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:
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
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
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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