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06-10-2019, 11:55 AM | #1 |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Uneven Cell Discharge - SMC 3S Lipo
Good Afternoon Folks, A friend of mine recently purchased a True Spec Premium 3S battery from SMC for his stock TRX-4. He is experiencing some oddities with this battery pack. After running a full balance charge and running the truck there is quite a difference in cell voltage. Once LVC kicks in, he'll let the pack cool, and hook it up to his Hota charger. Every balance charge and discharge cycle, Cell 2 is always sub 3V. I told him to reach out to SMC to get their input and they told him this is not a fault battery, pointed out the stock ESC's lipo cutoff, but suggested that he get a lipo alarm, set the cutoff to 3.6V, and see where all 3 cells end up after the next discharge. I suspect he is going to see Cell 2 at the cutoff value and Cell 1 and 3 much higher in voltage. Personally to me, this sounds like a bad cell from the factory and I dont see how the ESC would possibly be pulling more power out of Cell 2 but I'm still quite new to the RC/Lipo game and wanted to collect ya'll input on the matter while he awaits the delivery of the lipo alarm. Thanks! Last edited by CodeEzra_RC; 06-10-2019 at 11:57 AM. |
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06-10-2019, 12:44 PM | #2 |
RCC Addict Join Date: Nov 2014 Location: Nowhere
Posts: 1,326
| Re: Uneven Cell Discharge - SMC 3S Lipo
As far as I understand the LVC is not a "smart" system in that it only knows the total voltage / the number of cells, so you can still end up with that exact situation where one cell is lower than the rest. The ESC cannot target individual cells, cell 2 is just bad so it always discharges faster. The LiPo alarm is needed because it will read the individual cell voltages and start screaming once cell 2 reaches the cutoff, and then you can check what the others were at. An LVC of 3.3V for example(my guesstimate, I don't recall what Traxxas sets their LVC to) is really low in the case of the bad cell, so I don't think that battery will be recovering. Edit: Added emphasis on "smart" (not a dummy for using LVC), and also I believe that it's a bad battery that SMC should warranty. Last edited by Sneetches; 06-10-2019 at 01:47 PM. Reason: Fix |
06-10-2019, 12:50 PM | #3 | |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
I see no possible way the truck is pulling more voltage out of cell 2 than its pulling out of 1 and 3. Would you agree? Sent from my SM-N950U using Tapatalk | |
06-10-2019, 01:31 PM | #4 | |
I wanna be Dave Join Date: Jan 2006 Location: Corruptifornia
Posts: 12,107
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
Edit: Our group has had better battery life and as good or better perfromance from 'cheap lipos' than several more expensive brands. Last edited by Natedog; 06-10-2019 at 01:36 PM. | |
06-10-2019, 01:41 PM | #5 |
I wanna be Dave Join Date: Feb 2009 Location: Detroit
Posts: 2,075
| Re: Uneven Cell Discharge - SMC 3S Lipo
When discharging a lipo through driving, you don't take from each cell individually. SMC is starting to lack the good customer service they used to have... I do not purchase any rc king or smc batteries any more. (I get upset about a .05v difference.) if it take 15 minutes for a battery to "balance out" 1 cell to get level with the others from .05v... the cell is not on par. Your friend needs to get on SMC and light a fire under the customer service groups butt. A ESC or a light bulb could not pull more juice from once cell over another unless the cell is bad. |
06-10-2019, 01:41 PM | #6 |
RCC Addict Join Date: Oct 2006 Location: crawlifornia
Posts: 1,612
| Re: Uneven Cell Discharge - SMC 3S Lipo
I've experienced this with various batteries over the years. If the batteries are new, but they don't want to warranty them due to his concerns, I would just run them conditionally a few more times. By conditionally I mean keep an eye on the split. If it starts coming off the truck consistently at 5-10 tenths of a milliamp different then the other cells, then give it a swim in the salt bath. First thing I would do is go into ESC settings and change the lipo cut-off to the highest setting. It sounds like it might be set a little too low if the odd cell is under 3v. I set my HW1080 to the highest cut-off setting so the batts come off the truck around storage voltage (3.7'ish). I have had batteries get back in line with each other after more usage and good proper maintenance. edit to clarify my position: I also believe the battery should be warrantied by SMC . Last edited by rockhugger; 06-10-2019 at 01:52 PM. |
06-10-2019, 01:45 PM | #7 | |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
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06-10-2019, 01:46 PM | #8 | |
RCC Addict Join Date: Nov 2014 Location: Nowhere
Posts: 1,326
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
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06-10-2019, 02:02 PM | #9 | |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
@rockhugger Unfortunately, he is still running the stock Traxxas ESC. We've got some lipo alarms coming via Amazon tomorrow. Im a bit worried too as I recently purchased some 3S lipos(higher mAh though) from SMC but haven't gotten a chance to run them yet. You guys have any suggestions for future lipo suppliers? Thanks! | |
06-10-2019, 02:08 PM | #10 | ||
I wanna be Dave Join Date: Jan 2006 Location: Corruptifornia
Posts: 12,107
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
Quote:
I understood you, but thanks for clarification. | ||
06-10-2019, 02:24 PM | #11 |
Moderator Join Date: Sep 2008 Location: PA
Posts: 13,922
| Re: Uneven Cell Discharge - SMC 3S Lipo
Yet another SMC battery issue. I've heard far too many lately. They used to be known for quality, but that seems to have changed.
