I don't know of a command on rlm (there may be something to list sensors, but not sure on nvram), but I would "system console" to get to the console, login to the controller, then "sysconfig -a" and "environment status" to see NVRAM... even better, setup an alert with ops manager and make sure autosupport is enabled which will send an alert if the battery is low.
It is my experience the RLM is the same as having a serial cable plugged into the filers serial port. If the filer is dead and will not power on, the rlm access will not help. Get around this problem by asking a local 3rd party to connect a serial cable to a local machine and using Windows RDP or other remote control product to access the console session on this machine. This assumes the filer can be made to try and start by the 3rd party.
If the filer will power on and the RLM is correctly configured but DoT will not boot. You should be able to connect to the RLM and then connect to the filer system console. You should then be able to bounce the filer while the RLM continues to report what is happening. Interrupt boot process to access the special boot menu and run diagnostics. Either full system or just on the NVRAM system, just select from menu.
Our battery was not charged when we got our FAS3140. In some time after, it was charged enough, we could load Data ONTAP and monitor the battery through Data ONTAP. It looked OK, so we stopped trying to see the battery from the RLM.