NEC MobilePro 790 - Manual (Page 14)

Getting Started 1-7
Note
Before using the system for the first time, charge the MobilePro
continuously for 6-10 hours to fully charge the main and backup batteries.
Allow the 6-10 hours to pass even if the indicator light changes from
amber to green before the 6-10 hours have passed.
To protect your data, do not use the system until the backup battery is fully
charged. Recharge the battery whenever the battery charging LED lights
amber.
You must repeat this full 10-hour charge cycle only if the backup battery
becomes fully discharged.