Actually, you can use resistors to calculate exactly what you want your cutoff to be..
Any resistors added to the current coil will increase the mininum voltage it will stay on for.
You'll first need to find the minimum voltage the relay runs at (usually printed on the unit), and the resistance of the relay coil (multimeter can tell you).
Use this formula:
Ra is Resistance you need to add - the desired number
Rc is Resistance of the relay coil
Vc is the desired cutoff voltage
Vr is the normal cutoff of the relay
Ra = Rc * (Vc/Vr) - Rc
In my case, I had a 79.1 Ohm coil in the relay, it cut off at 2.52V. I wanted a 5.4V cutoff.
Ra = 79.1 * (5.4/2.52) - 79.1 = 90.40
So I needed around a 90-Ohm resistor, so I just got a 100Ohm variable resistor, and I dial it in myself. It also offers some flexibility if I wanted a bit lower or higher..
I likewise bought a 6V relay, and used the same formula to make myself a 3.6V 4-cell cutoff unit.
There was one guy's site a while ago that had all of this wicked stuff.. laser timers, and tons of other stuff.. Sanj's Yokomo page.. seems to be gone now.. but at least I kept my spreadsheet that calculates it all for me.. just don't ask me to explain it anymore!