For existing install
1) With the evohome powered on the dock/wall plate
2) Remove face plate and remove batteries and wait until the battery symbol with an X appears top right (should appear after >5 seconds)
3) Re-install batteries and face place
4) Battery symbol should now show the charging flash
5) Leave the unit on the charging cradle until the battery is fully charged and the symbol will disappear
This should allow the evohome to fully analyse the battery status properly and engage the charging cycle and then once fully charged allow operation for 3-4 hours off mains power when required.