Old #"Charles' Law"# originally specified the relationship between VOLUME and TEMPERATURE. It assumed (i) CONSTANT pressure; and (ii) a SPECIFIC QUANTITY of gas, what we would now call a molar quantity....
#"Charles' Law:"# #VpropT#...where the #prop# sign denotes proportional to....another way we could write this is as...
#VpropT-=V=kT# where #k# is a so-called #"constant of proportionality"#...the which here has units of #L*K^-1#... And as a constant it should be valid for any and all conditions (of course the gas should not condense). And this constant of proportionality may be incorporated into the Ideal Gas Law...#PV=nRT#...where #R#, the universal gas constant, has various expressions. For chemists, probably the most useful is #R=0.0821*L*atm*K^-1*mol^-1#...because here the units of pressure are intuitive units of atmosphere....#760*mm*Hg-=1*atm#...