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Feature Request: Li-Ion battery option in Battery Type w/ 2 cutoff values


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I have built and use a 3 x 18650 mod as an ADV and would welcome a Li-Ion Battery option in the battery drop down with 2 different cutoff voltage settings:

  • Cell Soft Cutoff: as it is now, for voltage value under load
  • Cell Hard or Idle Cutoff: for a higher, idle (voltage w/o load) value

My understanding is that:

Li-Ion has a significant and drain dependent voltage drop (due to its internal resistance?)
Under load voltage can go as low as 2.7V as long as the idle voltage goes back to 3V or above.

For safety and longevity point of view I would like the board eliminating firing when idle voltage couldn't go back to or above 3.2-3.3V but I wouldn't mind if it goes down to 2.9-3V  under load.

ATM I can't really do this.
If I set the soft cutoff to 3.2 I got weak battery warning (and reduced output) relatively early.
If I set the soft cutoff to 3 or 2.9 the board won't prevent me to discharge my batts close to this numbers, would give the weak battery warning though.

I admit that it is not some super crucial issue, especially with the 3 x LG HG2 I am using. I get an insane 28.4Wh out of them even if I play it super safe (w/ high soft cutoff settings).

But with two values the Li-Ion discharge could be controlled pretty well regardless of the drain and the actual internal resistance without  getting the annoying weak battery warning and power reduction unnecessarily.

BTW, here is my discharge curve for 3 x LG HG2 3000 mAh batteries @ 50W w 3.09V soft cutoff.txt LGHG2-3cell@50W 3090mV SCutoff.txt (in txt format for csv is not supported by the page:)
FYI: I have an external quick blow glass fuse in the circuit with unknown resistance therefore my curve is more likely a bit steeper than it should be.


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I hear what you're saying, but if your motivation for this feature is to somehow extract even more capacity from your battery, then this isn't going to help much, if at all. Sorry if I misunderstood your intention or motivation. But the capacity of lithium-based cells simply doesn't advance much at all past about 3.3v, particularly at the discharge rates that are typical for vaping. You would be significantly sacrificing longevity of your cells for a very minimal gain in capacity. Whether that trade off is worth it is up to you, but IMO it is not. This typical lithium cell discharge curve, which I grabbed from a quick Google image search, makes my point better than my own words. [image]

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I don't want to squeeze everithing out. On the contrary. I want to optimize the draw. What this curve is doesn't show is the idle voltage it goes back. You might want to Google a pulse discharge curve. You will find a few. What is interesting in the chart you attached is how it starts. The higher the draw the bigger the initial drop. But if you cut the drain the difference between the idle voltage will be way smaller than the difference between the voltage under load. And that is the point.E.g. in this chart initial drop goes up to cca 0.4V down to 3.8. But if you cut the load after a few sec it would go back above 4 probably pretty close to 4.2.In other word it doesn't mean that you drained your battery to 3.8V. Besides, if you check my battery analyzer settings you would find that my amp draw was around 5A which is not outrageous for a good Li-Ion

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All of what you say is true. But... The curve generated by the battery analyzer is resting voltage, not voltage under load. The analyzer unloads the battery every few seconds, allows voltage to stabilize, and then records the resting voltage. If you click the "Record" button at the start of the test, you will get a much larger .csv file that includes literally thousands of data points (many samples per second) of cell voltage and power output (basically, the raw data that is shown in the live graph while the test is running). And you can readily determine from comparing that data to the discharge profile .csv file that the latter is comprised of *resting* voltage. Voltage sag under load is just the allocation of the total voltage potential of the voltage source (the battery in our case) to all resistors in the circuit, one of which being the internal resistance (IR) of the battery itself, and the allocation is proportional to the resistance. In our case, the total voltage potential is divided primarily amongst (i) the external load (the atty/coil/mod) and (ii) the IR of the battery. And the higher the resistance of the external load (the atty/coil/mod) relative to the battery's IR, the higher proportion of the battery's total voltage potential that will be applied to the external load. The rest of the voltage potential is applied to the other resistor in the circuit (the IR of the battery itself). When we talk about "voltage sag," we are talking about the voltage potential across the *external* load relative to the total open-circuit voltage potential of the battery. The higher the resistance of the external load relative to the IR of the battery, the less "voltage sag" will occur, since proportionally more of the total voltage potential is allocated to the external load.

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Also, once the battery gets to the point that it cannot sustain the requested load (at your specified LVC), the DNA 200 will dynamically throttle the power output to whatever load the battery can sustain (down to no less than 5W continuous) in order to optimize the capacity of the pack, albeit at reduced power output. John posted about this a while back, but I can't find the post at the moment. But, frankly, once you're at that point, most of the pack's capacity is gone anyway. We're only talking about a difference of maybe a small handful of puffs.

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@bmclaurin
Thanks to your answers but I really don't get your point even though I  happened to hit the record button and checked the result including how the soft cutoff works.

Again, it is not for maximize puffs it is for optimize the discharge.

Let me give you an example.

Example #1
Let the target idle/rest voltage is 3.3V for the sake of battery longevity. Voltage under load is not a real issue as long as it stays above min 2.7V and the battery can recover to 3.3V

Let the voltage drop 0.1V for every 50W load (means 0.4 for 200w) just for the sake of the argument.

It means that I should set my soft cutoff to 3.2V for 50W, 3.1 for 100W, 3.0 for 150W etc. And if I do it AND stop vaping when I hit the weak battery warning I will get the nice and safe 3.3V rest/idle

And that is my point. That I would need different cutoff values for different target drains and would need to stop vaping when hitting the weak battery otherwise I would drain my batteries below target.

