How many new 1.5V cells can you connect to 7.5 lamp to obtain the brightest light without destroying the lamp?

How many new 1.5V cells can you connect to 7.5 lamp to obtain the brightest light without destroying the lamp?
Five standard cells or six rechargeable cells - A standard new battery is just over 1.5V but will drop down to 1.1V before it is essentially spent. A Nickle-Metal rechargeable battry is at about 1.25V charged and averages 1.2V during service capacity...
You could probably use six standard cells on the lamp, and it would work, but maybe not last as long because it burns hotter (Higher resistance because higher amperage/current - not voltage).