How does battery discharge work?

beezaur

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Apr 15, 2003
Messages
1,234
Just exactly whet goes on when batteries discharge in normal use in a light?

For the sake of defining the problem a bit better, say you have a Surefire 6P with two fresh lithium batteries. Contrast this to something regulated, say an A2.

At first there is a high voltage, low resistance, and therefore high current. When you hit the button, lots of light. Compare that to an A2 with its "soft start." What is it that makes the A2's start "soft?" In other words, what is going on inside the P6 at "startup" that is not going on in the A2 or vice versa.

Now say you have a manufacturing defect or whatever and one battery is at half capacity, say half of nominal stored energy. I know this is bad, and that it can cause overheating, etc., but how does that happen? What is the voltage/resistance/current situation in each battery? What exactly is "thermal runaway" in a battery, and how does it happen? Is there a good way, i.e., design feature, to prevent that situation, or to prevent it from going really bad?

Scott
 

ACMarina

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Sep 10, 2004
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Brookston, IN
I believe the A2 kinda ramps up the current going to the bulb, instead of shocking it with a blast of current like a 6P would do. I don't think it's for the batteries so much as it is for the bulb. Kinda like warming up your car before you drive it, except the soft start on the A2 isn't even noticeable (at least to my eye)
 

Doug Owen

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Jan 30, 2003
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1,992
Voltage is electrical pressure, current is flow of electricity. For any given voltage, the current is determined by resistance (opposition to the flow). There is some small resistance inside each cell as well as the flashlight body, switch and other parts the current must flow through. However, most of the resistance (and therefore control) is in the bulb. For practical purposes all to start.

Now the tungsten filament in the bulb, like most metals, has a 'positive temperature coefficient'. That is it's resistance goes up as it gets hotter, and since it gets very hot this can be a fairly large change.

So, when you slam the switch down of fresh cells with a cold filament the highest current ever to flow goes. This is often called 'inrush' current and can be several times running current. This is why bulbs always seem to blow when you first turn them on. In a very short time temperature (and therefore resistance with current following in step) becomes stable. From here things slowly degrade as the cell voltage very slowly falls, and depending on the numbers the 'internal resistance' of the cells goes up further 'robbing' voltage available to the bulb. The bulb cools off, the resistance goes down a bit, current goes up. Since power is the product of voltage and current (multiply volts times amps to get watts) you can get the same power by higher current at lower voltages. So, to a first order, tungsten filament bulbs are 'self regulating' (at least within a voltage range).

In the end bulbs die when enough tungsten filament material evaporates off and it overheats at that point....but that's another story.

Doug Owen
 
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