i connect a load with my Battery(any battery Li-on, Lid-acid) battery drop the voltage

Discussion in 'Power Electronics' started by sky_limit, Aug 26, 2016.

  1. sky_limit

    Thread Starter New Member

    Aug 7, 2016
    I notice that when i connect a load with my Battery(any battery Li-on, Lid-acid) battery drop the voltage(mean if i connect a volt meter across the battery).
    I notice that when i connect a Led in series with a resistor. the voltage drop in this case is from 4.1volt to 3.8volt. it also happen when i connect a transistor with a led.

    Do i design the circuit incorrectly ?

    Mods Note:
    Please don't hijack other member's thread, now you have your own,
    This thread was split from -- Is this a proper analogy to understanding how current, voltage, and resistance work?
  2. DGElder


    Apr 3, 2016
    And this site is called All About Circuits, not Hogswart. We don't do magic and we don't read minds. Show your circuit if you are going to ask questions about it!
  3. Tonyr1084

    Well-Known Member

    Sep 24, 2015
    4.1 volts is your battery without a load on it. When it drops to 3.8 volts it just reflects the fact that you're pulling some power out of it.

    A battery under load - lets assume 5 volts (odd, I know) is connected to a load with a resistance of 200 ohms. To know how much current is flowing you divide the voltage by the resistance (5 ÷ 200 = 0.025 amps [or 25 milliamps]) This also means that 0.025 (amps) times 200 (ohms) equals 5 volts.

    Now, this "5 volt battery" - suppose it's a lead acid battery of 6 volts. Sitting at 5 volts under load suggests the batterie's state of charge is depleted. Without knowing the forward voltage of the LED and the value of the resistor we can't tell for sure how discharged your battery is. However, it has no meaning to how much current is being drawn. I have a 12 volt battery in my car that is able to deliver over 700 amps during start-up. I have several 12 volt batteries in my sock drawer. They're used for remote controls for things like the ceiling fan and other stuff. Those batteries are at best capable of delivering maybe a few hundred milliamps. So even though they're 12 volt batteries I'm sure they could not start my car. Nor could they even turn on the headlights, or the radio. There's not much they can do other than transmit a weak signal to a nearby receiver. So if I were to hook one of these batteries from my sock drawer to a car headlight their 12 volt reading might drop down to 2 volts (just guessing at this). So just how much your battery dropped when you tested it in this fashion means nothing.

    Test the voltage across the resistor. Tell us the resistor value and the voltage across it. (called a "voltage drop") Knowing the voltage drop we can then calculate the current. And then can tell you the forward voltage across the LED. You might even tell us the specs on the battery (i.e. voltage and amp hour (aH)). Small batteries can't do as much as big batteries.