are at least 12v, and they can't hold voltage below 9v. In fact you shouldn't use a 12v battery below 10v, or 1v per element, or you will damage it.
So do check what voltage and current will your mixer output. It may feed just a few milliamps, which won't do with the R44. Just now did I have a look at the R4-Pro manual (which is the one I'm thinking of buying) and it demands 2 amps, which shouldn't be much different in the R44.
To be sure of the actual consumption you should connect a DMM in line with the R44 and the power source, battery or not. Then you should precisely know what you are dealing with, instead of just guessing.