How to calculate the draw current?

[RTJ]

Hi folks,

how can you calculate how much mAh a flashlight draws from an 18650 cell? I have read that you can 'stress' an 18650 up to 2C (that's twice the capacity right?)

So for instance, if I have a flashlight with an 18650 battery, how do I know what the minimun mAh has to be of the cell?

Thanks,
RTJ

 

[HKJ]

You cannot really calculate it, there are to many unknowns involved.

In my battery comparison thread I write something about how you can do a rough estimate.

Few flashlights get near the maximum for the batteries, even if you use 2200 mAh batteries. The only light that can really load a battery is lights with boost drivers and my above method does not cover that.

 

[moderator007]

Hi folks,

how can you calculate how much mAh a flashlight draws from an 18650 cell? I have read that you can 'stress' an 18650 up to 2C (that's twice the capacity right?)

So for instance, if I have a flashlight with an 18650 battery, how do I know what the minimun mAh has to be of the cell?

Thanks,
RTJ

If you mean how much current a flashlight will draw then read here. 2c refers to the current for dischage or charge. If the capacity is 3000mah and says max discharge rate is 2c ,then it should handle 6amps of discharge max. So even a 2000mah battery that can be discharged at 2c would be able to handle 4amps. This is enough current to handle a 3amp cree xm-l without problems in a single cell light. It would only handle 4amps for less than half a hour before the battery would be drained or the protection circuit would kick from the low voltage under that load.

 

[Justin Case]

Hi folks,

how can you calculate how much mAh a flashlight draws from an 18650 cell? I have read that you can 'stress' an 18650 up to 2C (that's twice the capacity right?)

So for instance, if I have a flashlight with an 18650 battery, how do I know what the minimun mAh has to be of the cell?

Thanks,
RTJ

Your thread title asks how to calculate draw current, but the quoted text above asks about mAh draw, which are two different things. If you want to estimate the current draw such that you have an idea of what mAh cell capacity you need based on the 2C max current draw criterion, then you can do so by the following for a light that uses a boost or buck driver:

Power draw by the LED = Power delivered by the driver = Power delivered by the batts * efficiency

With a driver, efficiency is always <100%. The question is what is that value. It differs for different drivers and different driver conditions (e.g., Vbatt to Vf ratio). However, you probably can reasonably bracket the value and do a sensitivity analysis. I'd assume an efficiency of 75% to 85% for a typical driver.

Then,

Power draw by the LED = Vf * If
Power delivered by the batts = Vbatt * Ibatt
Power deliverd by the driver = Vbatt * Ibatt * efficiency

You get If from the specs of the driver. You can estimate Vf from the LED's datasheet. Vbatt depends on your battery configuration and battery type. If they are Li-ions, you can assume each cell starts at 4.2V and sags to 3.7V nominal, with say 3.3V at the end of the run time. Assuming you have N batteries wired in series, the Vbatt is then N * either 4.2V, 3.7V, or 3.3V (or whatever you pick). So for a 2-cell light, you can have 8.4V at the start, 7.4V nominal, and 6.6V at the end.

For an XM-L driven at 3A, Vf from the datasheet is 3.35V. Thus,

Power draw from the LED = 3.35V * 3A = 10.05W
Power delivered by the driver at the end of the run time (which is worst case) = 6.6V * Ibatt * efficiency (let's use 75% again to be conservative) = 4.95V*Ibatt

Thus 10.05W = 4.95V*Ibatt

Solving for Ibatt gives Ibatt = 2.03A.

To do a simple sensitivity analysis, you can plug in different values for driver efficiency to see how much Ibatt changes (that's the sensitivity part). In this case, it's pretty easy. The dependence is linear. You can get a little fancier by changing Vf (maybe your XM-L has a higher Vf or a lower Vf by luck of the draw). Or you might pick a different value for Vbatt. Or some combination of all of the above variations.

If you have a linear driver like the very common 7135-based Chinese drivers, then tail current draw is the same as the driver's rated drive current. For example, if you have an 8x7135 driver, then the drive current is nominally 2.8A and the batt current will be 2.8A.

If you are using direct drive, then the batt current is given by the LED's If vs Vf curve from the datasheet and the voltage that the batteries can deliver.