39 is about fully charged (slightly less)
(fully charged is around 2.2 volts per cell)
at 36 volts even you are completely dead

now you have to monitor the battery voltage while you tilt to the stop and hold it

if you drop 2-3 volts youre probably OK more than that you might want to call a battery guy to check it out

you may need a new battery if the machine wont last more than a couple of hours or wont work at all
(if it loses its charge rapidly)

a good battery should last 6 hours of continuous use
(8 hour shift with breaks and stop time)

if you charge too often you can diminish the capacity to hold the charge effectively too