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Old 15-04-2009, 08:43   #16
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"For the simple 4.5v lights that use three AAA cells in series, larger capacity cells with the same voltage may well overdrive the LED. "
So unlikely that I'd guess it borders on simply impossible. While "9 volt" aka transistor radio batteries are designed to self-limit to about 125mA of current (mainly because of the big exposed metal terminals being a fire hazard) even a simple AAA battery has about 1Ah of power available to a dead short. There is no current limiting and you can indeed set a wire on fire with them.
Once you've got that much power available, moving up to a C or D or even F cell won't matter, the LED *is* the current limiting device.

And from long experience, I know that if you match an LEDs voltage rating with the supply voltage, no current limiting device is necessary. Just open up any LED micro-light and take a look, NONE of the manufacturers use anything besides the LED, the matched battery, and a switch. (Except the fancy ones like Photon, who are using a flasher module--not a current limiter--in the models that do sequential flashing.)
Look at all the rave ornament LEDs. Same same, no current matching is needed IF the voltage is matched, and the available current sufficient for an even power flow.
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Old 15-04-2009, 09:43   #17
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I suppose the real problems happen on boats with AGM systems and/or chargers that can do an "equalize" charge, as those currents are going to be in excess of the normal top load of 14.4v found when charging lead-acid batteries. I wonder how expensive it is (in terms of power use) to put a large device right on the batteries' output to ensure that the voltage remains constant or at least never exceeds a certain set value.
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Old 15-04-2009, 10:28   #18
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We recently changed out our bulbs to led's, inside and out. We got them from www.mastlight.com and can highly recommend them. We made a mistake when we ordered, they promptly sent the correct bulbs and did not charge for shipping. A very good company.
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Old 15-04-2009, 10:30   #19
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LEDs driven with a constant voltage source and not in some way current limited can indeed be overdriven (a battery or cell, especially a small one, is not a constant voltage device under load). As an LED heats up, the Vf will fall and current will increase, potentially leading to a cycle of increased heat/lower Vf/increased current until the LED expires. The simple 3 cell lights with no current limiting rely on the internal resistance of the cells to avoid high current and thermal runaway. Because of the internal resistance of these small cells, as current increases voltage will drop which will ultimately limit current. If you will take the same flashlight and attach it to a constant voltage power supply at 4.5 volts with no current limiting, you will almost surely witness thermal runaway and failure of the LEDs. Larger and larger capacity cells will more closely approximate a constant voltage supply.

If you do the same wire dead short experiment mentioned above with AAA, AA, C, and D cell alkaline cells, you will find that as the battery size/capacity goes up, so will current through the cells and the speed at which it heats up. The difference has to do with the differing internal resistances of the battery cells, and every cell has internal resistance. Of course, the experiment isn't really a valid parallel to an LED. As a wire heats up, its resistance also increases which limits current. An LED is just the opposite.
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Old 15-04-2009, 10:30   #20
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And from long experience, I know that if you match an LEDs voltage rating with the supply voltage, no current limiting device is necessary. Just open up any LED micro-light and take a look, NONE of the manufacturers use anything besides the LED, the matched battery, and a switch.
It is possible to run LED's directly from a battery. While simple it results in the LED having lower brightness and/or a shorter life and sometimes lower efficiency (if a resistor is used). The extent of these problems depend on the stability of the voltage from the battery. With cheap flashlights the results can be acceptable, particularly as a short LED life is not a great problem with devices like micro-lights and some dry cells have a very stable voltage with discharge.
With the large voltage fluctuations found with a 12v marine battery however the results will be poor, although possibly still acceptable. If you do this be aware that for a few dollars more you could have achieved much better results with a current limiting circuit.,
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Old 15-04-2009, 15:55   #21
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With the large voltage fluctuations found with a 12v marine battery however the results will be poor, although possibly still acceptable. If you do this be aware that for a few dollars more you could have achieved much better results with a current limiting circuit.,
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Old 15-04-2009, 21:39   #22
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"With the large voltage fluctuations found with a 12v marine battery however"
Yes, but that is not the question that was asked. The question was substituting larger primary cells, i.e. D for AAA, and not how to adapt LEDs to ships' power, which is a much larger issue.
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Old 15-04-2009, 21:44   #23
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"With the large voltage fluctuations found with a 12v marine battery however"
Yes, but that is not the question that was asked. The question was substituting larger primary cells, i.e. D for AAA, and not how to adapt LEDs to ships' power, which is a much larger issue.
Yup. That WAS the original question.
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Old 15-04-2009, 23:11   #24
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The question was substituting larger primary cells, i.e. D for AAA, and not how to adapt LEDs to ships' power, which is a much larger issue.
The answer is: If the design relies on the internal resistance of the battery to control LED current, then substituting a larger battery will cause the current to be much higher.

However, I would be remiss if I didn't point out that this is usually a very bad design practice. I can't imagine a case where it would be a good idea to use the battery resistance in this way -- unless you are trying to save a couple of pennies per LED, and don't care about reliability or stable operation.
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Old 15-04-2009, 23:52   #25
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"With the large voltage fluctuations found with a 12v marine battery however"
Yes, but that is not the question that was asked. The question was substituting larger primary cells, i.e. D for AAA, and not how to adapt LEDs to ships' power, which is a much larger issue.
When discussing 12v batteries I was trying to provide some advice for this part of the original post.
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I've also contemplated transforming down to 5 volts or so to operate the LED's from the boat battery.
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