Lights, batteries, amps & capacity

ChattanoogaDan · 2019-07-28T17:45:49.000Z

As I continue to read more about lithium ion batteries and their capacities, I have also learned about discharge rates and their relation to the batteries quality. Certain batteries do you really well at a low power draw but once you crank up the amps the batteries front out of juice quicker. I’m sure this has something to do with the internal resistance but my question is for directed towards flashlights. How do I find out how much draw a flashlight creates on a battery. It’s like go through battery reviews by HKJ I don’t know if I need a battery that necessarily performs well at a high average if my flashlight only draws .5 amps. I have a Sofirn SP33 V2, SP31 V2 an SF14 V2 and a Lumintop Tool as 2.0. I imagine the amount of amps the flashlight draws will change depending on what mode the light is in. If it is in turbo I would think it would draw more amps draining the battery faster.

amishbill · 2019-07-28T18:47:26.000Z

You are absolutely right. The more lumens out the front, the more continuous amps needed in the back.

You are also right that some batteries work better under high load conditions than others. It’s not a hard and fast linear equation, but in general, a battery will be good for high mAh capacities, or for high continuous amp loads. If you pay a little more, you can get ones that are better at both, but there’s still a balance between capacity and load.

I was looking into this a while back. Based on what I found then, an SP31 maxes out at about 3 amps. I’m a big believer in safety margins. If this light maxes out at 3 amps, I’d look for a battery with a rating of 5-8 or more amps. Going with a 15A battery like the popular Samsung 30Q will not cause any problems. The tradeoff is a 3000mAh capacity. You can also get a 3500-ish mAh battery with a lower continuous load rating. Since you only need 3A for this light, it’s not a big deal.

WalkIntoTheLight · 2019-07-28T19:31:18.000Z

My rule-of-thumb is that for every 1000 lumens, you need about 10 watts to power it. For a lithium-ion cell, that’s about 3 amps per 1000 lumens.

Of course, some lights are a bit more efficient, and some less efficient. But, it’s a good rough estimate.

Almost all modern 18650 cells are capable of 6 amps or more of continuous current. Very few single-emitter lights will draw that much current.

Boaz · 2019-07-28T19:47:35.000Z

Not long ago it was clear that you had to make a choice between higher capacity and low amperage discharge rates or high discharge batteries with low capacity. So if you ran a normal light you'd get longer runtimes on a cell with higher milli amp hours Mah.(capacity)

Lately the fanboys have decided that output is the only number that matters . never mind that because of heat it can be achieved but not sustained .... The rest of the flashlight world has loved to sell using numbers since buyers generally are to stupid to know anything other than output numbers . Which leads to lots of lights on Ebay claiming to do 700,000 lumens .

Lots of flashlight manufacturers have adopted this super high output number philosophy but to achieve it the light has to have a turbo mode that will toss out a huge number but only for minutes .. or even only seconds before dropping to a lower "high mode " These lights need a high discharge cell to get those numbers . they will run fine on any battery they just won't produce the big numbers.

In the last few years Batteries have gotten much better and the high discharge cells now have much better capacities too so it's not like you have to give up what you used to...life is good .

WalkIntoTheLight · 2019-07-29T14:56:33.000Z

Boaz:

These lights need a high discharge cell to get those numbers . they will run fine on any battery they just won’t produce the big numbers.

That’s true of lights that use FET drivers to achieve high output, which is the case for all budget lights.

However, lights that use boost drivers can suck more current out of a low-drain battery than is safe. For example, my Zebralight SC600w MkIV Plus has a “turbo” mode of 2300 lumens. It uses a boost driver for regulated output, so it produces 2300 lumens regardless of whether the battery is low-drain or high-drain.

I can use an old low-drain laptop 18650 cell in it, and it runs fine at 2300 lumens. But the battery heats up quite fast, and gets pretty hot. (This is not the case for high-drain cells that can sustain higher currents.) I think it’s drawing around 8 amps from the battery, where it’s only rated for 4 amps. That explains why it gets so hot. I wouldn’t recommend doing it for long. The light’s thermal controls will kick in to ramp down the light eventually, but that detects heat in the driver not in the battery.

For FET drivers and linear drivers, that is not a problem. The battery’s voltage sag will cause the output to drop right away, and limit current.

Phlogiston · 2019-07-30T23:03:51.000Z

WalkIntoTheLight:

For FET drivers and linear drivers, that is not a problem. The battery’s voltage sag will cause the output to drop right away, and limit current.

I wouldn’t rely on that, if I were you. It will shorten the cell’s service life, and it pushes the cell into an operating regime that wasn’t designed for by the manufacturer, which increases the risk of a catastrophic failure.

WalkIntoTheLight · 2019-07-30T23:18:40.000Z

Phlogiston:

WalkIntoTheLight:

For FET drivers and linear drivers, that is not a problem. The battery’s voltage sag will cause the output to drop right away, and limit current.

I wouldn’t rely on that, if I were you. It will shorten the cell’s service life, and it pushes the cell into an operating regime that wasn’t designed for by the manufacturer, which increases the risk of a catastrophic failure.

I only mean for single-emitter lights. In those cases, the current is limited to about 3-4 amps max. Yeah, it’s at the upper range for a low-drain cell, but they can handle it. I certainly wouldn’t recommend a low-drain cell for a quad-emitter light.

But before someone does this, I recommend they take out the cell after a few minutes and feel it. If it’s getting hot, then don’t use it in that light. A hot cell means it’s being stressed too much, as you mention.