Inquiring minds want to know - mine does anyway:
Do Fenix TK21's automatically 'downshift' from turbo to a lower output. If so, when does it occur, why does it occur, and does my TK21 exhibit the behavior?
Test equipment:
1. My TK21 (which I had selected for minimal green tint from the local flashlight store.)
2. Two (2) Trustfire TR18650 2500 mAh rechargeable batteries.
3. Radioshack DMM.
Initial test:
Fully charged both batteries. One indicated ~4.21 VDC, and the other indicated ~4.18 VDC. (Note that this is open circuit voltage, not voltage under load.)
I used the first battery to run the TK21 continuously on turbo until I observed a downshift. This occurred after about 75 minutes. After it occurred the first time, I checked the open circuit voltage on the battery, and it was about 3.80 VDC. I put that partially depleted battery back into the TK21, and continued to run it on turbo, which resulted in progressively less runtime before downshifting, initially less than a minute, and soon occurring in less than 5 seconds. After the final time, the open circuit battery voltage was about 3.77 VDC. (I also noted that the tint appeared to become slightly greener with run time. It doesn't seem like that would occur with a tightly regulated driver circuit…)
I did the same test with the other fully charged battery, which had the slightly lower initial open circuit voltage. Although I didn't time this test as carefully, the time until downshift was less than the 75 minutes of the other battery. After the first downshift, it also exhibited the progressively shorter runtimes before downshifting, with a similar final open circuit battery voltage.
I fully charged both batteries again, and used the lower voltage one to run the TK21 on turbo until the light first downshifted.
I then headed to the local flashlight store with my TK21, the two batteries (one fully charged to 4.21 VDC and one depleted to an open circuit voltage of about 3.80 VDC), and the Radioshack DMM.
Using a similar test procedure, I went through about 8 different TK21s at the store. Since I wanted a minimal green tint TK21, that was an initial selection criteria, but the store proprietor made a good suggestion that perhaps the TK21's with a whiter tint were being driven slightly harder, which could mean that they might downshift sooner than a greener tint.
To summarize what I determined after an hour or so of testing:
1. TK21's do downshift from turbo.
2. They downshift at varying battery voltages, and ALL of the lights I tested downshifted at a lower voltage than did my original TK21, which would allow a longer run time.
3. There seems to be no relationship between the downshift voltage of green tinted vs. white tinted lights - i.e., some white tinted lights downshifted at a lower voltage than did some green tints.
Since I wanted the longest turbo runtime possible, I then selected the whitest tinted light with the lowest downshift voltage that I had tested, and traded in my original TK 21 for it.
To confirm the above, I repeated my initial tests with the new TK21:
Fully charged both batteries. Again, one indicated ~4.21 VDC open circuit voltage, and the other indicated ~4.18 VDC.
This time, the first of the batteries ran the TK21 on turbo until the downshift occurred at about 94 minutes, 19 minutes longer than the original TK21. The open circuit voltage was then about 3.68 VDC, which was over .10 VDC lower than the downshift voltage of the original TK21. I extended the test with that partially depleted battery on turbo until it downshifted in just a few seconds (which occurred more quickly than on the original TK21), resulting in an open circuit battery voltage of ~3.66 VDC, again more than .10 VDC lower than with the original TK21.
With the other fully charged battery (with the lower initial open circuit voltage), the time until downshift was 83 minutes, at least 8 minutes longer than the original TK21 (recall than I didn't time the initial test on this battery as carefully.) The extended test downshift battery voltages were similar to the above, as would be expected.
My interpretation of this is that the TK21 turbo downshift appears to be battery voltage dependent, not time or temperature dependent. However, the actual voltage at which the downshift occurs varies in different TK21s. I don't know how Fenix sets this voltage, but because different batteries have slightly different output voltages, and different TK21's apparently have different downshift voltage settings, a particular battery and TK21 combination will result in variable runtimes before the downshift occurs. (Note that intermittent usage, instead of continuous usage like in my test, should result somewhat in longer overall runtimes than I measured, because the battery voltage should recover somewhat with 'rest periods.')
This would also explain why 2 CR123s don't exhibit the same behavior - their depleted voltage would be expected to stay above the TK21 downshift voltage for longer than a single 18650.
