Computer shutsdown, as if unplugged

[turbominnow]

So last night I am grininding levels and viewing the interwebz when my PC crashes. Crash is an understament it turns off as if unplugged. Its not a new build and I cant think of a reason why this would occur. Its on battery backup and I am running a Seasonic PSU that is only a few years old. Computer runs fine when it is running. Any suggestions?

 

[xdivenx]

Sorry for the late reply.
Plug it straight unto the wall, skip the battery backup.