Half my 6s Lipo brick goes out of balance quickly. advice?

[Kin]

Ultimately, you're right. I guess it needs to be taken in mind, though, that the cost is just just $$$/wh but rather $$$/wh * (1.1) [10% to account for duds],


How unethical (and perhaps unfeasible) would it be for me to purchase two more 6s 5ah 20c Lipos from Hobbyking, hopefully get good ones, then RMA the original duds they gave me...?

 

[ZOMGVTEK]

I've purchased 34 5AH 6S 20C packs and 100% were essentially perfect out of the box. The biggest voltage difference I had was maybe .05V, most were .01-.02V...
If you ask me, 10% extra is a bit high... If you order 50 packs, you probably should get 1-2 extra, but 5 is probably excessive.

I'm assuming they actually use the little warranty sticker on the side of the pack. You would need to swap the old packs with the new stickers, and they better be in nearly flawless condition. You're supposed to let them know about duds within 24 hours of receiving, correct? I don't see any big moral issue with trying this, its only 2 packs. What's the worst that can happen, they reject it and keep your 2 duds?

 

[Kin]

Yeah, I decided its not worth the annoying twinge of dishonesty to try and buy more packs and RMA them.

On the upside!! One of my packs (the slightly better one) seems to be good until 3.6v, I'm watching if it gets drastically out of balance from 36-3.5. If it's good until 3.5...and just goes haywire from 3.5-3, then perhaps I can consider it full 'good' since I should generally never dip below 3.5v.

The other one is still sucking, but I'll keep testing. At 22watts discharge with my Eco6-10, and like <5watts with my battery medic, this takes a while.

 

[wesnewell]

I intended this light cycle to be a full 3.0-4.2. I did charge first, and I should clarify, for the post, that my initial pack was in balance, discharged, out of balance, rebalanced, and discharged again (and out of balance again).

At 3.6V you're very close to 100% dod. You can't expect lipo to stay in balance below 3.6V. It just isn't going to happen, at least not anywhere close to 3.0V which is way over discharging it and will cut lifespan tremendously. Someone correct me if I'm wrong but I'm pretty sure you hit 100% dod somewhere around 3.55V

 

[Kin]

I intended this light cycle to be a full 3.0-4.2. I did charge first, and I should clarify, for the post, that my initial pack was in balance, discharged, out of balance, rebalanced, and discharged again (and out of balance again).

At 3.6V you're very close to 100% dod. You can't expect lipo to stay in balance below 3.6V. It just isn't going to happen, at least not anywhere close to 3.0V which is way over discharging it and will cut lifespan tremendously. Someone correct me if I'm wrong but I'm pretty sure you hit 100% dod somewhere around 3.55V

Uh, 100% is 3v for general 300 cycle operation range, and 4.2 the max [In extreme i.e very limited cycle you could do 2.8 to 4.35 but that is frankly stupid]

Where you are correct, I think, is that 3.6v *is* the majority of the charge. I believe 3.6v-4.1v is considered an extended-range discharge profile, so what most of the LiPo people here do, so their packs last long and stay in balance. It represents something like 75-85% of the capacity.

It looks from my tests that after 3.6v *rapidly* the voltages of the three "bad" cells in the "good-dud" drop to 2.8. Which kind of sucks. But, I charged up to 3.6v, and I'm testing how much capacity from 3.6 to 4.1v I'll get. Then, I'll consider the 5ah pack to be that. [Looks like it'll be about 2.8ah :/]

Looking at discharge curves it looks like the voltage should be plummeting from 3.4/3.3v -3v, not from 3.6v. (e.g, http://prototalk.net/forums/attachment.php?attachmentid=13&stc=1)

 

[miuan]

Looking at discharge curves it looks like the voltage should be plummeting from 3.4/3.3v -3v, not from 3.6v. (e.g, http://prototalk.net/forums/attachment.php?attachmentid=13&stc=1)

The curve refers to 6C discharge. Few of us do that on an ebike. At fast discharge, voltage sag means you can no longer tell the state of charge just looking at the voltage.
When talking voltage vs. SOC, you should always consider resting voltage. While in the above curve 3.6V happens before 50% DOD, in my packs 3.6V resting voltage is pretty much 90-95% empty.

 

[Kin]

Looking at discharge curves it looks like the voltage should be plummeting from 3.4/3.3v -3v, not from 3.6v. (e.g, http://prototalk.net/forums/attachment.php?attachmentid=13&stc=1)

The curve refers to 6C discharge. Few of us do that on an ebike. At fast discharge, voltage sag means you can no longer tell the state of charge just looking at the voltage.
When talking voltage vs. SOC, you should always consider resting voltage. While in the above curve 3.6V happens before 50% DOD, in my packs 3.6V resting voltage is pretty much 90-95% empty.

That's actually extremely valid point, I completely overlooked that. Perhaps I could find a better curve. I am still surprised that represents 90-95% empty, but it helps. It doesnt change to some extent the fact I can only pull out ~3ah from my 5ah pack before they hit 3.6v, but it does give me a better picture than I had before about what I actually have.

 

[wesnewell]

I was talking about resting 3.6V, not loaded with a 6c discharge. Yesterday I charged 29000mah into a 30000mah pack that was at 4% charge. IIRC, the cell voltage was either at or just below 3.6V.

 

[ohzee]

I did not want to start my own thread so just tried to find a similar one to ask a question.

Currently I have 18s4p 25c zippy flightmax that have been solid. Today after charging I was just double checking
to see how balanced they were. All looks good minus one pack. All cells were 4.15 and one was 4.0. I hooked it
up to the charger to balance and the low cell shot right up to 4.15 right away , but once the charger would balance
it went back to 4.0. I could even repeat this same behavior.

I am about to go take a ride and going to keep it on a cell log so I can see what happens under load. Sound like a
bad cell ? My concern now is it goes over the cliff and I have issues. Depending on what happens after this ride I may
order a new battery and swap it.

thanks in advance for your knowledgeable advice.

 

[Kin]

Hi Ohzee, I also added your reply on the first page, assuming that's cool. I haven't done much with this pack since the original situation. I'd been running it, just not pack 3ah discharge. That's running the regular cells at 60% (yay) but the crap cells at 90% (oh well). If you have a charger that discharges without balancing until the end, you can try to discharge the cells and see how many Mahs are put in before the "bad" cell reviews its dropoff. It does sound like it's a weaker cell, but I'll leave it up to those who know more to say more.

 

[ohzee]

Thanks Kin I very much appreciate it.

Did a nice 30 mile trip today used about 14ah out of my pack. Problem battery I ended up disconnecting mid trip as the bad
cell dropped to low. I put a slight charge on it all cells are 3.88 minus the bad cell is 3.62.

Once I am doing bulk charging all my good cells I will do some discharge/charge testing. Still not looking good come payday
I will order some new batteries.

My pack is taped up pretty well so hard to tell if the cell is puffy or not.