Do Fenix TK21's automatically 'downshift' from turbo to a lower output. If so, when does it occur, why does it occur, and does my TK21 exhibit the behavior?
Test equipment:
1. My TK21 (which I had selected for minimal green tint from the local flashlight store.)
2. Two (2) Trustfire TR18650 2500 mAh rechargeable batteries.
3. Radioshack DMM.
Initial test:
Fully charged both batteries. One indicated ~4.21 VDC, and the other indicated ~4.18 VDC. (Note that this is open circuit voltage, not voltage under load.)
I used the first battery to run the TK21 continuously on turbo until I observed a downshift. This occurred after about 75 minutes. After it occurred the first time, I checked the open circuit voltage on the battery, and it was about 3.80 VDC. I put that partially depleted battery back into the TK21, and continued to run it on turbo, which resulted in progressively less runtime before downshifting, initially less than a minute, and soon occurring in less than 5 seconds. After the final time, the open circuit battery voltage was about 3.77 VDC. (I also noted that the tint appeared to become slightly greener with run time. It doesn't seem like that would occur with a tightly regulated driver circuit…)
I did the same test with the other fully charged battery, which had the slightly lower initial open circuit voltage. Although I didn't time this test as carefully, the time until downshift was less than the 75 minutes of the other battery. After the first downshift, it also exhibited the progressively shorter runtimes before downshifting, with a similar final open circuit battery voltage.
I fully charged both batteries again, and used the lower voltage one to run the TK21 on turbo until the light first downshifted.
I then headed to the local flashlight store with my TK21, the two batteries (one fully charged to 4.21 VDC and one depleted to an open circuit voltage of about 3.80 VDC), and the Radioshack DMM.
Using a similar test procedure, I went through about 8 different TK21s at the store. Since I wanted a minimal green tint TK21, that was an initial selection criteria, but the store proprietor made a good suggestion that perhaps the TK21's with a whiter tint were being driven slightly harder, which could mean that they might downshift sooner than a greener tint.
To summarize what I determined after an hour or so of testing:
1. TK21's do downshift from turbo.
2. They downshift at varying battery voltages, and ALL of the lights I tested downshifted at a lower voltage than did my original TK21, which would allow a longer run time.
3. There seems to be no relationship between the downshift voltage of green tinted vs. white tinted lights - i.e., some white tinted lights downshifted at a lower voltage than did some green tints.
Since I wanted the longest turbo runtime possible, I then selected the whitest tinted light with the lowest downshift voltage that I had tested, and traded in my original TK 21 for it.
To confirm the above, I repeated my initial tests with the new TK21:
Fully charged both batteries. Again, one indicated ~4.21 VDC open circuit voltage, and the other indicated ~4.18 VDC.
This time, the first of the batteries ran the TK21 on turbo until the downshift occurred at about 94 minutes, 19 minutes longer than the original TK21. The open circuit voltage was then about 3.68 VDC, which was over .10 VDC lower than the downshift voltage of the original TK21. I extended the test with that partially depleted battery on turbo until it downshifted in just a few seconds (which occurred more quickly than on the original TK21), resulting in an open circuit battery voltage of ~3.66 VDC, again more than .10 VDC lower than with the original TK21.
With the other fully charged battery (with the lower initial open circuit voltage), the time until downshift was 83 minutes, at least 8 minutes longer than the original TK21 (recall than I didn't time the initial test on this battery as carefully.) The extended test downshift battery voltages were similar to the above, as would be expected.
My interpretation of this is that the TK21 turbo downshift appears to be battery voltage dependent, not time or temperature dependent. However, the actual voltage at which the downshift occurs varies in different TK21s. I don't know how Fenix sets this voltage, but because different batteries have slightly different output voltages, and different TK21's apparently have different downshift voltage settings, a particular battery and TK21 combination will result in variable runtimes before the downshift occurs. (Note that intermittent usage, instead of continuous usage like in my test, should result somewhat in longer overall runtimes than I measured, because the battery voltage should recover somewhat with 'rest periods.')
This would also explain why 2 CR123s don't exhibit the same behavior - their depleted voltage would be expected to stay above the TK21 downshift voltage for longer than a single 18650.
and your previous post makes much more sense.
