I have a small device operated by four NiMH batteries in series. The device also has a USB plug with boost-circuit to charge the batteries in the device, without need to remove them.

My goal is to replace the NiMH battery pack and operate the device using a permanent power supply because I do not operate the device mobile.

Is it safe to replace the NiMH battery pack by a 5.5V DC power supply with a (Schottky) diode in series (to prevent the charging circuit to feed back the charging voltage into the power supply) or do I have to consider something else?

  • \$\begingroup\$ Is your 5.5 V regulated? If not, how high is it unloaded? \$\endgroup\$
    – winny
    Feb 7, 2022 at 10:43
  • \$\begingroup\$ @winny The power supply is regulated to 5.5v, yes \$\endgroup\$
    – Hans Ott
    Feb 7, 2022 at 11:39
  • \$\begingroup\$ Then you are probably good to go. \$\endgroup\$
    – winny
    Feb 7, 2022 at 11:55
  • \$\begingroup\$ How many mAh are the cells? What format - AA or ... ? AA Cells under about 1800 mAh can (probably) be trickle charged. Those over that have no Hydrogen recombination chemistry and will die if trickle charged even at extremely low rates. 5.5 V for 4 cells is probably safe. Slightly lower is safer. What is the current rating of the supply? Vmax depends on charge rate. || Removing the cells is viable UNLESS you value the battery operation capability. Ensure there is a large enough capacitor at input. \$\endgroup\$
    – Russell McMahon
    Mar 14, 2022 at 11:55

1 Answer 1


My inclination would be to keep the NiMH pack, and just leave it on trickle-charge. Using that 5.5 VDC wall-wart and Schottky diode should drop the voltage to that of a fully-charged battery pack, i.e. ~5.4 V, and reduce the impedance of the power supply, to handle current surges that might strain the wall-wart. This still leaves the option of mobile use, if desired.


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.