Just faced a problem regarding setting the critical battery level on Windows 7.
It got stuck at 98% and I could not set another value, although “low battery level” could be set to any value.
Finally I came up with a solution using a command window:
run cmd.exe using administrator priviledges and type the following command:
powercfg.exe -setdcvalueindex 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c e73a048d-bf27-4f12-9731-8b2076e8891f 9a66d8d7-4ff7-4ef9-b5a2-5a326ca2a469 15
the value 15 is indicative here (one may use another value) and corresponds to the percentage of battery to be set as the critical battery level.