I had to write this up offline, because the darn site logged me off and I lost it (GRRRR). Sorry if this a bit long and math heavy. I'm an engineer and I beg your indulgence.
I had the idea of making a new sort of battery mod that ran the atomizer directly from a large-ish capacitor charge at 5Vd, and then recharging this from an internal battery and a step-up converter. My reasons for wanting to do this are 3 fold... 1) Improved consistency: each hit of the capacitor will be the same regardless of battery state. 2) Reduced battery wear: Batt is discharged at around 250 mA continuously, rather than in high current pulses. 3) Battery protection: On a short or stuck button, maximum discharge is 200mA @ 5V or about 1W... not enough to start nasty fires.
So, some details:
I'm looking to run a 5V mod off of a single 10440 cell. I choose this because of its convenient size and sufficient storage to last me a day or two (I know not all of us can make it a full day on this, but then just scale-up to a 14500 or larger). I chose 5V as the working voltage for several reasons. 1: Seems most folks like it. 2: Convenient for USB pass-through. 3: Higher voltage means more Coulombs (amps*seconds) stored in the capacitor (Q=C*V). 4: Higher voltage gives more current for the same resistance atomizer (I=V/R), and more energy can be delivered to it in the ~3 seconds of inhalation. 5: Higher voltage means more energy per amp-second delivered to atty (P=VI). Put it all together and voltage is king for storing energy in capacitors (E=0.5C*V^2) and for output power (P=V^2/R).
SO, how much energy do we need to store?
Well, lets take a common example of a 3 second inhale on a 3.7V system running a 2 ohm LR atty. That's 3.6V*1.85A = 6.8W which gives around 20J of energy delivered. So that's where we start. In practice we need a fair bit more (about 1/e or 37%) because the current gets too low to be useful as the voltage drops, so we'll make it a round 30J. At 5V, this works out to 2.4 farads... typical super-capacitor range. Now ~2.5V caps are far more common (and 2.7V is better for safety margin reasons) and we can string 2 in series to get out 5V, but that drops our capacitance by 1/2, so we need to jack it up to around 5F. We also need to make sure the one we pick has a very low resistance (ESR) so it can handle high loads efficiently. Fortunately this is all conveniently easy to track down. 2 of these (Digi-Key - 589-1001-ND (Manufacturer - ESHSR-0005C0-002R7)) will do nicely and when put together are just smaller than a 10440 cell!
Now we need to get the 5V supply. Running it on USB is simple, and converting up from 3.7V is not that bad. This works well and is suitably small (Pololu - Pololu 5V Boost Regulator NCP1402), though I wish it had a touch higher current rating. At 200mA constant current it will recharge the cap from 3.35V (I will explain this number later) back to 5V in only 20 seconds, which is good enough for me though I know not for all. Maybe someone can suggest one better.
I'd also add one of these (LiPo Charger Basic - Micro-USB - SparkFun Electronics) to recharge the battery while on USB. I prefer micro-USB over mini cuz my phone uses it too, so I have the AC adapters and UDB cables every where, but they carry both versions. Now add a button, master switch, and the connector of choice and voila!
Ok, so how does it work?
We'll to start, you put a fresh battery in it, or hook it up to USB or both and the cap charges to 5V (takes about a minute from dead). Now, lets assume a stardard 3 Ohm atty. You hit the button, and imediately the energy begins to flow into it the atty very rapidly, and the voltage on the cap starts slowly dropping...
Detailed study of out vs. time follows... At t=0 we have the following: 5V, 1.67A, 8.33W and E (energy delivered) = 0J. At t=1s we have: 4.38V, 1.46A, 6.38W and E=7.3J. At t=2s: 3.83V, 1.28A, 4.89W, E=12.92. At t=3s: 3.35V, 1.12A, 3.74W, and E=17.21J. About now it's time to stop and inhale all that vapor before I run out of space. Our wattage has already dropped considerably anyway, so there's not a lot to gain by hanging on much longer. It should also be noted though that the DC/DC puts out 1W on it's own, so that an extra 3J on top, or right around 20J total delivered in 3 seconds... just as we targeted in first place. Not bad! And, now you know why I say we only need to recharge from 3.35V (or about 67%), which keeps the recharge short.
The interesting thing about capacitors is that most of the energy is delivered in a big spurt right up front and then tails off to a trickle shortly there after. This is actually nice, because a typical atomizer will heat up more and more and more as the juice starts to run dry and there's nothing to vaporize. This is what gives you that burnt flavor, and leads to safety issues. In this case, the energy goes mostly into heating up the cold atomizer and evaporating the PG/VG and is almost gone once things go dry. While you're waiting to recharge the cap, the juice is re-wetting the atty. Of course on USB, you can run at 500mA, so recharge in about 8s... which is plenty fast for anyone and no wear and tear on the battery at all, it will just charge between puffs.
If folks still think this performance is low, you can always upgrade the caps to 10F, for only a 30% penalty in length, and switch to a dual-coil 1.5 Ohm atty, but recharge time will increase by 2X.
