The manual says that electric loads are shut down only under two conditions.
1. Battery charge status is in critical range
2. Alternator at maximum output
If either of these conditions exist the following occurs.
Heater blower reduced to 75%, then 50%, then 25% then off.
Mirror heating turns off
Rear Window defogger off
Seat heating reduced to stage 2, then 50% then off.
I can't find where a voltage is specified for critical range.