The RAM on these boards will retain data down to 2 volts.
This coin battery can provide 3V but you must install a blocking diode to prevent system from trying to charge the battery. The 'forward voltage drop' of this blocking diode directly reduces the voltage potential provided to the memory. A diode with a low forward voltage drop is required. Leakage current is also an issue - it reverses current flow back into the battery. Some batteries such as coin lithiums do not like this leakage current and tends to shorten the life. Normally the lower the forward voltage drop, the higher the leakage current.

Three diode types come into play - Germanium, Schottky or Silcone. Germanium costs too much and has too low of current rating.
Typical choice for diode schottky diode would be a very low forward voltage drop with a fairly low reverse leakage current - something like BAT49.
Typical choice for silicon diode with fairly low forward voltage drop and very low reverse leakage current - something like 1N4148.

A BAT49 type schottky diode has a very low forward voltage drop (about 0.3V) and fairly low leakage current (about 200 microAmps). You can also use a common 1N4148 as well -- this one has a higher voltage drop (about 0.6V) but a tiny leakage current (less than 1 microAmp). Both of these two can pass more than enough current through them to keep memory alive when machine is turned off.
Me - I'd probably go for the 1N4148.

You need to install this blocking diode with the anode (non-banded end) oriented towards the positive connection of the battery.

These coin lithium batteries have a very long life compared to NiCad batteries. But, like all batteries, they eventually do leak. Just takes a heck of a lot longer.