in the specs for a relay (download the datasheet) is the current drawn by the coil.
Knowing that, you can put an appropriate resistor in line (pay attention to power disipation in the resistor, not just resistance) to get the correct current through the coil of, say, a 24V relay for use in a 36V system.
of course, if it's a DC relay, you can just measure the coil resistance and calculate the approximate current required to operate it. . . and choose your resistor accordingly. if you're off by 10% one way or the other, most DC relays won't care.
If, for whatever reason, you're dealing with an AC relay, it's best to go to the data sheet, as DC resistance (what your $10 DVM measures) has very little to do with the AC impedance of the coil at 50 or 60 hz.
Regardless of the coil rating, the contacts don't care about voltage much so long as you don't exceed the current rating . . . though you'll need to know what rating to use (DC, AC, inductive load) Usually that information is on the relay case somewhere; it's also usually listed in the catalog if you're buying new parts.
there's also the question of "if you're dumping the power, what does efficiency matter?" (ignoring for the moment that you'd really like to pick where the heat ends up. . .)
the answer usually is: Dumping power BEFORE the inverter is better for the mill and batteries from a reliablity standpoint, and easier on the inverter, because it's not worked as hard, as often. efficiency doesn't enter into it. . .
Hope that helps-
-Dan