battery and bulb question

Azreal911

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i'm just wondering if i'm calculating this right. say i got a 2aa flashlight and wish to convert it into a higher power bulb. so i run off to radio shack and pick up a bulb that's 2.2-2.4 volts with a 700 mah rating (would be bright!). so would the burntime for my light with say energizers at 2850mah capacity in their cells last (2850 / 700) = 4hrs? i'm just checking if this sounds right? cause i play with tons on batteries in my radio controlled applications and have no prob with 6-8 cell calculations but not sure if it's the same here.
 
D

**DONOTDELETE**

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This kind of calculation can only give you rough approximation.

At first, when you apply higher voltage to the bulb, it will sink more than 700mA, but as the battery voltage decreases, the current will drop. On the other hand, at high-load applications, batteries lose in their mAh ratings. In addition, you will want to replace the batteries before the cutoff point is reached.

If you got 4 hours in theory, I would expect something like 2 to 3.5 hours in practice. The only way to "calculate" the right value, is to perform an experiment
grin.gif
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PeLu

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Azreal911:
so would the burntime for my light with say energizers at 2850mah capacity in their cells last (2850 / 700) = 4hrs?<HR></BLOCKQUOTE>

Theoretically yes, but do not expect to get the nominal rating in practical life. For 700mA you will be happy to get 2.5 hours. When the cells get weaker, voltage drops and you have (a little) less than your 700mA.
 

Azreal911

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Toronto
Ahhhh thanks for the enlightenment, i found one flaw in my calculations, the ones i was using deals mainly with sub c cells for my rc applications. and the batteries are capable of high amperage drain right down to the last drop. i made a mistake not factoring in that alkalines behave differently and have a slow drain and lower current as it keeps working throughout it's lifetime.
 

PeLu

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Azreal911:
Ahhhh thanks for the enlightenment, i found one flaw in my calculations, the ones i was using deals mainly with sub c cells for my rc applications.<HR></BLOCKQUOTE>

You compared two extremes. SubC cells have the lowest capacity loss at high currents, you may have only 10% at 2C.
Alkalines got much better the last 5 years, but give only about a quarter of their rated capacity at 1C (and they will have a very low voltage then).
 

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