I'm experimenting with a linear-mode fast-charge circuit for NiMH batteries. I was surprised to find that the voltage measured across the battery terminals while fast-charging is higher than the theoretical series cell voltage. By that, I mean rechargeable cells are theoretically 1.2V each (as opposed to non-rechargeable cells that are 1.5V each). Charging four AA NiMH cells in series I measured a voltage of around 6V across the pack, rather than 4.8V.
What is the explanation for this higher voltage while charging? Can I predict the maximum voltage, V, I will measure while charging N cells in series with a current of J mA?