Is a lowered rest voltage is a sign of bad or worn out battery ?

[Jeff23spl]

Hello
I'm playing with lithium cells for a while, harvesting and building packs thinking i know the base stuff and safety but i would like to go a step further:

I do IR test and capacity discharge test but sometimes a cell is testing ok for IR and capacity but once fully charged to 4.2v,
it rest at 4.10-15 after a while and some other (different brands/models) rest at 4.18-19.

Curiously i experience this on older cells that could have more wear and i wonder if it is a sign of that wear or it is just a coincidence with cell construction seating slightly lower than another ?
I do see some at 3.6v nominal instead of 3.7 but datasheet still talk about 4.2 fully charged for both....???
I know used cells could be self discharging but for the one I'm talking about, they drop to that lowered voltage quickly and remain there for weeks/month so it isn't like a cell that self drain in a couple days or less...

I did see worse with lifepo4 A123 26650 that seat at 3.30-3.4v instead of 3.65 but i know they are at 75-80% capacity remaining. (those well known 12s8p modules) I though it was related to the type of cells but i got some less used one that seat a bit higher at 3.45v instead... (don't worry, i didn't mixed them with the others) I did never try a brand new A123 to see if it would rest at 3.6-65 or less, those are more often avalable at 10+ years old and work great even with 20-25% capacity lost.

I wonder if for those cells, we should charge them accordingly? I mean, should we lower the max voltage at which the charger stop ?
Is the cell keep gaining capacity above that rest voltage or just make heat? It was fine with older chemistry to overvolt them to fully charge (pB,nicd/nimh) but i was under the impression that lithium didn't need this? I'm i right

I ask because i lowered the charge voltage with my A123 pack used for home electricity backup. I have a +/-3kwh 16s32p connected to inverter supplying emergency loads.
The inverter has annoying cooling fans and i look for ways to hear them less often. I did assume it may constantly supply extra voltage to the cells to bring them to 3.65 generating more heat for the fans. I lowered the ''charged'' voltage from 58.4 to 55v to help it but the changes are not obvious to see improvements on grid bill or on the fans duty cycle....
I figured out my inverter stay active and supply the load from the DC bus/batteries even if grid is ON without swithing. Furniture clocks never reset but it generate more heat that a system with a transfert switch and run the fans more often...(It is located beside my work counter.)
Was it a good idea to lower the voltage or i just lose capacity from my pack by doing this ?

Also i just bought a Tesla S module and would like to have a quick idea if it is a good deal or if i bought worn out cells, before i break it appart to harvest the cells. i don't have the gears to capacity test a 6s74p (I could patent one but it would take forever to drain 225-250Ah)
This bring me to ask if looking at resting voltage is a sign of wear or just a variable that doesn't matter ?
That pack came well balanced at 3.7per cell, i charged it to 4.2 at 15a rate (ISDT air 8 balance charger). The cells lowered to 4.15v after a short time. Since i don't know exactly how the ISDT figure out the state of fully charged and i did miss the time it stop charging, i made another attempt to charge but at only 1.4a to help with balancing and make sure it won't stop too soon.
And same: I came right after i heard the ''charged'' signal. The instant rest voltage was 4.17-18v but very soon (2-3hours) it returned to 4.15 again.
I was told the pack is 2014 and the car was still displaying range in km around 86-87% of the original range.
Is it a bad sign, or just normal ?

 

[BatteryMooch]

Cells with higher IR (older/damaged or just high-IR when new) will have a greater temporary voltage rise when being charged. This is the opposite of the temporary voltage sag we see when discharging.

This greater voltage rise can cause a charger to stop a bit earlier, thinking that the cell is full. This means the cell will drop down to a lower true “resting voltage” once it settles.

It can be a way to sort out lower grade or damaged cells but it can also be caused by bad spot welds, inaccurate voltage readings for different channels/slots, etc.

It’s not a big issue unless the differences between the fully charged resting voltages of the cells in your packs are getting wider and/or the BMS is spending a lot of time balancing the pack.

 

[Jackal21]

Cells with higher IR (older/damaged or just high-IR when new) will have a greater temporary voltage rise when being charged. This is the opposite of the temporary voltage sag we see when discharging.

This greater voltage rise can cause a charger to stop a bit earlier, thinking that the cell is full. This means the cell will drop down to a lower true “resting voltage” once it settles.

It can be a way to sort out lower grade or damaged cells but it can also be caused by bad spot welds, inaccurate voltage readings for different channels/slots, etc.

It’s not a big issue unless the differences between the fully charged resting voltages of the cells in your packs are getting wider and/or the BMS is spending a lot of time balancing the pack.

