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Mooch Musing: Is pulse testing a battery more “real world”?

Discussion in 'Batteries and Chargers' started by Mooch, Dec 14, 2019.

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  1. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    I’m often chastised for doing continuous current testing of batteries instead of pulse testing because many vapers feel that continuous current testing is useless. They feel that since we use short periods of battery current when vaping, a couple to a few seconds long, pulse testing would much better represent how these batteries perform.

    Is that true?

    I did some pulse testing of a Molicel P26A, a great performing 18650, using several different “profiles” and compared that to the vaping time we would get if continuously discharging it. Here are the results at 40W, each sequence repeated until the battery dropped to 2.8V:
    • 5 seconds on/5 seconds off = 12.9 minutes vaping time
    • 5 seconds on/15 seconds off = 12.6 minutes
    • 5 seconds on/115 seconds off = 12.4 minutes
    • 5 on/5 off, repeated six times and then 120 seconds off = 12.3 minutes
    • Continuously = 12.1 minutes
    There is only a 12 second (0.2 min) difference in vaping time between a “typical” profile (a burst of vaping and then a rest) versus continuously discharging the battery. This is less than a 2% difference.

    The other discharge profiles show a bigger difference between them and a continuous discharge but still only a few percent for the largest difference (a very ridiculous profile of 5 seconds on/5 seconds off for the entire discharge).

    So a continuous discharge can provide some pretty accurate information about how a battery would perform when vaping in addition to allowing us to directly compare one battery to another. Why is this happening? Why isn’t there a much bigger difference between running a battery continuously and “pulsing” it?

    For any discharge over a few milliseconds long our batteries don’t do much of anything different. They’re just shuttling ions the same way across the battery. It’s only for those first few milliseconds that different pulse lengths would mean anything since that’s when the ions are distributing themselves in all the different densities and flow patterns that exist across the battery. Once they’re set up, they’re set that way for most of the rest of the discharge.

    For our batteries, the pulse must be shorter than a few milliseconds to have a big effect versus a continuous discharge. Basically, the battery thinks that anything over that short amount of time is a continuous discharge, just of a different length. One second or 1000 seconds is all the same, until near the end when there are no more ions to shuttle across the battery.

    You might have noticed that the numbers above show that we get more vaping time when using the battery harder and wondered why.

    Harder use heats up the battery a bit more, making the chemical reactions more efficient and lowering the battery’s internal resistance. This reduces the voltage sag which means it takes longer for the battery to drop to a certain voltage level. So, we get more vaping time.

    The continuous discharge heats up the battery the most but that quick discharging, with its continuously dropping voltage, overwhelms the decreased voltage sag. The net result is a shorter discharge time.

    The results at 20W and 80W are about the same as these 40W results. This is also true for different pulse lengths typical of vaping since they’re all just considered to be continuous discharges of different lengths by the battery.

    The battery temperature can be very different between pulse and continuous discharges though! Obviously, a continuous discharge heats up the battery a lot more than pulsing it.

    If pulse and continuous discharging give us about the same results then why don’t I just do pulse testing?

    Continuous discharging is the industry standard and allows us to easily compare our results with those done by others, especially the battery manufacturers.

    Continuous discharging is also important for safety as it tells us what might happen if there is a mod malfunction or accidental button press that discharges the battery continuously. The short testing time for a continuous discharge, versus up to several hours for the profile that uses a set of 5 sec bursts and then a 120 second rest, is also great because it lets me do a lot more more testing in a day.

    If you miss using the pulse discharge graphs I used to do then you can use the Wh and DC internal resistance (DC IR) specs I include with every battery test report. These numbers let you directly compare the amount of energy a battery can deliver (the Wh), which determines run times, and the voltage sag for different batteries.

    To calculate the voltage sag at the start of a discharge just multiply the DC IR spec by the amount of current you are drawing from the battery. Sound familiar? It’s just Ohm’s Law...Voltage Sag = (DC Internal Resistance) x (Current). Subtract the voltage sag from the voltage the battery started at, typically 4.2V, and you get the voltage the battery sags down to when used at that current level.

    If the battery is warm internally then there will be a bit less voltage sag. If the battery is below room temperature there will be a bit more voltage sag. If the battery is cold there can be a LOT of voltage sag.

    You cannot use the AC internal resistance (AC IR) spec given in many battery datasheets though for that Voltage Sag equation though! The AC IR only tells us the resistance of the liquid in the battery, the electrolyte. This is only part of the total internal resistance of the battery that causes the voltage sag we see when using a battery. The AC IR spec is always lower than the DC IR spec.

    Why is the AC IR listed in the datasheets then? It’s an industry standard because it is quick and easy to measure and still offers a rough guide of how the cell will perform. We just can’t use it to calculate voltage sag.

    So while it can provide valuable temperature data for certain uses, pulse testing isn’t really more “real world” than continuous current testing. The industry standard continuous current discharge will remain the mainstay of my testing.

    Is all this making your head spin? There’s no need to worry about continuous versus pulse if you just want a good battery. There are some great choices in my Recommended Batteries tables:
    Mooch's Recommended Batteries | E-Cigarette Forum
     
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  2. Barkuti

    Barkuti Super Member ECF Veteran

    Pretty informative, thanks.
    Not the first time you speak about this, does some people often disagrees with your testing methods?
    Do you think some strain reducing extra cooling (a small fan blowing air) over the cell during continuous discharge could compensate the very slight reduction in output time versus intermitent profiles? This may be completely unimportant, but thought about it.

