PLC Battery Life

Is there a way to check the energy left in a PLC CPU battery? I know of a few systems which set a status bit when it's time to change it. Do they just look at voltage? What I'm getting at is, prior to the status bit being set, is there a way to test how far the battery is from this point? Using the AB SLC as an example, the battery has a nameplate rating of x. Is there a published or known value below which the status bit will be set?
I'm looking at this from the standpoint of scheduled maintenance.

Talk to the technical people at your PLC manufacturer/distributor and see what their recommendation is.

Typically, lithium batteries are used for PLC's like SLC500's. Unlike other battery-operated devices (flashlights, etc), PLC's use a known amount of power and fail after roughly the same time frame.

I seem to recall that the life was about five years. Then I would schedule changing them every four years.

Also, don't bother keeping a spare purchased when you get a new machine. It's shelf life will run out concurrently while the working battery's life runs out.

Another major factor is how long is the PLC without power?

A machine delivered recently had its batteries expire (2 Control Logix PLCs) on arrival, so there might be some merit in ordering spare batteries with a new machine, but put them in when the machine has been commissioned, as Jimmy says the shelf life would be running out.

We normally replace our batteries every 12-18 months (we aim for 12 but don't always get to it on time), its cheaper and quicker to replace the battery early than have down-time. We normally run between 8 and 16 hrs/day so perhaps we are using the battery more than if we had continuous production.