Hi, so IR could be why my charger showed 4.20v early on, on the 12th cell in a 12s half pack and 4.14 for the other cells. Testing with a multimeter right after charge the same cell has only 4.05v while the others have 4.17-4.18v.
What I don't understand is how did this happen suddenly after 18cycles of a suposed high end, expensive semisolid lithium battery.
I have never went bellow 3.4v under load and never realy push it. Max temp it reached was like 47C.
I have no bms, I ballance charge it. It didn't show any signs before, all cells ended up at same voltage. Battery is a 24s 30Ah (2x12s format to be able to charge in parallel with a 12s balancing charger).
I've charged half of the pack, 12s, at a time just to rule out the charger as a problem. It did ballance charge half of it good, 4.17-4.18 all cells after charge.
I've even charged that weak last cell separate to same voltage as other cells but after a slow 2-3 hrs discharging to storage voltage, the cell went far off like 3.62v and all other cells were 3.80.
The 11th cell caught my attention too but after charge was like other cells. Charging photos show cells from 7-12th at 22min and 48min charging from storage, at 10Ah charge current.
You mentioned bad connection, could it be that? It seems they are soldered together..
I wasn't aiming for 300 cycles but I sure was hoping for more than 18.

 

[Jeff23spl]

You made good points but i think it may not be the case here:
i tested that charger against all my others more than once and the measurement seem accurate, it is not a slot charger but a balance charger designed for RC batteries with a balancing pigtail. It could fall off but i check for this occasionnaly.

For cell quality, i could be wrong but it is ev cells that probably passed quality tests and are still working 10+ y later. They have wear and age but should not be low quality. (The Panasonic and even the A123)

I agree that charging at higher rate may bring a phantom voltage on the cell that didn't had time to absorb and trigger a BMS to stop the charge too soon. On my second example, it was the reason why i made a second charge at 1.4a to make sure it wasn't the case. And at the end, each group voltage is the same so seem unlikely that one peaked above 4.2 0.05v faster than the other. And at 15a/74 = 200ma and at 1.4a is 19ma for +/-3000mah cells, it seem a very gentle charge to help against this.

For connections, the Tesla pack is still by factory tesla fuse wires.
For the A123, i did try many chargers and cycle individual cells many times to be sure and all of them rest lower regardless of the charger i did used and it weren't connected inside packs but individuals.

 

[Jeff23spl]

Hi, so IR could be why my charger showed 4.20v early on, on the 12th cell in a 12s half pack and 4.14 for the other cells. Testing with a multimeter right after charge the same cell has only 4.05v while the others have 4.17-4.18v.
What I don't understand is how did this happen suddenly after 18cycles of a suposed high end, expensive semisolid lithium battery.
I have never went bellow 3.4v under load and never realy push it. Max temp it reached was like 47C.
I have no bms, I ballance charge it. It didn't show any signs before, all cells ended up at same voltage. Battery is a 24s 30Ah (2x12s format to be able to charge in parallel with a 12s balancing charger).
I've charged half of the pack, 12s, at a time just to rule out the charger as a problem. It did ballance charge half of it good, 4.17-4.18 all cells after charge.
I've even charged that weak last cell separate to same voltage as other cells but after a slow 2-3 hrs discharging to storage voltage, the cell went far off like 3.62v and all other cells were 3.80.
The 11th cell caught my attention too but after charge was like other cells. Charging photos show cells from 7-12th at 22min and 48min charging from storage, at 10Ah charge current.
You mentioned bad connection, could it be that? It seems they are soldered together..
I wasn't aiming for 300 cycles but I sure was hoping for more than 18.

To my understanding IR would matter more if you run at high rate (above 1C) what is capacity vs your load ?

you can think of this like a bucket of water for each cells or group. if you refill them at same level and drain them equally but get a different remaining level at the end, it is because the bucket volume isn't the same (capacity).

Have you tested for capacity each cell or just rely on the seller ?


To generate unbalance if the cells are same capacity, you would need to either have a different level of charge from the beginning or not draining them at the same rate.
Assuming the balance function of your charger work well, (what is it for 12s ?), Have you tested it? do you see perfectly equal voltage if you measure it with a separated multimeter at end of charge ? Because once 1 group start to unbalance, it just get worst at every cycle until you balance the pack again carefully.

Also cells could discharge unequally if you have different resistance with a high rate of charge/discharge. Instead of a team work, the ones with less resistance will work harder, heat more drain more. Then on the charger, the group with less resistance will get less voltage than the other groups so the more drained will recharge less amplifying the unbalance effect....


