- Aug 11, 2003
at aroiund 1/10 a amp how many l91s would it take to have the runtime of one 21700 cells?
Wasn't the 1/10Amp just indicating the expected drain current of the cells?When trying to compare cells with different voltages, compare Watts and Watt-Hours--not Amps. Watts = Amps x Volts
The only thing specific we have to go on here is two battery types and specs and 100ma current drain which I have to consider is for either option. I agree that the power output at 100ma for a single 21700 vs L91 is about 3x as much but we cannot assume much based upon the limited information given to us. 100ma drain is plenty in even a 1AA light to power out lumens in a case where there is no power and the likely light pollution is minimal. Personally I would opt to go with 18650 based lighting over 3AA when 21700 solutions are not available. I'm not an overall fan of the battery type for general purpose as there are too few options for its use right now and plenty for 18650s. I assume in the future that will change perhaps we may even see single cell 21700 powered lanterns and work lights that use replaceable batteries in them. Even with the higher amount of power that a single cell can contain in it the 21700 isn't mainstream yet but gaining in popularity. There may be new types of go to batteries on the horizon that may replace both sizes in many uses.Wasn't the 1/10Amp just indicating the expected drain current of the cells?
Though you're generally right regarding Watt-hours being the best measure, that does rely on what is being powered using the voltage/current efficiently - if using 3x L91s in a 3xAA light (or a 21700 in a single-cell light) which had a linear driver, runtime would be dependent on single-cell Ah, not total-cell
I took this question to mean similar run time, at similar output levels.at aroiund 1/10 a amp how many l91s would it take to have the runtime of one 21700 cells?
While that is likely true, unless you directly query him about what his uses are and then reply to that information the reality could be exactly as he is asking or something completely different..... we don't really know.Unless he's running an analogue clock he's going to need more than that. And I was talking about differences. Not necessarily exactly as op said. Also he's trying to decide on his survival selves. So everything is relevant
Long term storage of any rechargeables requires either occasional recharging or recharging prior to seasons where there would be possible bad weather etc. This is why L91s are good and vs CR123s they can be used in a lot more items than them are easier to source also and many devices using AAs can use alkaleaks (ugh) in an emergency you could go rob all your remotes and clocks and so on for AAs.Agreed. I can only presume though from the thread title it's for emergency use. The long term storage of the different chemistries is an important consideration.
I believe the op mentioned .1A so losses from internal resistance wouldn't be factored into the final capacity. A simple look through HJKs reviews/graphs will show around a 40% capacity advantage for a top 21700 compared to 3x l91 in series so I don't think the only criteria is mAh or only a .1A draw hence why I mentioned other considerations
If you are using a 1.5v source with a boost circuit you will probably get about 15 to 20 lumens using that much current from the battery. Plenty useful in a power outage to conserve battery power where there is little light pollution.Am I reading these graphs correctly?
If so, it appears that 0.1 amps (100ma) will deliver about 40 lumens.
The following graphs were taken from the Cree XPG Datasheet.
considering that 130 flux is the base @ 350 ma.
View attachment 18547
100 ma should output 25% of 130 flux or about 40 lumens.
View attachment 18546
and the LED should pull 0.1 amps or 100 ma, when the voltage is at 2.75 volts without regulation.
View attachment 18548