Why is my Deebot not charging

My Deebot 900 (number 1) has a fully charged battery after charging it with a different Deebot 900 (number 2). However it says it has a low battery when turned on and the Auto button glows red. Have check the charger, and used a different battery (from number 2) with the same results. The other Deebot (number 2) does take a charge from my charger using either battery. The unit will not charge!! I removed the brushes from my unit and the dust chamber. The unit does flash red and says to install dust chamber, after installing dust chamber it glows red and says a low battery and will not charge. Have checked the contacts as well, and all connections are good. We just started using this unit and have only 5 hours run time, even though we bought it more than a year ago.