# Battery Voltage While Charging

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?

The charging voltage will be 1.4 V -1.6 V per cell. They must be charged at constant current, not 1.2 V per cell.

• thanks Leon, any explanation as to where this voltage increase comes from? Commented Apr 10, 2011 at 15:02
• I don't know what you mean by "voltage increase". When fully charged, the cell voltage is 1.4V or so (open circuit). Cell voltage will be about 1.25V when they are on a heavy load and discharging. Commented Apr 10, 2011 at 15:34