    Now that I dropped by, I will share a device which can help technically inclined people to better know their batteries.
    Weeks ago HenrysCat at BLF (here, there and in the ZKE Tech EBD-M05 help thread) shared information from a small, affordable testing device which I myself have bought for testing too. The device has PC software which plots discharge graphs. Of course, just up to 5 A discharge may not be that much, but with curve plotting it is useful to infer a battery's DC internal resistance along with other traits. ZKE Tech also has more powerful battery testing devices, I will also share a link to the unit which the guy at Thunderheart Reviews uses. Links:
     
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  3. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    Many battery wrapping companies and vendors take issue with my testing and the results. This is, IMO, due to them not understanding the difference between a rating a just something the battery can do without venting/exploding. Quite a few vapers (that have strong feelings for a certain cell) take issue with my results too. Though it has gotten a lot better than it was 3-4 years ago.

    I’m not sure I understand your cooling question but any fan cooling of a cell being continuously discharged might actually shorten the running time as the slightly cooler exterior of the cell would have a higher internal resistance than the interior of the cell.

    Since the hotspots wouldn’t run any cooler if the cell was fan cooled I don’t think there would be much strain reduction using a fan unless the exterior temperature was significantly reduced. The hotspots would be just as hot but at least the other parts of the cells wouldn’t reach as high a temperature.
     
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  4. Barkuti

    Barkuti Super Member ECF Veteran

    This is the thing which made me ask the cooling question. If a hot battery performs better, there is no reason for a lesser discharge time. At least not if considering just these variables, there must be something else indeed. [​IMG]
     
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  5. Mooch

    Mooch Electron Wrangler Verified Member ECF Veteran

    Supporting member
    May 13, 2015
    I think I understand what you are saying.

    A warmer cell runs at a higher voltage at some points in the discharge but that doesn’t always mean it “performs better” for every definition of that phrase. Higher performance can mean increased run time or increased cycle life too.

    There can be other losses at different points in the discharge, inefficiencies that cause a reduced run time, gradients in the ion distribution that cause different voltage drops. The decrease in internal resistance is not the same at all points in the discharge.

    The test results were clear. What is going on inside I cannot fully explain though without a lot more testing...for which there is no time. It would be great if you could do that testing! A 5A discharge in a small box would allow the cell to heat up enough to, I believe, make a enough of a difference in its thermal resistance. Or you could insulate the cell with some material.
     
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  6. somdcomputerguy

    somdcomputerguy vaper dedicato Verified Member ECF Veteran

    Supporting member
    This is a very interesting thread. I believe it may get more interestinger too. :cool:
     
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  7. Punk In Drublic

    Punk In Drublic Vaping Master ECF Veteran

    Aug 28, 2018
    Toronto, ON
    Great post @Mooch

    Regardless of what ever testing mythology you employ, there will always be someone who will find subjective faults with it. Can’t please everyone.
     
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  8. Rossum

    Rossum Surly Curmudgeon Verified Member ECF Veteran

    Supporting member
    Dec 14, 2013
    NE FL
    I liked the pulsed testing at various current levels because it gave us some idea how low a resting voltage we could run a battery down to at a given current level before we were pushing it below 2.5V under load.

    You pull a cell out of a mod and pop it in the charger. Charger says 3.60V. Cool. That cell is still at ~40-45% charge, right? Well maybe. But what voltage will it produce under a 10A or 20A load? The older pulse-test graphs would show us the no-load voltage at which we should stop using a cell and re-charge it -- at a glance.

    Yes, sure, we can calculate it based in DC-IR. Theoretically. ;)
     
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  9. Barkuti

    Barkuti Super Member ECF Veteran

    By my hawk eye powers I'd say the SoC of a typical high discharge cell at 3.6 no-load volts is slightly below 30% already.
    Mmmkay. Mech user ;) Rossum has a point here. A very low discharge rate curve (or pulse curve) would solve our needs. Henrik, the lygte-info.dk guy, tests at 0.1 or 0.2 A depending on cell size, but still I wonder what is the actual output voltage difference between a rather small load and no-load because of capacitive (or other [​IMG] unknown to me) factors. By looking at the difference between the very start voltages of the lowest rate curves and comparing it to the theoretical no-load full charge voltage of batteries (should be very close to 4.2 V but can't say for sure as it depends on how the cells are charged) I presume there's something going on, i.e. a small difference.
    Once I get my small ZKE Tech battery tester I will know better. :)
     
  10. Rossum

    Rossum Surly Curmudgeon Verified Member ECF Veteran

    Supporting member
    Dec 14, 2013
    NE FL
    What do you consider a "high discharge" cell? I generally use 15-20A cells like 30Q or HG2, or VTC6.

    No, I don't think it would; at least it's not what I'm looking for. My desire is pulsed discharge curves at something close to the amperage I use, but I'm not expecting Mooch (or anyone else) to do that for me. I may just break down and buy WMR CBA with higher current capacity than the HR model I already have. FWIW, I also expect the results to change quite quite a bit as cells age with use.
     
  11. Barkuti

    Barkuti Super Member ECF Veteran

    Well, I took a look at the Vapcell INR18650 G26 curves, that cell is an LG H26 rewrap:

    [​IMG]

    Looks like Henrik set the x-axis scale wrong. :confused:
    At 3.6 V that cell is at ≈1.7 Ah out of ≈2.42 Ah (for 15 A or so discharge). At the 1.7 Ah point the red curve is a little bit below 3.6 V. 1.7 Ah / 2.42 Ah = 0.702479338843, which means a smidgeon below 30% charge left.
    Every cell is different, of course.

    In my previous post Iinked a testing device which goes up to 20 A and output graphs via PC software (I also found an even more high end unit, but looks like they are asking some serious o_O money already).
     
  12. dripster

    dripster Ultra Member ECF Veteran

    Feb 18, 2017
    Belgium
    5.2A = low
    10A = medium-low
    15A = medium
    20A = medium-high
    25A = high
    30A = very high
    35A = even higher
     

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