[Nut-upsuser] Synthesize low batt (LB) fron SNMP UPS which does not support this?

[Greg Troxel]

My $0.02:

  LB is baked in to nut behavior.   So if a UPS doesn't report LB, then
  it makes sense to synthesize it.  Synthetic LB is the cleanest fix at
  the earlier processing point.

  One could have a config that is basically
    synthetic-lowbatt-percent
  that generates LB if (as Jim says) on battery, not calibrating, and
  batt% <= that.

  One could have a similar config
    synthetic-lowbatt-voltage
  same thing except voltage.   The mapping from voltage and current (and
  history) to % is difficult, but this lets people have another option
  and is easy to implement.

  I think it makes sense to allow this even when the UPS will report LB.
  I want to shut down earlier than max runtime to be nicer to the
  battery.  Once the outage is over an hour it's usually 4, for me.

  It might make sense to also be able to ignore UPS-provided LB via
    ignore-hardware-lb: true
  but probably there is little need as most UPS units seem to throw LB
  later than wanted vs early.  But again easy.

  Yes, this could be in upsmon, but that's not synthetic LB, it's
  choosing to shutdown if battery is below X.

  We should be clear on "critical" vs "LB" and clean up the language to
  be consistent.

  Beware that I'm a bit fuzzy on details beyond my comments and even
  some on things I commented on, so take them as such.