The LS takes ~3V input. A 123 supplies this.
The Arc LS comes with a 123 (CR123) tail. A 2AA (2 NiMH or Alk batteries) is available, but it makes the Arc LS look like a minimag.
Some people have suggested that taking two 1/5A rechargeables (400maH) and putting them in the 123TSPworks, but only gives about 80 minutes of runtime.
I personally would love to see a 2x2/3A tail so that we could run the LS on 1100maH Nimh batteries. It would be considerable shorter than a 2xAA tail, and provide
much better runtime than the 1/5A rechargeables.