Battery Overcharge Protection Circuit

Discussion in 'General Electronics Chat' started by Andy Gee, Jul 12, 2014.

  1. Andy Gee

    Thread Starter New Member

    Jan 17, 2013
    Hello, I am trying to build a circuit (see attached diagram) which charges 3x1.2V NiMH 2000 mAh batteries to 3.6V and then stops charging after they are full. I have never worked with transistors before so I would be most grateful for any advice. This is how I want the circuit to work: my voltage source, the LT1073 5V regulator, limits its output current 400mA. I plan to use this regulated 5V 400mA output to charge 3x1.2V batteries. A 1N5817 diode is placed after the LT1073 to prevent the battery discharging back in to the regulator, which would destroy it. The 1N5817 has a maximum reverse voltage of 20V, and a maximum forward current of 1A. It has a forward voltage drop of roughly 0.35V, which I read from the data sheet. Therefore, 4.65V reaches the batteries, which is enough to charge them. Once the batteries reach 3.6V, the Zener diode allows current to flow through it and so the transistor base receives current, and the 5V flows to ground instead of charging the battery.

    I’m not sure about the power ratings that I need for the Zener diode and resistor.
    If the Zener diode sees 4.65V and 0.4A across it, P=IV=1.86W so will I need a 2W Zener diode instead?

    Likewise, for the resistor, I assume the 4.65V drops a further 0.65V across the Zener diode, so 4V / 100 Ohms = 40mA. The maximum base current of the BC635 is 100mA. Calculating power dissipation across the resistor, P=V²/R, P=0.16W.
    Are my figures right, will the circuit work, and if not, what can I do to fix it?

    Kind regards,
    JohannTattenbach likes this.
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    The base current will be 4.65v-(3.6v+0.7v)=0.35v/100R=3.5mA, which will be the zener current.
  3. Alec_t


    Sep 17, 2013
    At 1.2V a NiMH cell is not fully charged. is worth a visit for charging profile info.
  4. Andy Gee

    Thread Starter New Member

    Jan 17, 2013
    Thank you, I will change the Zener diode to a 3.9V.

    Ok thanks.
  5. ronv

    AAC Fanatic!

    Nov 12, 2008