NiMH batteries are not the easiest to charge and a battery with 12 cells becomes harder. The dV/dt method will probably be useless because each cell will have a slightly different state of charge and the dT/dt method may need sensors on every cell.
Not sure what the best method is for 12 cells - maybe to charge at c/4 till about 1.36V a cell (might get you to about 50%) and then drop down to 0.05C for 15 hours?
The NiCd batteries were much easier in battery packs as the individual cells could withstand a lot of overcharge, and in my experience, many of the NiCd batteries were still working fine after 20 years. Otherwise, everyone is now looking for lithium battery solutions in new designs.
Basically, it will be pretty hard to charge 12 NiMH cells to near 100% in 5 to 6 hours. If you can get to 95% in 24 hours without stressing individual cells, you have done very well.