When you put new batteries in
a string with old batteries,they will
take on the characteristics of the older
batteries and will require replacement
much sooner and cost more
in the long run

The charger will see the string as a
single battery and older batteries
may cause over or under charging of the
new batteries causing premature failure

It will also cause the equipment
to be less reliable

Batteries used in most standby equipment
such as UPS systems, emergency lighting
alarm systems, etc will have a low voltage disconnect (LVD)
that will disconnect the battery from
being discharged below 87.5% of nominal voltage