Chris,
For my way of thinking, a battery would not be acceptable. I am trying to physically show what is being generated not a measurement of it.
I think I might go for small incandescents in series, they can start off dim and grow brighter as it comes up to speed. As you know LED's also have a slight voltage range, a much smaller range than incandescent, and that is how I did it last time. They all started dim, and grew brighter. Just that I didn't have enough in series to cope with the extra voltage produced, and they went over their limit and committed suicide.
But please, do show us a sketch of the battery method, as I am sure Ralph is biting at the bit to see how it is done, and you never know, I might be able to be swayed in that direction.
Many thanks.
Bogs