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Maximum Current
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TinWhisker Offline
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Post: #1
Maximum Current
I'm currently using the IPV 4s (dual 18650, 120w max) with the LG brown 3000mAh cells. Even though I've studied this stuff, I still lose confidence in myself sometimes, so hopefully Don/someone can chime in and put my mind at ease.

I did a twisted 26g kanthal dual coil wrap, and it's reading at .17 ohms. I just want to be sure on how high I can set the watts to without issue.
17-11-2015 08:41 PM
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Don Offline
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RE: Maximum Current
You have 120W maximum output. From Ohm's Law you have an 8.4V supply and 120W output, so amps = wattage divided by volts. Which gives you around 14 amps.

However: The cells don't stay at 4.2V but drop in voltage as the charge is used up. The low limit is probably around 3.7V per cell so we need the numbers at 7.4V instead.Which is around 16A.

Next point - this is a "best case" DC-DC regulators are not all that efficient - a decent guess would be around 70-80%. Let's say 80% as the Yihi chipset is one of the more efficient ones out there AFAIK. so we need to divide the worst case current by 0.8 (80%). So we get 16/0.8 which is around 20A.

I believe the output side is hard limited to 35-40A output at around 3V.

I'm reckoning that in a non Temperature Control mode a cell capable of 22A would be OK. I just looked up the amp limits for those cells and it appears to be 20A continuous discharge rating so you should be fine with them. Just make sure the cells don't get out of balance. Ideally they should be balance charged, but if you don't have a balancing charger I'd charge them in whatever you use but finish off the charging in the same slot of your charger.

The closer together they are in final charged voltage the better. It is also a good idea to check the voltage of the cells when they are low. If the two cells have differences of more than a few hundredths of a volt (on a reliable meter), get a new pair of cells. I'd worry about a discharged level difference of 0.2V and would only use those cells in a single cell mod after that.

Temperature Control seems to really eat charge though - current draws seem to be much higher in TC mode. In that case, my personal choice would be Sony VTC3 cells as they are the most capable cells available retail just now. They are quite low capacity but will reliably do 30A continuous discharge. Sony VTC4 cells are less capable but are still higher current rated than any other round cell available - the VTC3 cells will do better than the 4's except in length of use - the VTC4s are 2100mAh, the VTC3s are 1600mAh.

TVF tested battery info to be found here

http://www.preventsuicideapp.com

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17-11-2015 08:57 PM
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TinWhisker Offline
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Post: #3
RE: Maximum Current
You're so cool they should name an Ice Age after you.

I doubt I'd go any higher than 100w with that build, or any. It's just that the Velocity (yes, I cobbled one working unit out of the two cheap clones) doesn't seem to want to wake up until at least 60w, and I figure I'd like to see what 100w is like if I can do it. Thanks as always!! Just didn't want to blow my face off.
17-11-2015 09:00 PM
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TinWhisker Offline
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Post: #4
RE: Maximum Current
That was fun. I've never tried more than 60w before. Fun.

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17-11-2015 09:09 PM
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