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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?

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  • \$\begingroup\$ Is your 5.5 V regulated? If not, how high is it unloaded? \$\endgroup\$
    – winny
    Commented Feb 7, 2022 at 10:43
  • \$\begingroup\$ @winny The power supply is regulated to 5.5v, yes \$\endgroup\$
    – Hans Ott
    Commented Feb 7, 2022 at 11:39
  • \$\begingroup\$ Then you are probably good to go. \$\endgroup\$
    – winny
    Commented 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 ♦
    Commented Mar 14, 2022 at 11:55

2 Answers 2

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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.

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NiMH cell nominal voltage is 1.2 volts and, in a 4S configuration, is 4.8 nominal. Does that mean it has to be 4.8 volts? No, it does not. Any adequately designed battery-operated gizmo should operate over the entire operating voltage range from Charge Voltage to fully discharged. For example, a 12-volt flooded lead acid operating range is 10.5 volts @ 0% SOC to 14.4 volts charging. NiMH chemistry is no different. NiMH fully discharged is 1 volt per cell or 4 volts in a 4S configuration, although you would not want to use that low voltage for a power supply. Charge voltage is 1.4 vpc or 5.6 volts. 5 volts is more than adequate and easy to supply. You could use a voltage range from 4.5 to 5.5 volts.

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