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2.4V Ni-Cd Charger

Discussion in 'General Electronics Discussion' started by abuhafss, May 17, 2017.

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  1. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Hi

    The PCB of my shaver with SMD components got damaged.
    I decided to build a simple charger circuit for the battery pack (2 x 1.2V 600mAh).
    The original adapter is 220v - 3.5V (1A).

    I built following circuit but it did not worked as it simulated.

    Screenshot 2017-05-17 00.30.09.png
    The green LED would light as soon as power is connected and the charging current is only 4mA.
    BTW, the battery pack is almost drained out.

    Where am I wrong? Or if someone could come up with an alternate solution, it shall be appreciated.
     
    Last edited: May 17, 2017
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    A resistor in series with D5 and another in series with D6 might help.

    When charging, NiCd's come up to voltage very quickly. The voltage is not a reliable measure of charge level. It's certainly not while the battery is being charged.
     
  3. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    I have labelled the schematic. There are total five diodes (excluding the LEDs). Perhaps you are referring to D4 and D5.

    So do you mean it is not a reliable circuit?
     
  4. 73's de Edd

    73's de Edd

    2,872
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    Aug 21, 2015
    Sir abuhafss . . . . .

    Are you presently using an adjustable power supply to get the initial 3.5 VDC input voltage being shown ?

    If I were using a wall wart to get that voltage, I would want about a minimum 5 VDC or so to get some minimal voltage headroom.

    That is . . . . considering the unit to be charging at a healthy / long- life preserving rate of 60 ma.

    How about trying your unit, as it is, and initially with that 3.5 VDC input and pull the battery duo and just use a 47ohm load resistor in their place, to see what charge current is then being present thru it.

    Then if it's being well, above your present 4 ma, up the voltage input on up to 5 or 6 VDC to determine the charge rate then.

    If the charge current into the load resistor is then on up to being a healthy 60 ma or so . . . . . you may have problems with the two rechargeable cells.

    Come back for some ni-cad / ni-mh battery tips / evaluation procedures.


    73’s de Edd
     
  5. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Edd

    As mentioned in the first post, I am using the original wall wart that came with the shaver. Its output lead is compatible to the male-input on the shaver.

    I don't have a 47Ω with me instead I used 33Ω. About 34mA is passing thru it.
    So that means I have problem with the cells?
    How should I confirm it?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
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    Jan 21, 2010
    I am now :)

    I wouldn't think so.

    NiCd chargers tend to either rely on the user to terminate the charge (typically with a 14 hour charge rate of C/10 or use rather more sophisticated circuitry to detect cell heating or a voltage dip.
     
  7. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Hmm, that means the LEDs won't work as desired and will give false indication.

    How about this circuit which detects the cells temperature using a thermistor?
    http://www.stefanv.com/electronics/usb_charger.html
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
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    Nov 17, 2011
    This is an example of a charger IC, specifically for charging 2 NiMh or NiCd cells.
    This is another one.

    These ICs require only a few external components and have a built-in charging algorithm which detects the change in voltage (not the voltage per se) as a ctriterion for end of charge.
     
  9. 73's de Edd

    73's de Edd

    2,872
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    Aug 21, 2015
    Sir abuhafss . . . . .


    Hey that’s SOMEWHAT more than that prior paltry 4 ma isn’t it?
    So your charge circuit does seem to have some merit.

    I don’t know if the battery pack is pulled from the unit, but next do this.
    You need to still have the pack disconnected from anything, or just carry the whole conglomeration out to your automobile.

    With the 2 series cells outer positive connection not connected to any charger circuitry.

    You will need 2 test leads with clip on connectors at each end.
    (Or hand fashion same . . . . with 2 wire leads and clips.)

    Pop up the hood and get access to the 12V battery terminal posts . . . and maybe have a smal 1-2 ft sq/rect of plywood as a workbench / support.
    You need the – battery connector of the series pair of the cells to connect to the – of the car battery.
    You then need to connect the + of the car battery post to one clip connector lead of another test lead.
    That will thereby leave its other clip end free

    You then need to analyze the shared common connection buss between the two 1.2 V cells in order to determine its best exposed and accessible temporary contact connection.

    What you then want to do is a quick drag of that free clip lead end, a.c.r.o.s.s that junction, just as quickly as if you were striking a wooden kitchen match.

    Then you pull the first clip lead that you used and transfer its clip to the car battery + and the leads other clip to the + terminal of the series battery pair.
    You then connect the other leads clip to the car battery – post and you will do the identical “striking” of that free clip across the same connection junction.

    ( A h h h h h h h . . .prior experience, with that first cell . . .simplifies things . . . .doesn’t it ?)

    With those two actions to the two cells, the brute electrical strength . . . of milliseconds duration bursts of having applied 12,000 millivolts at a peak 450 amps across each cell should have vaporized any time developed intercell metallic dendrite leakage paths.

