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Preventing current backflow

  1. Oct 5, 2012 #1
    Hi All,

    I am new to this forum and not all that skilled in the electrical department. I need help with choosing the correct diode or method to prevent the back flow of current.

    I have built a flashlight with an internal charging circuit that is of course pre-wired permanently. so once you see the light dimming all you need to do is plug in the charger cord and wait till the light turns green.

    My question here is, when the charger is not in use do i need to have something that will prevent the power from the battery going to the charging circuit? Also i noticed that the circuit tends to get really warm when i am charging, would a diode solve this heal issue as well?

    Thanks in advance.
  2. jcsd
  3. Oct 5, 2012 #2
    Hrmm, is this one you built? or is the circuit made by a company? To the best of my knowledge some devices will have an internal switch so that when the battery reaches its capacity the circuit will activate the switch and stop the current from flowing into the battery. If the light is one color while charging and then turns green when charged, this should be a signal that the battery is at capacity. Some devices will stop current flow at that point and others will continue to pull current into the battery when its full as a resistor. Heat would be normal as the battery is still a resistor, but if the input current and resistance value of the battery are calculated correctly, then there should be no catastrophic failure or super high heat.

    In other cases of power generation: lets say you are using an electric motor powered by a bicycle to charge a 12V battery. The bike will have to speed up to meet or exceed the current of that 12V battery in order to charge it. If the motor was wired directly to the battery and the speed was below 12V then the battery would try to actuate the motor. In this case a high value diode would be placed in-between the leads of the battery and the motor only allowing voltage to flow into the battery and not out.

    If your power input source is equal or greater than your battery, then voltage should never flow in reverse, and the power input should not exceed the resistance of the battery or it could overheat and fail. Hope this helps.
  4. Oct 6, 2012 #3
    Hi MachX,

    this is one that i purchased pre-made. the issue i am having is when i charge the battery with out a diode the circuit board does not get hot however i noticed the voltage is not as high (i figured after reading about it that it does decrease a bit).

    is there any way i can run a diode on either the positive or negative charge terminal with out hindering the voltage drop?

    its ideal to have it stay at 4.1-4.25v ... at all times as i am charging a single cell.
  5. Oct 6, 2012 #4


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    Staff: Mentor

    Hi deltaspace, http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]
    Factory built, okay. But you did say "I have built a flashlight ...."? :confused:

    It may (or may not) be possible to add a diode to prevent the battery discharging via the charging circuit, but until you establish that it is needed, there is no point in trying to add the diode. If you leave the flashlight unused for 4 days, does it still work when switched on?
    4 volts? This is a Lithium cell?
    Last edited by a moderator: May 6, 2017
  6. Oct 6, 2012 #5


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    Science Advisor

    It sounds like the origin of the "problem" (circuit board getting hot) may have nothing to do with the back flow of current. Deltaspace is this something that you just "guessed" was the problem or do you have some evidence of this.

    A more likely scenario is simply that, with the inclusion of the diode, the charger is fooled into thinking the battery is fully charged much sooner than it really is. Meaning that the charger supplies less current and therefore doesn't get as hot (plus the battery never gets anywhere near full charge and also stays cool).
  7. Oct 6, 2012 #6
    yes, its a lithium cell.
    Last edited by a moderator: May 6, 2017
  8. Oct 6, 2012 #7
    hi UART, i am just guessing here... reason is because once the it finishes charging the board cools down..

    keep in mind when i say it gets hot, its still ok to touch, its not burning hot.
  9. Oct 6, 2012 #8
    yes, 4 days is not a problem, actually i fully charged it last night... it was at 4.19V, this morning it was at 4.17... the loss is very little
    Last edited by a moderator: May 6, 2017
  10. Oct 6, 2012 #9


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    Staff: Mentor

    Secondary cell voltages take time to settle after charging, even when no current is draining from them.
  11. Oct 7, 2012 #10
    on a side note....

    does anyone know where i can find/purchase a pre-made circuit board just to control a flashing LED...

    every half second would be ideal..
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