I agree with Nick. But if you plan to use rechargeables, bring a spare pack because they tend to have a shorter life. The Variax seems to require batteries that are nearly fully charged, and the rechargeables tend to lose their charge more quickly than alkaline when used in the Variax.

I notice that this is a rather old thread, but I have been working on this issue for awhile and just recently got a solution that I think works. I have the electronics from a 500 in a beautiful walnut Warmoth custom body and maple neck that I'm not about to part with (to buy a new Variax with the nice rechargeable Lithium Ion battery pack). But I'm sick of hauling around an external power supply or buying disposable AAs in 20 packs.The first thing I tried was loading the standard 6 pack of AAs with Eneloops. For the reasons described so well in this thread, that did not work at all. I guess 6 x 1.2 or 7.2 volts just doesn't cut it. I didn't want to mess with a voltage regulator also described in this thread, so I went with 7 Eneloop AAAs which theoretically should be producing about 8.4 volts. I bought a 4-AAA battery case (with a 9v snap connector) and a 3-AAA battery case (with solder lugs) and glued them back-to-back. I cut the wire that goes across the 4-battery case and soldered it up to the lugs of the 3-battery case. That combination fits in the orginal cavity in the guitar--no modification required. It works like a champ. I just started using it so I don't have much to say about battery life. I'm hoping that using 7 batteries will mitigate, to some degree, the smaller AAAs and the reduced life of rechargables. Thanks to thealiasofMark in this chain who really helped me figure this out. If anybody else out there has experience with a similar set up, I'd love to hear about it.