Double check my runtime calc?

[burpee]

I just want any comments regarding the relationship between discharge rate and battery run time.

For a give 4x18650 Li Ion battery, if I have three discharge rates (loads) when using the light will the rate of run remain somewhat proportionate?

For instance: the discharge rates are .35A .70A and 1.4A - so would this mean that for every 1 hour at 1.4A would mean I could have used the battery twice or four times as long at lower load settings?

I recently field tested my light/battery on an all night trip and found that mixing discharge rates exceeded the "calculated" run time of "on high" all the time. Just wondering why.

Is there a discharge "sweet spot" for a given battery configuration?

If you can find the thread on discharge rate versus run time theory I'd appreciate it.

 

[Mr Happy]

If you actually know the currents, then half the discharge current will give you twice the run time (or longer -- more in a bit).

However, brightness levels on a light cannot be directly related to discharge currents. For example, to get 200 lumens on high may take more than twice as much power as to get 100 lumens on medium.

Efficiency comes into all these calculations. For instance, all batteries are more efficient when discharged at lower rates, but the effect varies with the kind of battery. With alkalines the effect is dramatic, but with Li-ion much less so.

Similarly, electronic circuits and LED emitters are usually less efficient when run at high power levels.

So when all is said and done, running an LED light at lower brightness settings might significantly increase the run time beyond a simple proportional calculation.

There is a sweet spot, but it's not always easy to say what it is. In most cases the sweet spot is probably the lowest brightness setting, but only probably. It depends on the electronic details.

 

[45/70]

As Mr H said, there are a number of variables to consider when determining the "best" discharge rate.

There is a sweet spot, but it's not always easy to say what it is. In most cases the sweet spot is probably the lowest brightness setting, but only probably. It depends on the electronic details.

I think the "sweet spot", Mr H referred to is generally regarded as the point where the power consumed by the driver circuitry equals the power consumed by the LED, or "bulb", in a regulated light. This isn't necessarily the case, but output/watt is a good place to start.

For example, IIRC LiteFlux determined this point was at the 6% drive level, with their original LF2. This doesn't mean that the light will not run longer at a 5% drive level, but the output per Watt efficiency will become less at any drive level below 6%, as the driver is consuming more power than the LED.

That is to say, a drive level of 3% will run the the light longer than a drive level of 6%, but it won't run twice as long, nor will the light run six times as long at a 1% drive level, because the driver is consuming a larger and larger portion of the power, as the drive level is decreased below 6%.

The 6% figure will vary between different lights, having different driver circuitry and LED's, or bulbs. This was just one example, but hopefully, you get the idea.

Dave

 

[HKJ]

I think the "sweet spot", Mr H referred to is generally regarded as the point where the power consumed by the driver circuitry equals the power consumed by the LED, or "bulb", in a regulated light.

Another way to define the sweet spot is where the light has the best efficiency (power consumed to light out the front), this will be at a much higher brightness.
This is not the same brightness that will give the most light from a battery (lumen*runtime), because battery chemistry will play a role in that.

 

[KiwiMark]

If we compare 100 Lumen to 200 Lumen and suppose that producing 200 Lumen takes 110% more power because of inefficiencies & loss to resistance/heat. Consider that we are using over twice the power to create light that appears to our eyes to be around 50% brighter. Low & medium mode offer much better apparent brightness for the power consumed than higher modes - the lesson here is that we should only go as bright as we need to if we want a decent run time out of our lights.

 

[45/70]

Another way to define the sweet spot is where the light has the best efficiency (power consumed to light out the front), this will be at a much higher brightness.

That's true. Most LED's that I'm aware of are most efficient at lower outputs, although it may be at a higher drive level than the 6% in my example of the LiteFlux LF2. As I alluded to, there are different approaches and answers to this question.

If we compare 100 Lumen to 200 Lumen and suppose that producing 200 Lumen takes 110% more power because of inefficiencies & loss to resistance/heat. Consider that we are using over twice the power to create light that appears to our eyes to be around 50% brighter.

Careful there Kiwi. You are likely to be labeled as a "party pooper" to the crowd that just got rid of their 400 lumen BlastFireix MCEP7-12318650RCR V2.103-A "Extremeinator", so they could get the 500 lumen BlastFireix MCEP7-12318650RCR V2.103-B "Extremeinator" which is "25% brighter", but actually is only 12 1/2 percent brighter to the human eye, and thus practically unnoticeable.

Dave

 

[uk_caver]

I recently field tested my light/battery on an all night trip and found that mixing discharge rates exceeded the "calculated" run time of "on high" all the time. Just wondering why.

How is run time determined with your light?

That is, does the light switch to a lower level, does it noticeably dim, does the battery protection kick in, does something else happen?

 

[burpee]

Similarly, electronic circuits and LED emitters are usually less efficient when run at high power levels.

So when all is said and done, running an LED light at lower brightness settings might significantly increase the run time beyond a simple proportional calculation.

Thanks, I completely forgot about the flashlight's regulator circuitry.

I ran this battery pack until the light quit. The battery pack did indeed shut the light off - not the other way around. I read the terminal voltage as "0V" and then after 20 seconds of charging the terminal voltage read 6.7V.

This bicycle light is a great tool -I only wish I could modify the button switch/circuity to select various modes without cycling through strobe/SOS and "off" before returning to high/med/low.

Thanks for the help.