Preventing voltage going over 6.3v I've been scratching my head about this for a while.. but I just can't think of a solution!

I have a piece of equipment that has a max voltage of 6.4v, it's powered using a 6v lead acid battery or 4x AA batteries.

A couple of times after charging the lead acid battery, i've blown the fuse in the equipment, due to the battery being over 6.4v when i've removed it from the charger.. (needed it in a hurry)
Can i put something between the lead acid battery and the equipment to prevent the voltage going over 6.3v?
The minimum voltage of the equipment is 4.1v.

I've thought about a diode to get a 0.7v drop.. is this the best solution?

Not sure what to do!