4
\$\begingroup\$

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?

\$\endgroup\$

1 Answer 1

Reset to default
2
\$\begingroup\$

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.

\$\endgroup\$
2
  • \$\begingroup\$ thanks Leon, any explanation as to where this voltage increase comes from? \$\endgroup\$
    – vicatcu
    Apr 10, 2011 at 15:02
  • \$\begingroup\$ 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. \$\endgroup\$
    – Leon Heller
    Apr 10, 2011 at 15:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.