S7-315 dp2 low battery warning

I need the simplest way to access
the low battery warning on a Siemens
s7-315 PLC. I know you can do it
with the OB81 or SFC51 blocks.
The Siemens manauls always seem
to leave a step or two out of
the instructions. Or you have to
pay for an example program online.
Any help would be appreciated. Thanks!

Retrieve and give this a try.