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Help with a power problem

Discussion in 'Power Electronics' started by edg, Aug 23, 2018.

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  1. edg


    Aug 23, 2018
    s.jpg Hi

    Hope im in the right area..
    I working on a power supply for a 9v rechargeable battery.
    Someone was helping me but im still having trouble with the
    Ceramic Cement Power Resistor getting very hot..
    Please see the attached diagram with parts list and connection..
    Can someone tell me if this is good way for the charger to work..
    Thank you for any help..Tom
  2. Audioguru


    Sep 24, 2016
    Did you measure the charging current or measure the battery and power supply voltages then calculate the current?

    If the battery is fairly discharged its voltage might be 7V.
    The power supply voltage probably is not regulated and might be 10V.
    Then the 3V is across the 2 ohms resistor creating a power in the resistor of (3 squared)/2 ohms= 4.5W. Sure a 5W resistor gets hot with 4.5W.

    But what is the current? 3V/2 ohms= 1.5A (1500mA) which is way higher than the recommended charging current of only 58mA. When the battery is fully charged then it will get extremely hot and fail.

    A resistor is not a battery charger circuit. A battery charger circuit limits the current then detect the small voltage drop that occurs when the battery is fully charged then it turns off to prevent over-charging that causes heat and shortens the life of the battery.
    Bluejets likes this.
  3. Bluejets


    Oct 5, 2014
    Expensive little setup for a " not the way to go" arrangement.
    What is the "pcb" for??
    Perhaps tell everyone a bit more about what you are hoping to achieve.
    Plenty of lithium packs around with charger modules that would suit for less money.
  4. edg


    Aug 23, 2018
    Thank you..
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