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06-10-2019, 02:34 PM | #12 |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo
Ya'll are making me more and more nervous with each post. I own 2x 7400mAh 2S and 2x 5400mAh 3S from SMC. Ive never noticed an imbalance on the 2S with maybe 20 cycles on them. Haven't had a chance to run the 3S yet. No puffing(so far). I make sure to run them through a charge(4.19V) and discharge(3.8V) cycle once a week and always store @ 3.8V in lipo bags and an ammo case. To add to that, I feel even worse for suggesting SMC to my friend. |
06-10-2019, 02:49 PM | #13 |
RCC Addict Join Date: Oct 2006 Location: crawlifornia
Posts: 1,612
| Re: Uneven Cell Discharge - SMC 3S Lipo
You're doing good battery maintenance practices for sure. |
06-10-2019, 03:16 PM | #14 |
I wanna be Dave Join Date: Feb 2017 Location: My mothers basement
Posts: 2,128
| Re: Uneven Cell Discharge - SMC 3S Lipo
The cell that is always low probably has a higher internal resistance, that would cause it to lose power faster. If you have or know anyone with a battery internal resistance meter i would say check that, its good information to report back to SMC with.
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06-10-2019, 04:24 PM | #15 | |
Quarry Creeper Join Date: Dec 2016 Location: Cawston, British Columbia, Canada
Posts: 477
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
Just to add a liitle info to what voodoo says - anything under 6 milliohm internal resistance per cell is like brand new. 7 - 12 milliohm per cell is pretty common for a well used/maintained battery. 13 - 20 is a battery getting tired and at the end of its life. Over 21 milliohm and the battery is done for me. | |
06-10-2019, 05:12 PM | #16 | |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
Sadly no IR tester. I could probably nab a decently sized resistor and do it the hard way. My friend got a response back from SMC and they are willing to refund if he returns it but still sees it as a non-issue do to the small variance in a cells mAh capacity and IR. Ill agree on the theory but I would assume a .4-.6V ending discharge difference would indicate a substantial difference in Cell 2's IR versus the others. | |
06-10-2019, 05:20 PM | #17 | |
RCC Addict Join Date: Oct 2006 Location: crawlifornia
Posts: 1,612
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
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06-10-2019, 05:27 PM | #18 |
Newbie Join Date: Apr 2019 Location: Palmerton, PA
Posts: 38
| Re: Uneven Cell Discharge - SMC 3S Lipo We both have HOTA D6+ chargers. I would have bet money that there was a screen or menu in there somewhere that showed it but I'm not @ home to check and the manual I found online only notes that it checks IR during balance charging but no mention of a graphical output of it.
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06-12-2019, 02:26 AM | #19 |
I wanna be Dave Join Date: Feb 2017 Location: My mothers basement
Posts: 2,128
| Re: Uneven Cell Discharge - SMC 3S Lipo
Some of the new sky rc charges can check ir, mine is something like a d100 v2 it was $100. I think they have a smaller one for $50 that checks as well. Sent from my SM-N950U using Tapatalk |
06-12-2019, 05:53 AM | #20 | |
Moderator Join Date: Sep 2008 Location: PA
Posts: 13,922
| Re: Uneven Cell Discharge - SMC 3S Lipo Quote:
Here's a battery tester including IR for under $15. https://www.ebay.com/itm/291596586816 | |
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