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Battery charger/battery type NMH

Discussion in 'Power Electronics' started by pepi, Aug 29, 2020.

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  1. pepi

    pepi

    30
    2
    Sep 20, 2017
    Hello,

    Need some advice regarding a charger. The suggested charge rate 12dc 200mA ,
    The question any AC/DC power supply with an output of 12dc 200mA is usable ?

    Beers on me
    Pep
     
  2. WHONOES

    WHONOES

    937
    238
    May 20, 2017
    Simple answer is no. It is not that straight forward. A 12V battery can top out at up to 15 to 16V depending on the quality of the cells. You will need a power supply that can output a constant current of in your case of 200mA. That does not mean a 200mA PSU.
    I suggest that you have a good long read of the following link.

    https://batteryuniversity.com/learn/article/charging_nickel_metal_hydride
     
  3. Bluejets

    Bluejets

    4,154
    880
    Oct 5, 2014
    Plenty of simple circuits out there on the web for charging NimH using a basic LM317 regulator as a constant current device.
    You will require a power supply with an output voltage greater than the battery you are trying to charge though.
    Then again there are ways around this by using a small $2.00 boost converter with your power unit.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,223
    2,204
    Nov 17, 2011
    The simple constant current battery chargers imho all suffer from an inadequate means of terminating the charge cycle: time. If you leave the batteries in the charger for too long, they will be overcharged and their lifetime will be decreased.
    A goof charger detects end of charge e.g. by the dv/dt method and/or by registering the temperature rise of the battery near end of charge. You can find many dedicated NiMh charger ICs (often doubling for obsolete NiCd, too).
    Or you build one of the simple circuits and life with the drawback of having to time the charge.

    A rather standard question with a rather standard answer. The output rating of any typical power supply means:
    - voltage is output as stated, in this case 12 V.
    - current output is defined by the load, with a maximum current draw allowed as stated, in this case 200 mA.
    By no means does this imply that the output current will be 200 mA for any load nor does it imply that the output current is safely limited to 200 mA. You may be able to draw more than 200 mA but this will overload the power supply and in the long term lead to premature failure of the power supply due to overheating.
    My favorite comparison is a mains outlet in your home. You'll get 115 V regardless of the load. A small desk lamp will draw only a few Milliamperes whereas an appliance may draw a few Ampere - all from the same outlet. Current is limited only by the fuse in your distribution panel.
     
  5. pepi

    pepi

    30
    2
    Sep 20, 2017
    I've realized I forgot all the specs in order to define the question. Let me clean it up and clarify, and say sorry for the confusion

    Assuming I am after a wall charger, found on every portable phone. Need to be looking for one with the output of 12vdc; 200mA

    5 watt transmitter
    Transmitter label: 8.4Volts battery power pack
    Charger 12vdc; 200mA

    Battery pack label: 8.4Volts:1300 mA

    Typing this, I answered my own question LMAO, one of the great things about a
    forum.

    Nevermind
    Pep

     
  6. WHONOES

    WHONOES

    937
    238
    May 20, 2017
    You can use a simple current limited charger without wrecking your batteries.
    You charge the batteries at the 14 hour rate. For this, you set the charge current to 1/10th of the batteries capacity. This is so because the process is only 60% efficient.
    Using this regime you can leave them on charge almost indefinitely.
    I've been doing it for many years with my R/C equipment with no premature failures.
     
    pepi likes this.
  7. pepi

    pepi

    30
    2
    Sep 20, 2017
    That makes perfect sense thanks
     
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