Weak Cell

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Re: Weak Cell

Postby steves1977uk » 03 Nov 2021, 22:43

Only under load it will show the correct mOhm reading, so connect your dump load and measure each cell.

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Re: Weak Cell

Postby expresso » 03 Nov 2021, 22:54

the item i linked to from amazon is not good ?

or just under load i have to use it ?

i could use the pl8 with aligator clips also ?

i hate to have another gadget in the closet and never use but it can be useful and faster easier to check i think

i just cleaned out my closets and threw a bunch of shit out - tires i dont use headrests for chairs i dont own etc, i need to keep my place organized - dont want to collect things i may not use or just use once

if i can do it with the PL8 - connect dump load and look at the IR tab on the PL 8 -

but to check each cell loose etc, then the meter would be best and easiest to do - i have it saved - will see if i order it -

would this work ? for what i want to do - instead of the ones on ebay china etc,


https://smile.amazon.com/dp/B07WS7HR31/ ... _lig_dp_it
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 03:05

That things no use.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 03:07

which one - the one from amazon or the other one ?
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 03:09

Amazon doesent sell impedance testing tools. Its NOT a volt meter.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 03:19

Burgerman wrote:Amazon doesent sell impedance testing tools. Its NOT a volt meter.



the link i posted is from amazon

is that useless for what we need ?


the one shirley got and is using - is that useless also your saying ?
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 03:46

Get shirleys one. But its a tool. Its also useless unless you can understand what you are testing and why.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 03:50

yes i know - - the reason this came about is because of the IR -

that tool looks like it easy to check the IR of a cell fast without anything else

i am undecided because i hate to buy more tools i dont use or may use just once - but it does seem useful when needed

unless theres another way i can check the IR on the FLY or with PL8 ?

looks to work the same as a volt meter - nothing to really do but turn it on and touch the cells - giving you the volts and IR of that cell

isnt that a good thing
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 04:00

If you LEARN ohms law you can measure it with a multimeter and your load.

You can get an IDEA with the PL8.

You can get a half decent internal resistance tester for 600 uk pounds.
Or a cheap chinese one like this https://www.ebay.co.uk/itm/233774610738

But those are not needed You dont need an accurate figure, your cells go high resistance its obvious when you charge, drive, etc. With a VOLT meter. You have one of those to plug in that shows your cells right? Then accelerate and any cell that drops its voltage severely more than the rest, as you do so, is ready to throw away. Or learn to read the screen on your charger. It shows this problem compared to other cells as you charge, or discharge.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 04:15

using a load dump as i have connected to the PL8 would show me which cell drops fast or faster

thats how i figured something was wrong before also -


i hold off then buying anything - but does seem useful if you do build alot of pack and just want to check quickly on the fly with nothing else
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 04:26

Yes. But costs£££


This is from the PL8 Manual, I just dug it up...

Internal Resistance tab
The Int. Res. tab displays cell internal resistance when charging LiPo and A123 balanced packs. Internal resistance is not calculated or displayed during discharge cycles.

Note: Internal resistance can only be measured on packs charged from below 75% of capacity. It takes less than 3 minutes to calculate internal resistance. Internal resistance can only be calculated during charging, and only when the pack is connected using both the balance wires and discharge wires.

Viewing individual internal cell resistance enables you to more accurately evaluate pack quality and condition. If all cells show a low internal resistance, that’s a good indicator that the pack can be charged and discharged at a higher C rate. Here are two examples:

(Note to expresso, the hobby packs we use are not 1C or 2C but anything up to 150C! -- burgerman)
A pack rated at 5C and 40–50C discharge may have an internal cell resistance of 2 milliohms. This low resistance allows larger currents into and out of the pack with-out generating excessive heat.

(Note to expreso, the hobby packs talked about below may be 3Ah not 200... And LIPO - Burgerman)
A pack rated at 2C charge and 20–30C discharge may have 17 to 20 milliohms internal cell resistance. The higher resistance prevents the pack from charging or discharging as quickly, and the larger resistance will generate more heat during those operations.

A cell poorly matched to the other cells in a pack will show a substantially different internal resistance. Also, a cell with high internal resistance compared to other cells may indicate the cell is weakening, which can degrade overall pack performance.

Be aware that individual cell resistance will vary during charge. This is normal and results from the different voltages and currents applied to the pack during charging.
Use the PowerLab 8 as a tool, measuring and record the internal resistance of your packs over time. Look for trends that would indicate the IR of a cell or a pack is increasing over time. This is a good indication that the pack is aging or has internal damage and should be replaced eventually.


