Get the NIMH rechargeable battery packs that are available on the internet. Do a web search for Excalibur rechargeable batteries and you will find a 10 cell 1.2 volt per cell NIMH battery pack that fits nicely in your alkaline battery pod.
The reason why the standard AA NIMH rechargeables are not good for the Excalibur is that those batteries are only 1.2 volts and not 1.5 as are the alkaline.
So doing the math, 8 x 1.2 = 9.6 volts. Just barely enough for the Excalibur to run right. As they lose voltage with use, the threshold of correct voltage is passed and the Excalibur will either run poorly or will shut down. The alkalines math is 8 x 1.5= 12 volts which is the correct voltage for the Excalibur.
The NIMH battery pack has 10 1.2 volt batteries. 10 x 1.2= 12 volts. More than enough to run the Excalibur. I get almost 40 hours per charge on each of my packs.