just found my old lights in box not been used for at least 5 years is it safe to charge the battery or will the cells explode and end the world

fine, but only if wearing tin foil.
hth

In my experience of NiMh batteries, it’s safe but pointless. If they’ve been flat for most of 5 years, they won’t hold a charge. Do try it, I might be wrong, but be prepared for failure!