The charger won't turn on if the pack voltage is below 36v.
You can either pre charge each battery individually with a car charger at the 2 amp setting (even 8v batteries) until they get close to 7.5v each.
Or you can
jump the relay inside the charger so it comes on regardless the pack voltage.
If the batteries are really low, the Second method may trip the breaker in the charger.