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 Aug 01, 2012, 07:04 PM Space Coast USA Space Coast Joined Oct 2000 21,588 Posts Hi Norm, To make an IR meter you just need a known resistance and a volt meter. http://www.rcgroups.com/forums/showp...&postcount=272 I used 100W, 2 ohm resistors I got off the internet from China. IR Vu = unlaoded V Vl = voltage under load IR = (Vu - Vl) x R/Vl Don't forget to divide the answer by the number of cells in the pack. The IR will probably not match other meters but, it can be used to effectively grade your own packs. Mine varied from 8 mohms to over a 100 mohms.
Aug 01, 2012, 11:15 PM
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Quote:
 Originally Posted by hoppy Hi Norm, To make an IR meter you just need a known resistance and a volt meter. http://www.rcgroups.com/forums/showp...&postcount=272 I used 100W, 2 ohm resistors I got off the internet from China. IR Vu = unlaoded V Vl = voltage under load IR = (Vu - Vl) x R/Vl Don't forget to divide the answer by the number of cells in the pack. The IR will probably not match other meters but, it can be used to effectively grade your own packs. Mine varied from 8 mohms to over a 100 mohms.
Oh, that's easy. I'm embarrassed to admit that I'm an electrical engineer and should have been able to figure this out myself

I read the post you referenced but I don't know anything at all about LiPo chemistry; do you know if it matters how much current you're pulling or if the IR changes as the cells heat up? I've got a 47ohm 5W load but that's not going to pull much current, so its probably not too good for this. Guess its time to pick up some power resistors.
Thanks a lot.
-norm
 Aug 02, 2012, 07:48 AM Space Coast USA Space Coast Joined Oct 2000 21,588 Posts My tests at 10A and 20A showed little difference in the IR. My setup can give me either a 1 or 0.5 ohm load. A 1 ohm resistor would yield a 12A load with a 3s pack. That would be 24C for a 500mah pack and a 3C load for a 4000mah pack. Using the 0.5 ohm load would increase that to 6C with the 4000mah pack. The IR meter uses a 16A load. I assume the chargers with built in IR circuitry use less than a 5A load. Good luck.
 Aug 02, 2012, 07:49 AM Singapore, Singapore Joined Apr 2012 424 Posts I do believe that battery manufacturers specification sheets state AC ohs as the ohms is measured using AC 1khz. There may be a slight difference by using DC ccts.
 Nov 02, 2012, 10:15 PM Registered User Canada, ON, Toronto Joined Oct 2012 11 Posts Hoppy and others, thanks for posting and maintaining all this information on LiPos, chargers and their care. For a newby like myself just getting into RC and specifically planes, remember that there is a lot to take in. Glow engines are not this complex! I have been crazily reading stickies and boning up on my electrical math. The stickies on how to prevent a campfire while charging your LiPos was most enlightening (literally). These batteries are like nothing I have ever used, and their care and feeding have no similar counterpart with any other common battery type, and this may be the problem. To treat LiPos like you do other batteries might burn your house down. I continue to madly read but not post just yet.
 Nov 03, 2012, 07:27 AM Space Coast USA Space Coast Joined Oct 2000 21,588 Posts Thank you. Enjoy the hobby, it's a great time to be getting into electric flight.
 Nov 26, 2012, 10:37 PM Corflute Codger Australia, New South Wales, Fernmount Joined Feb 2011 581 Posts Very helpful info. Have managed to salvage one battery so far and am about to resurrect some more. Thank you all
Feb 13, 2013, 04:23 AM
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Ukraine, Crimea, Bakhchysarai
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hi. I have 3.7 battery that unlike other 1 cell batteries have 3 contacts. One is +, another is -, but can't understand what is 3rd one b/n them.