"Resistors to save battery life" question

[degarb]

Resistor question:


If I can't visualize in my head, with analogy to water, I have a difficult time understanding. So, I am a little fuzzy on one fundamental, maybe.

I always assume, that when you cut current with a resistor then you cut battery usage. This is based on my experience with a task-light based on the Lux I, running it straight for 12 hours with a 3 AA Garrity with resistors from ¼ power to %100 power (depending on task), and she was still pumping usefully on full power (12 foot unsanded trim boards, at 11 or 12 hour out).

But people keep saying a resistor burns power. I ask what they mean and to quantify it, but no one can respond. So, I am assuming this loss in power burned by resistor would be equal to current drop times voltage drop usually needed to run at lower current without resistor, if there were no thing called a forward voltage.

Or am I wrong, and a 2500 MAh 3 AA pack cut from 500 milliamp to 250 milliamp would not get double runtime (from 5 hour to 10 hour)? Would 4 AA cut to 250 milliamps not get 10 hours too?

I wonder because my tests yesterday, I saw a 3 AA pack drop from a peak of 500 milliamp to 300 milliamp in only 3 hours, driven at 300 to 250 milliamps, driving a Rebel 80. Is this quick current drop normal for these rechargeables? So as a result of this test, I am thinking of driving with 4 AA's, so I can more consistently manually regulate it to 250 or 300 milliamps over the 8 to 10 hours of use, by edging up the variable resistor every so often.

 

[Mr Happy]

Resistor question:

If I can't visualize in my head, with analogy to water, I have a difficult time understanding. So, I am a little fuzzy on one fundamental, maybe.

I always assume, that when you cut current with a resistor then you cut battery usage. This is based on my experience with a task-light based on the Lux I, running it straight for 12 hours with a 3 AA Garrity with resistors from ¼ power to %100 power (depending on task), and she was still pumping usefully on full power (12 foot unsanded trim boards, at 11 or 12 hour out).

Well, we can successfully use the water analogy here. Putting a resistor in the circuit is like putting a kink in a hosepipe. Suppose your hose has a nice powerful jet that reaches far. It will be using a lot of water and your water meter will be spinning round merrily and racking up your water bill. Now suppose you put a kink in your hose. Suddenly the flow of water will be reduced and your water meter will turn more slowly -- you are conserving water. However, at the same time the nice powerful jet has been reduced to a dribble. In a sense the kink in the hose has "burned" the power of the water jet and wasted the water pressure that was available at the tap. (This part of the analogy is not very good, but it is the best I can do without getting technical.)

But people keep saying a resistor burns power. I ask what they mean and to quantify it, but no one can respond. So, I am assuming this loss in power burned by resistor would be equal to current drop times voltage drop usually needed to run at lower current without resistor, if there were no thing called a forward voltage.

Really, the answer is that the resistor wastes some power. Suppose the resistor halves the normal battery current, so the battery power output is halved. What happens is the LED emitter gets less than half the power -- maybe only a quarter. The other quarter is burned by the resistor. (Incidentally, the formula for the power burned by a resistor is resistance times the current squared. So if you double the resistor current while keeping the resistance the same you quadruple the power burned.)

Or am I wrong, and a 2500 MAh 3 AA pack cut from 500 milliamp to 250 milliamp would not get double runtime (from 5 hour to 10 hour)? Would 4 AA cut to 250 milliamps not get 10 hours too?

Yes, double the run time.

I wonder because my tests yesterday, I saw a 3 AA pack drop from a peak of 500 milliamp to 300 milliamp in only 3 hours, driven at 300 to 250 milliamps, driving a Rebel 80. Is this quick current drop normal for these rechargeables? So as a result of this test, I am thinking of driving with 4 AA's, so I can more consistently manually regulate it to 250 or 300 milliamps over the 8 to 10 hours of use, by edging up the variable resistor every so often.

Yes, this is fine. Using a higher supply voltage (more batteries) gives you more scope to regulate the current. However, going from 3 AA to 4 AA will not give you more runtime is you keep drawing the same 250 mA in both cases. In this case the higher resistance with the 4 AA is just burning up the extra battery power.