No other drivers cap the battery level, so this one should not either.

The WS90 backup power, used when the solar photovoltatic charged
supercap is empty, is provided by 2 AA batteries.

The standard manufacturer specification is that the nominal voltage of
a fresh alkaline battery is 1.5V.

The voltage of a new alkaline battery ranges from 1.50V to 1.65V,
depending on the purity of chemistry and the quality of manufacturing.

The voltage of a new lithium iron disulphide battery is 1.72V.  The
lithium iron disulphide chemistry is good down to -50C and has an
operational life of 15 years, so it is often used for weather stations
such as the WS90.

It is useful to the user to be able to see a Battery Level >100% to
let them know that they have a pair of very high quality batteries
installed and that they are fully fresh.