Hi Glenn,

New cells should receive an equalizing charge of 1/10C ( 400-500 mA in this case) or less for 20-24 h prior to charging them with whatever you think appropriate.
Not all chargers will detect the Delta peak at this low charging current, just let it charge untill the pack feels warm.

Failing to do this, the individual cells all have a different charge level, meaning one or more will be 'full' before the other cells are, these cell will overheat and vent hydrogen gas, or as you've experienced, go with a bang (which is an indication you charged it with more than 1/10 C...).

As you have no idea how long the pack has been shelved, it's always wise to give it an equalizing charge.
The same goes if a pack hasn't been used for a number of weeks, the charging level of the individual cells will have drifted apart.

As high Ah NiMH cells are notorious for theirs self discharge, it's odd, most suppliers don't offer this information with each pack.

Regards, Jan.