0
\$\begingroup\$

I have an old calculator which contains 4 2/3 AA NiCd batteries in series. These batteries are very old and need replacing, so I'm going to replace them with 4 2/3 AA NiMH batteries in series, which have a capacity of 600mAh. The power supply is 6VDC and I want to charge the cells at C/10. What value of resistor would I need to use?

\$\endgroup\$
2
\$\begingroup\$

Sounds like you want to connect 6VDC charger directly across a series string of the cells plus a resistor, limiting current to C/10, or 600/10 = 60 mA.

Highest current will be when battery is at lowest voltage. If the cells have not been over-discharged, this will be around 0.8V per cell, or 4 x 0.8V = 3.2V for the 'battery pack'. V = IR, so R = V/I, R = (6.0 - 3.2)/.06 = 46.6 ohms. 47 or 51 ohms for a standard value resistor.

P = V^2/R, so resistor power rating should be at least (6.0 - 3.2)^2/47 = 0.16W. To be more safe use full DC supply voltage (as if battery voltage could be 0V), for power rating (6.0^2)/47 = 0.77W. Pragmatically a 1W or 5W power resistor would work.

Note that while NiCd typically maxes out at 1.5V per cell during charge, NiMH may go as high as 1.8V per cell. You may need a higher power supply voltage to fully charge the battery.

This is not at all a recommended way to charge NiMH safely! C/10 is a bit high for "trickle charge" and might be reduced to more like C/20. You have no protections for faster charge, such as full charge detection, time limit, or over-temperature, that a battery charger IC would provide.

If you're only looking to keep an old calculator working, keep a timer on how long the battery is charging and maybe don't charge all the way, say 8 hours at C/10. This will help with keeping the cells from dying early due to overcharge, which is likely with this charging scheme if left unattended for too long.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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