I purchased the Thunderbolt extended battery - and it fit and worked and lasted as long as the Rezound extended battery. So, I now have two working extended batteries - the Rezound ext battery and the Thunderbolt ext battery.
I purchased the Thunderbolt extended battery for about $10 with free shipping.
However, I find I don't like the size, weight, and case/holster - and am going back to using the standard battery.
I ordered a second standard battery to have in my pocket in case it is needed.
I figure two standard batteries should equal one extended battery.
However, I plan to just leave the phone charging while at work.
If some day I have the need to be away from a charger all day long, then I can switch to the extended battery, cover, case, and holster.
Howard