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Losing Lipo cells

Posted: Jan 14 2019 4:02pm
by wesleyb82
I collected a bunch of old 5AH Lipo packs that had bad cells, dissemble them and made a 14S2P 10AH pack. All the cells had at least 4AH capacity when discharging at 1C to 3.0v. I charged them all, joined the parallel wires then after 12 hours I connected a 14s BMS ( This is the charger I am using: Finally I am using this wattmeter:

The pack discharge wires connect through the wattmeter shunt to the controller and the wattmeter is setup with a 43v LVC that activates a buzzer to alert of LV condition. The wattmeter is power from ground + #4 (~15v). The charge port connects through the BMS so that the BMS can control the HVC.

After 5-10 uses @ ~4AH per use, all of a sudden after 4AH of usage the wattmeter LVC alarm is going off and is reporting <43v! I took apart the pack and cells 1-4 are reading [3.45,1.15,3.57,3.20] all other cells are 3.95v. My first guess was that there was a bad or weak cell in the #2 position which dragged down the surrounding cells. I replaced the cells with less than 4AH of capacity but after one more use the symptoms have returned, this time the readings are 1-3 [2.59,1.70,0.12] and the rest are 3.88v.

Anyone have any ideas what is going on here? My first thought was the 1-4 cells are powering the wattmeter but the power consumption is obviously extremely low. I usually just stack 6s lipos and use a balance charger and this is the first time using a BMS of this type and hope someone has some suggestions, thank you

By the way this is for a modded MX650, 1800w BLDC, 65A controller, 11t/80t. Taller spring and spacers in the front forks. It hauls. The controller is a neck snapper which makes it a handful. It is wild.
2019-01-07 11.21.51_1080.jpg

Re: Loosing Lipo cells

Posted: Jan 14 2019 4:36pm
by flat tire
Archers, loose arrows! Err, lipo cells! That's a joke, because you used the wrong word. "Loose" is what happens to your wife after she gives birth and it's also a verb meaning "release" typically used in the context of archery. The word you're looking for is "lose" which means to be deprived of something.

Anyway, it sounds like you have bad cells. Only solution is to get rid of them. Also, it's a extremely bad practice to tap a partial voltage even if it just unbalances the pack a little. Much better to get a voltage converter to lower your pack voltage.

Re: Losing Lipo cells

Posted: Jan 14 2019 5:06pm
by wesleyb82
Thank you. I am trying to understand if it is one bad cell, wouldn't the bad cell and it's pair be low but the others would remain at the same voltage? Why would this whole section of the pack sag from one cell? Thank you

Re: Losing Lipo cells

Posted: Jan 14 2019 6:33pm
by liveforphysics
Mechanical stress to the pouches causes cracks in the coating layers that makes self-discharge and gas production in the cells which causes capacity loss when the gas is trapped between layers.

Also, nice bikes! Looks like a blast!

Re: Losing Lipo cells

Posted: Jan 14 2019 11:39pm
by wesleyb82
Based on that information maybe my pack does not allow the cells to swell. I have the cells bound together with light electrical tape within a flexible plastic case which I hoped would allow the thing to stretch a bit but I just took the pack apart and the bad cell was curved from what I assume was a neighboring cell expanding. I guess I need to figure out how to allow movement even though the pack is 14 cells wide. If anyone has any suggestions I would greatly appreciate any input, thank you

Re: Losing Lipo cells

Posted: Jan 15 2019 2:16am
by motomech
(This should be in the Battery Technology section)
I thought the idea was to tape them tight to keep them from swelling.
At any rate, building packs from used ones w/ some bad cells and others swelling sounds like an exercise in frustration. They won't last any time at all w/ the type and rate you are dis-charging and charging.
Unfortunely, Hobbyking hasn 't been keeping lot's of LiPoly on sale like they used to, but some does go on sale now and then. You need to get on their alerts and watch(When broswering HK's awful website go straight to the last 3 battery pages). Look for the biggest bricks that will fit the space(for a reason I'll go into in a moment). The Multistar offers big bricks, 10Ah up to 20Ah, but I'm not sure they could handle the high discharge rates you mention. Maybe the new 12C stuff. I used to get the MS 6S, 10Ah bricks for $50 (free shipping) and make 12S, 20Ah packs for $200!
In the same vein, doing 12S (6S + 6S, or 4S + 4S + 4S) gives more and less expensive buying options. I run some 14S packs using the Zippy Compact 7S, 5.2Ah that were on sale for $50 ea. and 4 make a nice 14S, 10.4Ah pack.
Really, those bikes and your new packs deserve a serious charging system. Get a Mean Well HLG-320-54A, a really solid, potted CC/CV charger(it's really a power supply) that has an adjustable top charge Voltage. If you don't know, LiPoly should be stored @ 3.90V or below and only top charged before use. Not doing this will shorten the service life of the batt.s. Buy one Battery Medic per brick, that why going w/ the biggest bricks possible is way to go. For example, a 2S/2P pack would be only 4 bricks and only 4 BM's would be needed.
Using this adj. power supply and the BM's take care of the HVC and balancing, so if your LV alarm is reliable, you could lose the, often troubesome, BMS.
If you are interested in going this route, I could provide the link to the BM's and provide more details how I do it.
Note; My experience is leaving a device w/ a lighted display hooked up w/out an "off" feature will discharge the pack, sometimes pulling down the 7th or 6th cell down the most(electricity flows "backwards"). I use LCD display voltmeters that really do have almost no drain.

Re: Losing Lipo cells

Posted: Jan 15 2019 8:37am
by dogman dan
Old cells is your main problem. My use of old cells gets limited to .5c, or capacity suffers real bad. Usually this is a slightly puffed cell, but not really puffed. They just lose any ability to handle 1c or more, after about 3 years, regardless if they were used much or not. Yeah, you tested them at 1c, but I betcha you pull 2c from them on the bike, or more.

You just need a big 20 ah bundle of old lipo, to even hope to get 20 amps from them, and even that is pushing them too hard if over 3 years age.

Re: Losing Lipo cells

Posted: Feb 21 2019 6:16pm
by wesleyb82
Thank you for all your input. After replacing the cells in the #2 position 3 times after they sagged to 1v over and over I think I have a BMS that is stuck in discharge mode in this position. Does anyone agree? How would I test this BMS or future BMS's for proper function? Is this condition common?

Re: Losing Lipo cells

Posted: Feb 22 2019 7:48am
by dogman dan
Your bms could be stuck in discharge, to find out, charge full, and just watch it. If your cells keep dropping in voltage, discharging with no load, rather than sagging under load, the bms might be the culprit. Another sign would be the bms is warm all the time.

But your crappy old cells are likely to be discharging themselves anyway. to test that charge, disconnect the bms, then watch em. you might just have internal shorts caused by mechanical damage, or just dendrites that grew.

you start with crappy cells, they will not hold up under discharge, this is just lipo worn out, which I think is your real problem. Right now my best battery is 3 years old. Its shit now, barely handles a 1c discharge, and sags like hell under 2c. When these cells finally kick the bucket for good, its typical for the number one cell to die first. I really think this is your actual problem, your packs are dead.

Spring is a good time to buy new lipo. You want to use em more in summer, and typically by march the last year stock is sold out. When the price goes up, you are looking at fresh stock. Big sale in the spring means last years cells, which got a year of wear on them in the warehouse. Lipo is very sensitive to age IMO. Three years and done, unless stored really well. you just need new stuff.