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Relevance of amperage

  1. Jan 2, 2010 #1
    I am trying to understand a personal dilemma. I would like to hook up an automobile headlight bulb to a custom made battery pack. What I don't understand is how amps are related to the circuit. Lets say I took (2) 6volt lantern batteries and wired them in a parallel circuit. Now that would be 12volts correct? So now I know that I have the correct voltage to power the headlamp.

    Now lets say the headlight bulb is rated a 55watts. So 55/12=4.58. So that means the bulbs require 4.58amps, correct? I know the amperage of the fuse that the headlights came from was a 15amp fuse. I don't understand how to test/ know whether or not the 2 6volts batteries wired in parallel have the proper amount of amperage. Is it as simply as hooking up a multi meter and test the amps of the circuit "shorted"(without the headlight wired up?) or is there more to it?

    Also can any explain to me exactly what can burn the bulb out, in reference to electrical variables. For instance if I have too many amps running to it? Or if I have 24volts instead of 12?

    I am very new to electrical engineering and I sincerely appreciate anyone's insight.
  2. jcsd
  3. Jan 2, 2010 #2
    First of all (2) 6 volt bulbs connected in parallel require 6 volt and not 12 volt, since they are in parallel so they are connected to the same points, this means you need a source of 6 volt and can give power more than 110w, since each bulb needs 55w.
    Now you can calculate the current that traverses each bulb which is given by the following formula: P=V*I
    giving I=P/V...then the current in each bulb is=55/6=9.1 A
  4. Jan 2, 2010 #3


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    I guess that was a typo. The batteries give 12 volts if they are in series. They give 6 volts if they are in parallel.

    Batteries in series and parallel.PNG

    If you put a 12 V 55 watt lamp across 12 V it will draw 4.58 amps and dissipate 55 watts.
    On 6 volts it would draw less current but probably not exactly half of 4.58 amps.

    Testing batteries has to be done safely. If you just shorted out a car battery there could be an explosion, melted wires, a fire or other mayhem.
    So, you put a known heavy load across the battery and measure the voltage drop. Then you can calculate the short circuit current.

    More useful is the ampere-hour figure for the battery. You need to know how long a battery would last if it was supplying 10 amps, for example.

    A lamp will burn out if it gets too much voltage. You can't make a lamp draw too much current if it doesn't get more than its rated voltage.
    Last edited: Jan 2, 2010
  5. Jan 2, 2010 #4
    Hello DJM111188,

    Perhaps it would be useful to check what you mean by "lantern batteries". Are they non-rechargeable carbon-zinc or alkaline types, weighing a bit over 1lb (600g) each? If so, what you have planned probably won't work well, or for very long. There are a few issues:
    1. As explained in the previous post, you need to consider the Ampere-hour capacity. Disposable batteries of this class likely have only a few Ah capacity, rated at their intended discharge current of 1A or less. The effective capacity would be less at over 4A drain.
    2. Even when new, at this high current the battery voltage will be noticeably reduced by an effect called internal resistance: the battery behaves as if there is a small resistance in series with it. As the battery discharges, this internal resistance will rise and the voltage will fall even more.
    3. Particularly with the cheaper zinc-carbon types, some of the chemical reactions within the battery can't keep up with very high discharge rates. This causes a further drop in voltage after a short time in use, although this recovers if the battery is "rested"
    4. Last but not least, it is possible that the heavy lamp load may result in the batteries getting hot and perhaps leaking.

    The bottom line is that you need an appropriate battery for the job in hand.
    Last edited: Jan 2, 2010
  6. Jan 2, 2010 #5
    Perhaps the following site can help:

    For really high output applications, ni-cad battery packs, such as the ones used for RC cars, can output a tremendous amount of current for a relatively short time (i.e 2.5 amps for one hour for C sized). I've seen people operate portable movie lamps from these. There's probably much better technologies available now. You might look at:

    - Mike
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