PLEASE read this at least 3 times. Ask questions if you do not get any part of it! This is all that you need.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 04:35

i have to do that - from now on - when charging - check the IR with the PL8 -


could be the cell i was charging - was a Low C rate and i charged it at 25A - too much for that cell but the IR was 78 - so way out there

i dont care about those cells - i want to just see if i can connect to the pack to check the rest of the pack till i get new order in - next year

maybe good idea to slow down the recharge on these things -

i got used to fast charging - maybe not a good idea with these cells - fast charging me for me is about 30A - no less than 20A -

but a larger cell 100ah - 200ah - should handle that easily - even if its an 1C rate cell - lower charge rate - 15A may be useful for less heat at least - if someone dosnt mind the wait - or does it overnight -
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 04:39

i wasnt worried with the headways - there are better Cells -

i dont know or think these cells will last as long as the headways - mines are still going after 6 years - will last 10 for sure - since i stopped using that chair now - this past summer is the last i will use it -

only as a back up to the backup - or else its sitting there - i charged it up and removed some - about 20ah from it - next time i recharge it will be next year some time -
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 04:44

could be the cell i was charging - was a Low C rate and i charged it at 25A - too much for that cell but the IR was 78 - so way out there


Thats not the problem. Its an 80Ah cell. If its 2C discharge its at least 1C charge. Thats 80Amps. Even if its a 1C discharge, then charging at .5C (40A) should be fine. 25A was only too much because the cell was dead. Probably manufactured faulty and sold to some reseller. Who knew perfectly well it would fail.

Incidentally, on my lipo heli pack which is 4Ah (4000mAh) I charge at 10C or 40A all the time. 40A and its as big as a pack of 10 cigarettes. Thats a very high C rate cell. I30C peak. So I can draw 4Ah x 130C = 520 amps peak from it with maybe 15% voltage drop. So when used in my heli at a more sane 150A max, the voltage drop in negligable. That tiny battery can start a V8 in mid winter. All because its really really low impedance.

Heres a cheap turnigy nano tech LiPo bomb...
Its small enough to almost hide in your hand. It is 4.4Ah. But can generate 65x 4Ah = 286A continuously. And DOUBLE that for a couple of seconds. And 15C charge capable. Thats 66 Amps charge.
https://hobbyking.com/en_us/turnigy-bat ... xt-60.html
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 04:59

i reached out to the seller just to let him know - i didnt expect anything

was told it was new - it was user issue - and they dont have them anymore -

cell looked brand new to me - QC sticker - everything -

i am sticking to my GTK cells for this pack - i want to use the same Cell i have already - i dont like the seller but only one who has this cell
size phyiscal size i mean - and specs show a good C rate - so if true - not worried

like i said - i never felt heat coming from the cell during a charge or discharge like i did with this one - and i always charged them up at 20A at least - or else i be here for ever

just the IR says it all - i was a bit excited that it charged up first - and discharged down to 80ah - then on the recharge - got hot

reason i dont like to leave it alone when i do this - but didnt think nothing since never had this happen this way before - i had to step away for a few hours -

when i got back - and was checking it - was at 65ah - but i felt alot of heat - without even touching it - then i touched it - and was very hot - seemed like it swelled some - - anyway - done - will be in the trash soon

i though i was seeing things hard to tell but the cell on top didnt sit flush on it as i recall it did before - could be mistaken -

either way - it warped my hard desk mat i had - now its not flat anymore on the end - got me thinking of a chair pack - if they get that hot - little bit of concern there
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 10:55

cell looked brand new to me - QC sticker - everything -

They take old or used cells, and recover, new tops, new stickers etc. So the outside looks new.
Or the cells are still looking brand new but are in fact used, usually with a cell destroying BMS.
Or they take brand new but failed cells that didnt pass testing etc out of the back door. This happens for e.g. the room where they were assembled had too high humidity. Then the cells swell under load. Theres a lot of money to be made by the unscrupulous in china as theres millions of used cells, back door non A grade cells, etc around.

One of the best ways to detrmine health is internal impedance. Resistance. Impedance is just resistance measured with low frequency AC load. And a GOOD meter capabe of doing so isnt cheap. But if you know ohms law and have a load, and measure current. And measure voltage drop. You can accurately measure DC resistance yourself. But you must have something to compare that to. Or you do not know what is good/bad. Most large cells will be around 1 to 2mOhm. Maybe 5 with connections etc. If its more like your 78 then you have big problems.

The PL8 is reasonably good at doing this for you. But its only really a guide, as its very very hard to measure low resistances accurately. And you must really use short cables like 10 inches. Or you will always be measuring cable resistance on cells 1 and 8 as well as cell resistance. So again its useful, but you MUST understand how that works and be able to make sense of what you are seeing.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 16:24

i dont know ohms law - and the cables i have been using to charge etc, not short - in fact i made it longer - i needed a longer cable

mines may be now 5 feet that i used to charge etc, thats the reason Cell 1 and 8 was a bit more then the rest when i charged my 6 year old pack the other day - Cell 1 and 8 was about 2 - the rest were way less -


take a look at this - cell 1 - weaker of the bunch - this is an old 6 years 12ah cells headways - 36ah pack thats been used and abused

this was lent to my friend at the start to extend with lead - she used it for a year this way or more - made her decide to go full chair after experiencing this little ADD ON -
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 16:42

what is it about Cell 5 ?

i noticed on my packs - not just one of them - so far about 3 - seems like Cell 5 is always the lower cell when charging -

is there a logical reason for that ?

