Please read and go through the steps below.

Note! don't miss step 3, it is vital to make this work
"The battery charge level icon is low even if the unit has been charged overnight or the battery is totally dead, no life sign at all."
If the battery is totally dead you won’t find it under the battery status following the steps below. 
If that is the case the battery needs to be exchanged. 
If the battery is found the reset procedure solves the problem.

Step 1: Update the unit to the latest software version using this link, chose the correct unit for you: If the upgrade did not help, please try step 2.
Step 2: 
A. Pressimageand release.
B. Press HOLD for 2 seconds and release. 
C. Press LINE MODE for 2 seconds and release. 
What you now should see is the SERVICE MENU of the unit, if not, start over with Step 2. 
Go to BATTERY STATUS and confirm with OK.
D. Press and hold digit 1 on the keypad ‐‐> you should get a confirmation “BATTERY RESETED”
Step 3: (Important) Leave the unit OFF the charger for a MINIMUM of 60 minutes so the unit discharges for a while. 
Now let the battery get fully charged by putting the unit on the charger and leave it on until it is fully charged. 
If the unit gives a warning for low battery power, which is because of the procedure, ignore that until the unit has been fully charged.
Step 4: If the battery still does not work after the procedure, please enter the BATTERY STATUS menu again and this time write down the information found on that page and e-mail it back to