Yep, I'm affraid the XXI was never really completed, however there is a XXI Battery Fix available as a standalone mod as I assume you are not using any of the Supermods.
You may be able to find it in the archives or maybe on
http://u-boot.realsimulation.com/
As a note, there are many things wrong with the XXI in the stock game, however most have these have been fixed and incorporated into the supermods, GWX for example.