lithium ion batteries should not lose as much charge sitting around as nickel metal hyride batteries.
Lithium ion batteries should also benefit from a few power cyclings (fully charge and then fully discharge) to get them accepting a complete charge. Best thing to do is to set up your complete recording rig and record the tv/your stereo 'till the battery's fully discharged, recharge and repeat (say 2 more times). Moniter how long the battery lasts the final time and you should have a good idea of its total capacity.
Also, when charging, you need to let it charge 'till the red charge light turns completely green (it starts red and then flashes red/green).
Finally, it's best to store these batteries in the fridge, fully charged, to prolong their life.