Trial & error. I've found every one to be different, and you have resistance in the battery spring contact, switch, etc. Start with 10 ohms to be safe and work your way down. Use the 10 or 20 A setting on your DMM so as to introduce as little series resistance as possible, or use a 0.1 ohm current sense resistor and measure the voltage across it. You'll probably end up with something between 3 and 4 ohms with 4.8V source.
Rob
