Simple NIMH or NICAD trickler from USB circuit?

Discussion in 'General Electronics Chat' started by jdraughn, Jan 20, 2011.

  1. jdraughn

    Thread Starter Active Member

    Jan 30, 2009
    I have a radio shack "AA" battery holder that holds 4 "AA" cells. I rigged it so it only holds 3 cells and the fourth battery slot holds a 3.3v LDO regulator, this is very nice, I have an off/on switch and regulated 3.3v output in a nice enclosure for cheap.

    I want to cut a hole in the side of it so I can plug in a USB cable for charging. I would like to do this with discrete components that I already have on hand and not have to order anything if possible.

    Does anyone know of a simple trickle charging circuit for 3 NiCad or NiMh cells. USB only puts out 5 volts so I don't have enough overhead to use a L317 variable regulator.

    Later I would like to add more functionality like a smart charge controller, maybe a low battery light/cut off, excessive current light/cut off, and maybe make that usb port functional so I could monitor current and voltage in real time with an app on my PC, but for now I just want simple trickle charging functionality from 5 volts.
  2. Markd77

    Senior Member

    Sep 7, 2009
    Some NiMH can be constantly trickle charged at a current of C/40 (C=rated capacity). Data from energizer website.
    NiCad maybe more.
    If you want to charge at higher rate then you should monitor the voltage (search for delta V charge termination) and probably a timer in case that doesen't work.
    You could try the LED/transistor or zener/transistor method on this page.