Even the D cells used in Hybrid cars need to have special internal construction to deal with the 100A assist and 45A charge that they see on a regular basis.
I took some apart to see what was special.
Stock honda 6.5AH cells dissected:
http://99mpg.com/Data/resources/down...nstruction.jpg
MaxIMA 8AH cell construction (Bumblebee Batteries)
http://99mpg.com/Data/resources/down...nstruction.jpg
http://99mpg.com/Data/resources/down...nnerphotos.jpg
they need a very low internal resistance with multiple welds in multiple places to comfortably handle the currents without overheating.
The photos are an analysis of the differences between different cell construction.
This was a 10AH cell that is not even metal foil.
http://99mpg.com/Data/resources/down...ngkongcell.jpg
The cell self destructed at only 30A.
Another problem with parallel cells is that a short in one cell will suck heavy currents from the other 2 cells, making the pack a dangerous device.
You never see NIMH cells used in parallel for that reason.
Nice idea, but it is not a good or safe replacement for those very special Hybrid D cells in our battery packs.