Schumacher Pro Series Owner's Manual page 12

PROBLEM
Display shows "SUL"
Engine start does not
work.
POSSIBLE CAUSE
The battery is sulfated.
Vehicle drawing more than
the Engine Start rate.
Failure to wait 3 minutes (180
seconds) between cranks.
The charger may be
overheated.
Battery may be severely
discharged.
SOLUTION
A sulfated battery may eventually
take a normal charge if left
connected. The charger will continue
to charge with a low current for up to
10 hours to recover the battery. If the
battery will not take a charge after 10
hours, have it checked.
Crank time varies with the amount
of current drawn. If cranking draws
more than the Engine Start rate,
crank time may be reduced.
When the Engine Start LED
blinks, wait 3 minutes of rest time
before the next crank.
The thermal protector may have
tripped and needs a little longer to
reset. Make sure the charger vents
are not blocked. Wait and try again.
On a severely discharged battery,
use the Boost rate for 10 to 15
minutes, to help assist in cranking.