I tried the Relay out today on the HBS and Life Batt and shes a pearler.
To make an easy circuit I just did exactly this and it works very well.
I used a momentary centre off, throw on DPDT toggle switch that reverses the polarity, You should use a 470 ohm 10 watt resistor in case something gets stuck and put the resistor before the DPDT toggle, so the toggle switch isn't not hooked directly to the battery at 52+v. The DPDT switch only sees 14v@ 84ma max over a second. Its quite an acceptable circuit in terms of reliability and safety for your gear.
My pack was charging at 58v and I was getting 14v over the coil. This is fine IMO. The relay is rated at 1 watt this can produces little more than 1.008 watts of there abouts.
Math for a 48v pack.
470 ohms + 150 ohms = 620 ohms.
I had 58v
So!
58/620 ohms = 0.09355 divider ratio.
V drop over 470 ohm resistor= 470 ohms X 0.09355 = 43.9685v
V drop over the relay = 150 ohm x 0.094 = 14.0325v
That is spot on to what I got on the multimeter rounded off to 5 decimal points.
48v pack at LVC
@ 41v pack/ 620 ohms = 0.06613= divider ratio.
V drop over 470 ohm resistor= 470 ohms X 0.066 = 31.02v
V drop over the relay = 150 ohm x 0.066 = 9.9v
I tested the relay with a 9v batt and it still switches with a nice click..
Cheers.