these are 6 year old headways - 105ah - 45ah and 36ah - both ADD ONs were used and abused - 36ah for sure was used hard

45ah was used with the 105ah for a few years was never drained low enough this way - the 105ah was used all the way when i had it alone the first year - then i realized i needed more - and 45ah was added

now i dont use the ADD On anymore - other than the bounder -

tomorrow will check on my 200ah in my new chair - been sitting for some time
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 17:42

No logical reason. Cell 5 is just lucky... You cant tell much on resistance on long charge cables. That is meant to be used with short cables and the battery plugged in about 6 inches or so from the charger. Because cables have more resistance on 1 and 8 than the cells themselves. With long wires only cell 2 to 7 will be accurate.
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Re: Weak Cell

Postby ICEUK » 04 Nov 2021, 18:29

Burgerman, That's really interesting what you said about higher resistance on cells 1 and 8 due to long cables, how does the PL8 measure the resistance, on the positive and negative cable not the balance cables?

My pack has higher resistance on cells one and eight,
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Re: Weak Cell

Postby LROBBINS » 04 Nov 2021, 20:33

The PL8 shows the following internal resistance values during charge for my 8S 200AHr pack:
cell 1 - 0.2 mOhm
cell 2 - 0.3 mOhm
cell 3 - 0.3 mOhm
cell 4 - 0.7 mOhm
cell 5 - 0.2 mOhm
cell 6 - 0.3 mOhm
cell 7 - 0.3 mOhm
cell 8 - 0.1 mOhm
and these values are consistent from charge to charge. All are under 1 mOhm and probably none too accurate for such small values, but this is what a good pack should more or less look like. BTW- the balance leads are the usual length and the PL8 to chair anderson lead is ca. 1/2 meter long.
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 20:41

very nice pack :thumbup:

would you have a link to those cells ? in case i need to get some in the future

thanks
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Re: Weak Cell

Postby LROBBINS » 04 Nov 2021, 21:07

Bought on Alibaba from Shenzhen Youngpower Technology Limited, Andy Chen
https://www.alibaba.com/product-detail/lifepo4-battery-cells-LS-3-2v_62566756886.html?spm=a2700.galleryofferlist.normal_offer.d_title.261811251VmS05
(Current prices are likely to be higher than in this link.)
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Re: Weak Cell

Postby expresso » 04 Nov 2021, 21:15

thanks - good to know

how tall was it total - if you recall - the studs were installed already welded on top ?
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Re: Weak Cell

Postby LROBBINS » 04 Nov 2021, 22:29

M6 studs already installed - I don't know if welded or otherwise. I don't recall the height, but I'll try to remember to measure one tomorrow.
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 23:34

You frequently get more than. 0.1 ohm just on a test meter/multimeter test lead. So you need very good clean connections, terminals etc to get readings as good as lenny's show. All the plugs and connectors add or CAN add a fair bit of that. Esp on 1 and 8.
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Re: Weak Cell

Postby LROBBINS » 04 Nov 2021, 23:42

My el cheapo multimeter and old, beatup, oxidized leads give me ca. 1 to 2 ohm indicated. 0.1 ohm = 10 mOhm is probably John's Fluke. The PL8 seems to be pretty reliably reading down in the mOhm range. All of my battery and balance wire ring terminals are crimped and soldered and they're on M6 studs with tight Nyloc style nuts and no loctite.
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Re: Weak Cell

Postby Burgerman » 04 Nov 2021, 23:53

My el cheapo multimeter and old, beatup, oxidized leads give me ca. 1 to 2 ohm indicated. 0.1 ohm = 10 mOhm is probably John's Fluke.


If my multimeter read 2 ohms on shorted leads I would throw it away! :shock:

But yes my fluke. If set to "low" ohms, it measures very low resistance accurately. One of the advantages being that you can measure battery loom or motor resistances. So to set compensation correctly as a good starting point. And test your full battery charge loom, or spot bad terminal connections on a pack. People wonder why decent fluke test leads cost so much.

I can measure motor test leads zeroed) on low ohm setting, at say 6 or 7 different armature positions, and have the meter read instantanous, averaged, upper peak, and lower peak resistances. And end up with extremely accurate reading for motor mOhms.


Low ohms (LoΩ): allows you to measure lower resistance in circuits more accurately through compensating for the natural resistance in the probe leads, as well as through high resolution. The 289 has its own low ohms (50Ω) range position.


Quick review http://testmeterpro.com/fluke-289-review/
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Re: Weak Cell

Postby shirley_hkg » 05 Nov 2021, 01:48

LROBBINS wrote:My
0.1 ohm = 10 mOhm is probably John's Fluke. .




0.1Ω is 100mΩ drunk2
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Re: Weak Cell

Postby ICEUK » 05 Nov 2021, 10:15

LROBBINS wrote:M6 studs already installed - I don't know if welded or otherwise. I don't recall the height, but I'll try to remember to measure one tomorrow.



Yes it would be interesting to kmon overall height. I have looked at those cells in the past but you get told different sizes.
How did you do you put the packs together tape? Case etc?


Thx In adavance
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