Or: I set my cutoff to 3.3V and completely ruin my own vape in the 3.4-3.7 idle/rest V range (depending on the power setting) by the soft cutoff regulation.

Meanwhile with two values I can set my idle/rest and under load separately fully covering all the wattage (voltage drops) range AND fully eliminate any accidental over drain AND fully eliminate any unnecessary output reduction because the board thinks (wrongly) that my battery is weak:

Example #2
Target idle/rest voltage: 3.3V
Target min under load voltage: 3.0V

Desired operation:

If target idle/rest voltage <3.3V no firing at all but low battery warning only
If target voltage>3.3V firing as per now protected and limited if needed by the soft cutoff

Working in the low volts:
As long as I don't hit any of the thresholds the mod would fire as per normal.
In this example, below 3.4V idle/rest and above 150W setting I would get weak battery warning and power reduction for the last few puffs which is acceptable. (or if not, I would reduce soft cutoff to 2.9V:) )

Worst case scenario:
V rest/idle: 3.30..01V, power setting 200W and firing.

The mod would fire indeed and depending on the batteries even up to 200W more likely below though. Either way, after this very last puff it would cut off nicely and the voltage would recover pretty close to 3.3V


Bottom line:
Trying to obtain given target rest voltage for Li-Ion with a single under load value (soft cutoff) is not optimal IMO.

P.S.:
Again, it is not crucial at all. You can save your batteries with the present setting options too. It is all about the quality of the vape when you are getting low on charge without getting the soft cutoff engages unnecessarily.


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@vapealone, my apologies, as I think I read your first post and reply too quickly and missed your main point and proceeded to yammer on about trivial stuff that wasn't responsive to your idea. Yes, for a given LVC, the terminal resting voltage of the depleted pack depends on the rate at which the pack was discharged. At relatively low discharge rates, the terminal resting voltage will be lower as compared to what it would be at higher discharge rates, just as you describe, due to the aforementioned voltage drop. For that reason, I understand the conceptual appeal of a more customizable LVC feature that would allow compensation for differing rates of discharge. You are spot on in that regard, IMO. I will point out, though, that when you get down to the 3.5v range (resting), you're not doing much more vaping at any power setting without a recharge. Even at low power settings, you're only going to get a small handful more puffs before the pack is depleted. So the quality of the vape will only be impacted on those last few puffs.

I hope I didn't come across as being dismissive of your feature request, as I do understand its conceptual appeal.

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:)
I am happy that you think my proposed approach would be a bit more convenient than the present one.

However, I don't completely agree with your point regarding to the 3.5V resting threshold for I think that it depends on the load and the batts IR and it can easily be more than just a handful of puffs.

E.g.: my batts have ~15% left around 3.5 restV when rest cutoff is set to ~3.25-3.3V (should check the recorded csv and probably will later)
this 15% means 28.4Wh x 0.15= 4.26Wh=255.6Wm=15,336Ws=15,336Ws/(25W mean power x 2s mean puff) = 306 puffs for me (if my maths holds:)

And I wouldn't call it insignificant for it is more than 40% of the overall capacity of Evolv's 900mAh reference mod:)

Of course, for Li-Ion it can indeed be a lot smaller should I set my Wattage higher. 

And that is why I would like to have a bulletproof battery management system that looking after me regardless of my power settings and doesn't cost me precious Whs and/or neither ruins my last Whs if not necessary.

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I'm pretty sure it is undesirable (i.e. reduces cycles before the battery won't hold a decent amount of charge anymore) to allow a lipo to fall below 3V under load. The 3.09V cell cutoff was intended to follow that thinking with a very small safety factor of 0.09V. You could reduce the cutoff voltage setting to obtain more out of the battery before soft cutoff but your number of charge/discharge cycles before you need to replace the battery will fall. The best measure of battery charge status is voltage under (known) load. I don't think, IHMO, that refining a scheme for cell cutoff based on resting voltage is a step forward.

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bmclaurin said:

Lol, yes, I forgot about you lucky folks with monster capacity batteries. Meanwhile, I'm stuck with the stock 10 Wh stock battery in the Lavabox. For that battery, at 3.5v it is literally just a few puffs. I checked my .csv to verify.



:)
For I don't do high power I figured that multiple Li-Ions and their extended capacity would suit me better even with its significant voltage drops:)
And with this mod, my whole thing about the cutoff is really just a bonus feature.
One day I forgot to charge it during the night and when it took me home on the next day after work it annoyed the hell out of me with the power limitation and still had plenty of charge when I have put it on the charger :)
I don't want to limit myself vaping I want the board to simply cut it off and doesn't allow me annoying myself:)
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bmclaurin said:

Lol, yes, I forgot about you lucky folks with monster capacity batteries. Meanwhile, I'm stuck with the stock 10 Wh stock battery in the Lavabox. For that battery, at 3.5v it is literally just a few puffs. I checked my .csv to verify.



BTW, I have made my mini monster  (38mm x 50mm x 85mm) from scratches but you can buy (or will be able to buy soon) small, professionally designed  3 x 18650 mod for yourself if you wish:)
e.g. the Wismec Reuleaux with dimensions not much different from the Lavabox (Reuleaux : 40mm x 50mm x 84mm vs Lavabox: 28.15mm x 46.19mm x 94.87mm) 
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