Ducati DIAVEL Owner's Manual page 138

Note
The heated handgrips actually activate and
warm up only with engine running.
Note
This means that if heated handgrips are enabled
and engine stops, the heating is "temporarily"
disabled but the ON indication is still active. Heating
will automatically turn on when engine is started
again.
Warning
Ducati recommends NOT to use the heated
handgrips when ambient temperature is above 15° ÷
20°C since they might get damaged.
Note
Handgrip heating requires a high current draw
which, at low engine rpm, might result in the battery
getting soon flat. If the battery is not fully charged
(voltage below 11.9 V) handgrip heating is disabled to
ensure engine start-up ability; it will automatically
activate again when battery voltage is above the
specified value.
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