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Old 06-10-2019, 11:55 AM   #1
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Default 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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Old 06-10-2019, 12:44 PM   #2
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Default 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
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Old 06-10-2019, 12:50 PM   #3
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by Sneetches View Post
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.
I agree with the lvc being "dumb" but disagree with SMCs assessment that there are no issues with the battery.

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?

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Old 06-10-2019, 01:31 PM   #4
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

Quote:
Originally Posted by CodeEzra_RC View Post
I agree with the lvc being "dumb" but disagree with SMCs assessment that there are no issues with the battery.

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?
Yes, I agree. Bad cell in the lipo. I've got 4 very new GensAce 3s hardcase lipos that are the same way, they just outside warranty period (only about 10 runs on them) and they told me have a nice day, here's 10% off coupon for next purchase lol. Not buying from them again. Good luck with the 'warranty', hope you have better luck than I did. The punch is less than other brands too.

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.
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Old 06-10-2019, 01:41 PM   #5
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Default 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.
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Old 06-10-2019, 01:41 PM   #6
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Default 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.
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Old 06-10-2019, 01:45 PM   #7
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by Natedog View Post
Yes, I agree. Bad cell in the lipo. I've got 4 very new GensAce 3s hardcase lipos that are the same way, they just outside warranty period (only about 10 runs on them) and they told me have a nice day, here's 10% off coupon for next purchase lol. Not buying from them again. Good luck with the 'warranty', hope you have better luck than I did. The punch is less than other brands too.

Edit: Our group has had better battery life and as good or better perfromance from 'cheap lipos' than several more expensive brands.
Thanks for the confirmation NateDog. I'll relay the general consensus-so far and tell him to push back on SMC to see what they'll do.
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Old 06-10-2019, 01:46 PM   #8
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

Quote:
Originally Posted by CodeEzra_RC View Post
I agree with the lvc being "dumb" but disagree with SMCs assessment that there are no issues with the battery.

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
I just realized how that sounds without some airquotes so to clarify that's "dumb" as you mentioned, I don't mean anyone is dumb for using it, just that the system is only counting total battery voltage / cell count as a catch-all.
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Old 06-10-2019, 02:02 PM   #9
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by Sneetches View Post
I just realized how that sounds without some airquotes so to clarify that's "dumb" as you mentioned, I don't mean anyone is dumb for using it, just that the system is only counting total battery voltage / cell count as a catch-all.
@Sneetches No worries Sneetches, I knew what you meant.

@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!
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Old 06-10-2019, 02:08 PM   #10
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

Quote:
Originally Posted by rockhugger View Post
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.
Yes! I've upped my esc cut-off voltages after running and talking with Rockhugger, doesn't seem like we should have to, but it helps.

Quote:
Originally Posted by CodeEzra_RC View Post
Thanks for the confirmation NateDog. I'll relay the general consensus-so far and tell him to push back on SMC to see what they'll do.
You're welcome, I don't buy SMC for couple years because I had 4 hardcase 2s all puff in about 1 year, even with balancing, storage charge etc.

Quote:
Originally Posted by Sneetches View Post
I just realized how that sounds without some airquotes so to clarify that's "dumb" as you mentioned, I don't mean anyone is dumb for using it, just that the system is only counting total battery voltage / cell count as a catch-all.
I understood you, but thanks for clarification.
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Old 06-10-2019, 02:24 PM   #11
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Default 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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Old 06-10-2019, 02:34 PM   #12
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Default 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.
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Old 06-10-2019, 02:49 PM   #13
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

You're doing good battery maintenance practices for sure.
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Old 06-10-2019, 03:16 PM   #14
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Default 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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Old 06-10-2019, 04:24 PM   #15
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by voodoobrew View Post
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.
like!

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.
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Old 06-10-2019, 05:12 PM   #16
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by Voodoobrew View Post
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.


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.
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Old 06-10-2019, 05:20 PM   #17
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by CodeEzra_RC View Post
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.
what charger do you have? a lot of chargers have IR meters just like they have votage meters. Just look through the menu
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Old 06-10-2019, 05:27 PM   #18
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by rockhugger View Post
what charger do you have? a lot of chargers have IR meters just like they have votage meters. Just look through the menu
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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Old 06-12-2019, 02:26 AM   #19
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Default 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.

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Old 06-12-2019, 05:53 AM   #20
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Default Re: Uneven Cell Discharge - SMC 3S Lipo

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Originally Posted by CodeEzra_RC View Post
To add to that, I feel even worse for suggesting SMC to my friend.
That's exactly how I felt! I recommend SMC to a friend because of its long standing reputation and I personally don't even own any of the batteries. Two out of his 4 were junk after only a few cycles and SMC's policy is "GFY".

Here's a battery tester including IR for under $15.

https://www.ebay.com/itm/291596586816
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