How long should an 18650 battery last

[Annoying Twit]

Tricky question I know, but I just want to be able to tell the difference between real batteries and fake by capacity.

I've got a claimed (likely wrong) 4000ma/h 18650 battery. Using it in my 12w CREE light gives something near an hour of light before it fades too significantly. A bit of searching suggests that this isn't unreasonable. And that it's unlikely to be a tiny fake battery wrapped in paper.

But, does anyone know ballpark figures for how long a reasonable ma/h 18650 battery should last in this application? Just enough to tell real from fake?

[gerrymcm]

have you had a look on http://www.torchythebatteryboy.com/
There are some tests on this site and maybe you can extrapolate from one that's similar to your own setup.

[gaz]

Rough figure ignoring any other circuitry being driven by the cell.

To power a 12w LED ,a single 3.7v cell needs to deliver 3.24amps.

So a single 4000ma/h (4.0 a/h) 18650 cell in the above would last 1.23 hours.

Its unlikely that it's a single cell system, multiply that figure by the number of cells in the battery pack.

[Brucey]

well presumably you won't have just one 18650 cell in there, you'll have a pack. If the rating is 4Ah @ 3.6V then this gives about 14VA nominal capacity. A 12W lamp (if that is really what it is) for an hour requires 12VA, so you are probably pretty close.

In reality batteries very rarely give their rated capacity and if they do they don't always last long before they clap out. You can directly trade the battery management strategy (i.e. how fully you charge the battery) against its life with many Lithium cells.

Also you may find that your '12W lamp' doesn't really consume 12W all the time after all. To be sure of the true load/capacity, you should measure the current and voltage during the discharge.

cheers

[Annoying Twit]

Thanks all. It is a single 18650 cell torch. I only use it for extra lighting woomph when travelling at night. I typically use a 3W light for continuous visibility, and that gives me plenty of battery life on 3x AAA.

So, if it's 1.23 hours for 4000 ma/h, then the time I got out of it probably wasn't 4000 ma/h worth, but well above what a fake battery would do. (Which by those numbers would be an incredibly short time if it's say a tiny 600 ma/h battery in there.)

I don't have a fancy charger that can measure discharge rates etc. The best I can do is put it in the torch and run it on full power to see how long it lasts.

The torch came with a battery, but the supplied charger doesn't work. I bought a charger on ebay which came with an optimistically labelled 4000 ma/h battery. I was concerned that the battery is fake and could be dangerous. It seems to work OK.

[mjr]

I don't have a fancy charger that can measure discharge rates etc. The best I can do is put it in the torch and run it on full power to see how long it lasts.

But that doesn't tell you anything unless you know how much the torch is pulling. Prod it with a multimeter to see what current is being delivered although I'm feeling dim this lunchtime and can't think where would be best to prod.

[Mr Evil]

...I've got a claimed (likely wrong) 4000ma/h 18650 battery...

The highest legitimate capacity 18650 batteries are around 3400mAh.

...can't think where would be best to prod.

Unscrew the end of the torch and put one probe on the end of the battery, and the other on the threads of the torch.

[Roadster]

Yes, 4000mAh is a mighty big claim for a single 18650 battery - quite unbelievably so!

[edocaster]

And 12W seems a pretty wild claim for a single cell torch. That would need to drag almost 4A out of the cell, which isn't super-healthy. Also, if it only has one LED emitter it would be putting almost 4A through that too, which none of them area really designed for.

I suspect two inflated specs cancel each other out!

[Annoying Twit]

I don't have a fancy charger that can measure discharge rates etc. The best I can do is put it in the torch and run it on full power to see how long it lasts.

But that doesn't tell you anything unless you know how much the torch is pulling. Prod it with a multimeter to see what current is being delivered although I'm feeling dim this lunchtime and can't think where would be best to prod.

The only thing it tells me is that if the torch runs out in five minutes there's something seriously wrong. If it runs out in about an hour (which it did) then it's somewhere in the ballpark. It could still be a recycled laptop cell, or a battery with nothing like the claimed capacity.

[Annoying Twit]

And 12W seems a pretty wild claim for a single cell torch. That would need to drag almost 4A out of the cell, which isn't super-healthy. Also, if it only has one LED emitter it would be putting almost 4A through that too, which none of them area really designed for.

I suspect two inflated specs cancel each other out!

Quite possibly In any case, the torch puts out ample amounts of light for night riding. My 3W CREE (which could also be an inflated claim) is adequate for unlit roads night riding. Using the two together, switching on the '12W' when needed worked very well for a ride yesterday that started at 3am.

EDIT: I wanted to delete this post and merge it with the previous one. But, I only get a message saying 'You cannot delete posts in this forum.'

[The utility cyclist]

Only two things really matter, does it do the job you need it to, from what you've said an hour on full meets that, second, are you happy with the amount you paid if it has indeed fulfilled your requirement.
What the stated/actual capacity of the battery is or indeed what the stated rating of the bulb is, is irrelevant if the above are met from your individual point of view.
I bought a torch for the home, it's rated at 800 lumens and is probably very close to that, it uses a '3800' ma/h battery, i know it's not, probably not even half that but it works, has continued to work without exploding after multiple charges over the last 3-4 years and gets an adequate amount of lighting time for my needs.

[Annoying Twit]

Only two things really matter, does it do the job you need it to, from what you've said an hour on full meets that, second, are you happy with the amount you paid if it has indeed fulfilled your requirement.
What the stated/actual capacity of the battery is or indeed what the stated rating of the bulb is, is irrelevant if the above are met from your individual point of view.
I bought a torch for the home, it's rated at 800 lumens and is probably very close to that, it uses a '3800' ma/h battery, i know it's not, probably not even half that but it works, has continued to work without exploding after multiple charges over the last 3-4 years and gets an adequate amount of lighting time for my needs.

Is the 'continuing to work without exploding thing' I'm a bit concerned about. Your battery (and charger I presume) has been proven safe. Mine, not yet. I'm tempted to ditch both and go with branded items from a reputable source.

[The utility cyclist]

Only two things really matter, does it do the job you need it to, from what you've said an hour on full meets that, second, are you happy with the amount you paid if it has indeed fulfilled your requirement.
What the stated/actual capacity of the battery is or indeed what the stated rating of the bulb is, is irrelevant if the above are met from your individual point of view.
I bought a torch for the home, it's rated at 800 lumens and is probably very close to that, it uses a '3800' ma/h battery, i know it's not, probably not even half that but it works, has continued to work without exploding after multiple charges over the last 3-4 years and gets an adequate amount of lighting time for my needs.

Is the 'continuing to work without exploding thing' I'm a bit concerned about. Your battery (and charger I presume) has been proven safe. Mine, not yet. I'm tempted to ditch both and go with branded items from a reputable source.

I've got two cheapo chargers and had a few cruddy not so great 18650 batts, they were just rubbish, no sign of problems, the other one Ithat has done alright again was a cheapo job with a charger and never given any bother. I put them into a protected 4 gang extension in the living room that has the TV/VCR connected.