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Charging Led Circuit

Discussion in 'LEDs and Optoelectronics' started by AlistairL, Apr 1, 2017.

  1. AlistairL


    Jul 30, 2016
    Hi I am making a handvac and i tested my fans but i need part of the circuit to show via a red led that the batteries have charge then when it switches on red led off and green led and motor working. Would it be a zener diode to let electric through below certain level then when switch on red led off and motor and green led on? Please advise how to do this please. What schematics should i be looking up?

    Many thanks. Al.
  2. Audioguru


    Sep 24, 2016
    I replaced the worn out Ni-Cad battery in my hand-vac with some Li-Ion cells from a laptop computer. It doesn't need LEDs. I tried it a moment ago because it hasn't been used or charged for a few months.
    1) It runs but not at its maximum speed.
    2) After charging overnight it will run at full speed for over one hour then it will slow down and I know it will need to be charged.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Judging the state of charge of a battery from the battery voltage is difficult to impossible. Too many parameters have an influence like:
    • temperature
    • capacity of the battery
    • load current
    • etc.
    When can use the voltage as a coarse indicator. You'd need a comparator circuit with a stable reference voltage and some hysteresis to avoid flicker of the LED. However, you'd have to set the reference voltage rather low to avoid false indication of a drained battery when in reality the battery is still good for some work.
    Probably Audioguru's method of observing the fan speed is much simpler and reliable.
    To always have a fully charged battery at hand: with a quality charger and modern battery technology (e.g. NiMh) today it is possible to connect the battery to the charger whenever it is not needed without risk of damaging the battery (this is different from old NiCd batteries where regular charge/discharge cycles were recommended to avoid the so called memory effect).


    P.S.: Note that a charger that indicates a full batttery doesn't use the battery voltage or capacity as an indicator for the state of charge. A common method for a charger to determine end of charge is to check for the change in voltage, a change in temperature or absolute temperature when charging. These methods are obviously not available when measuring the steady state of a battery.
  4. Alec_t


    Jul 7, 2015
    If the red LED is on before you switch on the vac then it will be drawing current continuously in standby and flatten the battery.
    Harald Kapp likes this.
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