George Lukas where he told Mark Hamill to hold the lightsaber as if it where a heavy weapon
Yah, I remember watching that too.
Lucas wanted the actors to portray the lite-sabers (get it?) as though they were heavy English Broadswords. I think he was telling them that because the lightsaber props were very light weight compared to a real sword. He wanted the actors to make us believe that it required great effort to weild a lightsaber as we watched them dueling.
I don't think there was any room for batteries in the GRAFLEX Lightsaber, they had a motor down inside it to spin the blade, and the motor was wired... so no need for batteries. Now that's not to say that the motor itself and whatever held onto the blade weren't as heavy or heavier than 3 D-cells!
The only shots where there may have been an option for batteries to be in the GRAFLEX Lightsaber, would have been when Obi-Wan pulls it out of his infamous lightsaber chest. If I'm remembering correctly, once Luke has it in hand while standing, it's the motorized version. Mark paused once he held up his arm, they rushed in the blade, and continued filming as he began to move again waving the now lit lightsaber around.
The Obi-Wan Kenobi Lightsaber REAL PARTS static prop would easily be as heavy as any sword hilt/pommel, but, if I've learned this properly, the Obi-Wan Shared Stunt was an aluminum casting of a lathed wooden prop built to make the bladed stunt version of the Obi-Wan Kenobi Lightsaber. So, though it'd be lighter than it's mostly steel REAL PARTS static counterpart, it'd still be heavier than the hollow tube of a GRAFLEX, with or without its blade spinning motor setup.
So, poor old Sir Alec Guinness probably had to "act" a little less in ANH than Mark Hamill did in that regard.
Good question overall! I personally have my GRAFLEXes full-o-batteries, that way they can still be used as flashlights via their spot lights!