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What's more important amps,r watts or volts?

  1. Aug 5, 2012 #1
    Last night i did an experiment with a halogen bulb, rated at 12v 100w (8.33amps) dc.
    The exact model is this:
    I used to turn that bulb on, 3 serial batteries 1.5v "aaa" @ 0.4ah = 4.5 @ 0.4ah (1.8watts), and it turn on a little bit dimmed, then I used the lithium battery from a cellphone rated at: 3.7v @ 1.050ah = (3.89 watts), and it lit slightly stronger.
    Then I put the "aaa" batteries in parallel to get: 1.5v @ 1.2ah = 1.8w, but I didnt turn on not even weak (did I do something wrong? or something was loosen in the wires)?

    I have 2 questions:

    1. How could tiny batteries provide (at least) energy to power such a high amp demanding bulb?

    2. In this case, what factor would make the bulb to be much brillant? a higher voltage? or a higher amperage? (or even whatever that puts a highter number or watts)?

    This is still a little confusing. :(

    Gracias in advance.
  2. jcsd
  3. Aug 5, 2012 #2


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    Staff: Mentor

    When you connect small batteries to something that will need a lot of current, generally you find that the voltage from the battery drops. So even if you thought you had 4.5v applied to the lamp, were you to measure it while it was connected to the lamp you'd find that it was quite a bit lower than 4.5v, maybe just 3v. Different batteries drop by different amounts, depending on their chemistry, how new they are, and what they are designed to power. A good cellphone battery won't drop as much voltage as cheap AA cells.

    If the battery voltage didn't drop like this, that is, if all batteries were perfect, then all batteries with the same voltage would cause exactly the same brilliance.
  4. Aug 5, 2012 #3


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    Science Advisor

    All batteries have a voltage (which you can measure when there is no load on the battery) and a series internal resistance.

    When a current flows, there is some voltage drop across the internal resistance and this subtracts from the internal voltage and makes the output voltage lower.

    So, a battery will give a lower output voltage on load than it does open circuit.

    Flashlight batteries have a much higher internal resistance than Lithium or NiCd or NiMH batteries, so the flashlight batteries output voltage drops more for the same current than the other batteries.

    It is the voltage on the lamp that determines how bright it is.

    Incidentally a battery with a rating of 0.4 amp hours would last 1 hour if there was 0.4 amps flowing.
    The product of the time and the current has to be 0.4 amp-hours.
    If you had 0.04 amps flowing then the battery would last (0.4 amps hours / 0.04 amps) or 10 hours.

    This is because current * time = amp hours, so time = amp hours / current.

    This is only a guide and you can't rely on it exactly.

    So, a small battery might be able to light a lamp well if it has a low internal resistance, but it may not last very long if it has a small amp-hour rating.
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