I recently got a mifi 2200 and was wondering how you can tell when your battery needs a recharge.
If you are using it as a wireless access point open a web browser and enter http://192.168.1.1. This will bring up the admin console, which shows battery charge as well as other information of interest. The default password is "admin", which you should change after logging in.
The light on top of the MiFi will also turn orange when the battery is low.