That's all for now, I'll open it up for comments.
Thanks!
-Nathan
I had the idea of making a new sort of battery mod that ran the atomizer directly from a large-ish capacitor charge at 5Vd, and then recharging this from an internal battery and a step-up converter. My reasons for wanting to do this are 3 fold... 1) Improved consistency: each hit of the capacitor will be the same regardless of battery state. 2) Reduced battery wear: Batt is discharged at around 250 mA continuously, rather than in high current pulses. 3) Battery protection: On a short or stuck button, maximum discharge is 200mA @ 5V or about 1W... not enough to start nasty fires.
So, some details:
I'm looking to run a 5V mod off of a single 10440 cell. I choose this because of its convenient size and sufficient storage to last me a day or two (I know not all of us can make it a full day on this, but then just scale-up to a 14500 or larger). I chose 5V as the working voltage for several reasons. 1: Seems most folks like it. 2: Convenient for USB pass-through. 3: Higher voltage means more Coulombs (amps*seconds) stored in the capacitor (Q=C*V). 4: Higher voltage gives more current for the same resistance atomizer (I=V/R), and more energy can be delivered to it in the ~3 seconds of inhalation. 5: Higher voltage means more energy per amp-second delivered to atty (P=VI). Put it all together and voltage is king for storing energy in capacitors (E=0.5C*V^2) and for output power (P=V^2/R).
SO, how much energy do we need to store?
Well, lets take a common example of a 3 second inhale on a 3.7V system running a 2 ohm LR atty. That's 3.6V*1.85A = 6.8W which gives around 20J of energy delivered. So that's where we start. In practice we need a fair bit more (about 1/e or 37%) because the current gets too low to be useful as the voltage drops, so we'll make it a round 30J. At 5V, this works out to 2.4 farads... typical super-capacitor range. Now ~2.5V caps are far more common (and 2.7V is better for safety margin reasons) and we can string 2 in series to get out 5V, but that drops our capacitance by 1/2, so we need to jack it up to around 5F. We also need to make sure the one we pick has a very low resistance (ESR) so it can handle high loads efficiently. Fortunately this is all conveniently easy to track down. 2 of these (Digi-Key - 589-1001-ND (Manufacturer - ESHSR-0005C0-002R7)) will do nicely and when put together are just smaller than a 10440 cell!
Now we need to get the 5V supply. Running it on USB is simple, and converting up from 3.7V is not that bad. This works well and is suitably small (Pololu - Pololu 5V Boost Regulator NCP1402), though I wish it had a touch higher current rating. At 200mA constant current it will recharge the cap from 3.35V (I will explain this number later) back to 5V in only 20 seconds, which is good enough for me though I know not for all. Maybe someone can suggest one better.
I'd also add one of these (LiPo Charger Basic - Micro-USB - SparkFun Electronics) to recharge the battery while on USB. I prefer micro-USB over mini cuz my phone uses it too, so I have the AC adapters and UDB cables every where, but they carry both versions. Now add a button, master switch, and the connector of choice and voila!
Ok, so how does it work?
We'll to start, you put a fresh battery in it, or hook it up to USB or both and the cap charges to 5V (takes about a minute from dead). Now, lets assume a stardard 3 Ohm atty. You hit the button, and imediately the energy begins to flow into it the atty very rapidly, and the voltage on the cap starts slowly dropping...
Detailed study of out vs. time follows... At t=0 we have the following: 5V, 1.67A, 8.33W and E (energy delivered) = 0J. At t=1s we have: 4.38V, 1.46A, 6.38W and E=7.3J. At t=2s: 3.83V, 1.28A, 4.89W, E=12.92. At t=3s: 3.35V, 1.12A, 3.74W, and E=17.21J. About now it's time to stop and inhale all that vapor before I run out of space. Our wattage has already dropped considerably anyway, so there's not a lot to gain by hanging on much longer. It should also be noted though that the DC/DC puts out 1W on it's own, so that an extra 3J on top, or right around 20J total delivered in 3 seconds... just as we targeted in first place. Not bad! And, now you know why I say we only need to recharge from 3.35V (or about 67%), which keeps the recharge short.
The interesting thing about capacitors is that most of the energy is delivered in a big spurt right up front and then tails off to a trickle shortly there after. This is actually nice, because a typical atomizer will heat up more and more and more as the juice starts to run dry and there's nothing to vaporize. This is what gives you that burnt flavor, and leads to safety issues. In this case, the energy goes mostly into heating up the cold atomizer and evaporating the PG/VG and is almost gone once things go dry. While you're waiting to recharge the cap, the juice is re-wetting the atty. Of course on USB, you can run at 500mA, so recharge in about 8s... which is plenty fast for anyone and no wear and tear on the battery at all, it will just charge between puffs.
If folks still think this performance is low, you can always upgrade the caps to 10F, for only a 30% penalty in length, and switch to a dual-coil 1.5 Ohm atty, but recharge time will increase by 2X.
That's all for now, I'll open it up for comments.
Thanks!
-Nathan
