Battery Backup Failure - C7100V – Nighthawk AC1900 WiFi Cable Modem Router With Battery PB022

I have a C7100V – Nighthawk AC1900 WiFi Cable Modem Router with battery backup Model PB022. I purchased the battery almost two years after I purchased the cable modem router. I mention this is only to emphasize that the battery is new and I have much experience using the router without the benefit of the battery backup. At this time the battery is less than six months old.  

This week we had two brief power failures, each less than 10 minutes. In both instances, the cable modem router maintained power with assistance from the battery, but it lost internet connectivity and would not reconnect until I removed the battery and disconnected the cable modem router from power. I waited a minute or two and then reconnected power and the cable modem router was back online with full connectivity. 

The problem seems to be that the presence of the battery is causing the cable modem router to lose internet connectivity when the main power supply becomes interrupted and it cannot reacquire connectivity until I remove the battery and restart the system. 

I can recreate the problem easily when I simulate a power failure by merely disconnecting the unit from power. It restarts and performs perfectly as long as the battery is not installed. If the battery is installed, then no connectivity until I remove the battery and restart. 

Any suggestions would be very much appreciated.