    Close up auto-shop and move back inside to try out charging those cells then.

    Use your DVM’s DC current function in an ~ 100-300 DC ma range, with the leads inserted in series with either a – or + lead of that two cell pair. That then confirms your charge rate, and if seeming adequate, you then can pull that inline metering hookup.

    Charge the set for ~12 hrs from your charger circuitry and see if you can THEN . . . . . . . . . “Buzz the Fuzz” again.

    On my Norelco or Remington’s, I just rely upon my unplugging them after a typical 8AM-8PM charge period and then recharging them 5 days or so later . . . . when I hear a noticeable torque / speed decline.


    Thassssssit . . . . .


    73’s de Edd

    .
     
    Last edited: May 17, 2017
  10. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Hi Edd

    I followed your instructions and gave momentary 12V pulse from a 65A car battery to each cell.
    Then placed for charging with my circuit but no change. At this moment after running for 2 hours, the charging current is only 2mA. I had another pair of cells, I pulsed those two also and quite interestingly they also charging 1mA. Does that mean both the pairs are not usable?
     
  11. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Today, I bought a new pair of cells.
    They started charging at 7.5mA and within 15 min reached less than 4mA.
     
    Last edited: May 20, 2017
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    A NiCd cell with a nominal voltage of 1.2 V rapidly rises to 1.4 V and more during charging. Your pack of 2 cells then has >= 2.8 V.
    Your power supply has 3.5 V. Subtract 0.6 V for diode D1, and take into consideration the battery voltage then you get (3.5 V - 0.6 V - 2.8 V) = 0.1V across the 33 Ω resistor. 0.1 V / 33 Ω = 3 mA whic is in perfect agreement with your measurements.

    To create a reliable "current source" for charging batteries from a voltage source and a resistor you need to make the input voltage much higher. This will lead to a high voltage drop across the resistor and consequently a much smaller relative variation in current due to change in battery voltage.
    Or use a dedicated charger IC which will compensate these effects.

    Read more about charging NiCd batteries on the web pages of the battery university.
     
    abuhafss likes this.
  13. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    I got it. But one thing has been deceiving me. As I mentioned earlier, the shaver in which the cells are used came with a wall wart 3.5V-1A. So how that has been charging the cells?
     
  14. 73's de Edd

    73's de Edd

    2,872
    1,216
    Aug 21, 2015
    Sir abuhafs . . . . .


    Let’s find out . . . for your particular situation.

    With a 3.5VDC and 1 Amp capacity, that wall wart should not sag and pull down at the much lower current pull that we are wanting.

    I thought that I had included my marking up of your schema in the earlier posting . .. not so, still on Flash . . . as my host site was continually giving a try back later response.
    It’s up now and is placed below.

    The way you had it, each of the paths of D2—D3—D4 or D2—D3—D5 through their respective base-emitter junctions were creating more of a current drain pull of the power supply to ground, than just the intended turning on of the two LED circuits.

    Soooooooo . . . . Lets just see what that unit can do, by initially opening that diode voltage referencing and steering path by opening it up at any of my three YELLOW X options.

    Considering,that is all out of circuit now.

    At some time, if you don’t know already, test the diodes for their Vf on a DVM in its diode test mode. (Expecting about 700-800 mv.)
    (If we run short on available charging current, in later testing, our Ace in the hole, may be the
    use of a Schottky type for D1 to get about 10 ma more current available.)

    Now a full 3.5VDC input from the wall wart, passing through that series 33 ohm resistor should give ~106 ma available for charging ye olde battery pair. Downgrade that to 85 ma since it will then have to pass thru series diode D1 to reach the battery.

    NOW . . . you need to have the battery disconnected by one lead and get DVM in hand and place it in its DC Current test mode, and set up to read your closest DC current scale ABOVE 100 ma.

    One current monitoring negative lead goes to ground and the other one gets touched to D1 cathode just long enough for the ammeter reading to ramp up.
    • Power up.
    Are we at our expected / suspected benchmark of 85 ma or so ? . . . . . as, that level will place an easy charging condition on the cells of the battery. Preferrably charging them in the order of 60 ma, since they are 600 ma units.

    OR . . . has / is that wall wart sagged down from its professed spec ?

    If you are up in that desired urrent range hook in the battery and use the series current test with the meter to check its charge rate. Write down the rate and then use the meter to read the voltage across the battery . .log it.
    Start charge and check back hourly for evaluation readings.

    If all is well, then the full indicating circuitry, thru the lifted diodes can be engaged,for its initial evaluation during further tests.


    Schematic Deux . . . .
    Ni-cad__ Ni-mh Tapered Charge Circuit.png



    Thassssit . . . . .


    73’s de Edd
     
    Last edited: May 21, 2017
    davenn likes this.
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