RB911G Netbox 5 problems after 2days

Hi all

We have site running with 3 RB911 5HpacGG. The problem we are experiencing is tha after about 2 days the RB freezes and we have to hard reboot it. We have replaced 2 units with new ones, thinking it might me defective parts. After 2 days…same problem on both new units.
The units are powered by battery power. Voltage is running at 27.1V
Does anyone maybe have an idea what could cause this?

what is max voltage this router receives?