In your case i would put a good balancing BMS. I wont publish a brand name but you won't search long to find a smart one with balancing current of 1-2a. An active balance bms keep balancing during the use. so it will help to drain the pack lower and prepare it to be recharged higher by balancing all the time VS one that only top balance near the end of charge clipping the cells that reach 4.20 before the others.

 

[BatteryMooch]

You mentioned bad connection, could it be that? It seems they are soldered together..

You’ll have to CAREFULLY check.

 

[Super X Star]

Hello
I'm playing with lithium cells for a while, harvesting and building packs thinking i know the base stuff and safety but i would like to go a step further:

I do IR test and capacity discharge test but sometimes a cell is testing ok for IR and capacity but once fully charged to 4.2v,
it rest at 4.10-15 after a while and some other (different brands/models) rest at 4.18-19.

Curiously i experience this on older cells that could have more wear and i wonder if it is a sign of that wear or it is just a coincidence with cell construction seating slightly lower than another ?
I do see some at 3.6v nominal instead of 3.7 but datasheet still talk about 4.2 fully charged for both....???
I know used cells could be self discharging but for the one I'm talking about, they drop to that lowered voltage quickly and remain there for weeks/month so it isn't like a cell that self drain in a couple days or less...

I did see worse with lifepo4 A123 26650 that seat at 3.30-3.4v instead of 3.65 but i know they are at 75-80% capacity remaining. (those well known 12s8p modules) I though it was related to the type of cells but i got some less used one that seat a bit higher at 3.45v instead... (don't worry, i didn't mixed them with the others) I did never try a brand new A123 to see if it would rest at 3.6-65 or less, those are more often avalable at 10+ years old and work great even with 20-25% capacity lost.

I wonder if for those cells, we should charge them accordingly? I mean, should we lower the max voltage at which the charger stop ?
Is the cell keep gaining capacity above that rest voltage or just make heat? It was fine with older chemistry to overvolt them to fully charge (pB,nicd/nimh) but i was under the impression that lithium didn't need this? I'm i right

I ask because i lowered the charge voltage with my A123 pack used for home electricity backup. I have a +/-3kwh 16s32p connected to inverter supplying emergency loads.
The inverter has annoying cooling fans and i look for ways to hear them less often. I did assume it may constantly supply extra voltage to the cells to bring them to 3.65 generating more heat for the fans. I lowered the ''charged'' voltage from 58.4 to 55v to help it but the changes are not obvious to see improvements on grid bill or on the fans duty cycle....
I figured out my inverter stay active and supply the load from the DC bus/batteries even if grid is ON without swithing. Furniture clocks never reset but it generate more heat that a system with a transfert switch and run the fans more often...(It is located beside my work counter.)
Was it a good idea to lower the voltage or i just lose capacity from my pack by doing this ?

Also i just bought a Tesla S module and would like to have a quick idea if it is a good deal or if i bought worn out cells, before i break it appart to harvest the cells. i don't have the gears to capacity test a 6s74p (I could patent one but it would take forever to drain 225-250Ah)
This bring me to ask if looking at resting voltage is a sign of wear or just a variable that doesn't matter ?
That pack came well balanced at 3.7per cell, i charged it to 4.2 at 15a rate (ISDT air 8 balance charger). The cells lowered to 4.15v after a short time. Since i don't know exactly how the ISDT figure out the state of fully charged and i did miss the time it stop charging, i made another attempt to charge but at only 1.4a to help with balancing and make sure it won't stop too soon.
And same: I came right after i heard the ''charged'' signal. The instant rest voltage was 4.17-18v but very soon (2-3hours) it returned to 4.15 again.
I was told the pack is 2014 and the car was still displaying range in km around 86-87% of the original range.
Is it a bad sign, or just normal ?

Hey, what you’re seeing is actually pretty normal with lithium cells, especially older ones.


When you charge a cell to 4.2V, there’s always a bit of surface charge that makes the reading look slightly higher. After a few hours, it settles down — usually around 4.18–4.15V. For cells that have some age or wear, that resting voltage can drop a bit lower, like 4.10–4.15V. It doesn’t always mean the cell is bad — it’s just a sign that the chemistry isn’t as “fresh” as a brand-new cell.


For your A123 pack, lowering the charge voltage is actually a good move for longevity. It just means you’ll have slightly less capacity, but the cells will last longer. And no, they don’t really “gain” more capacity after that lower resting voltage — pushing them harder (to 4.2V or 3.65V for LiFePO4) just stresses them more over time.


As for your Tesla module, the fact that it balances well and only settles to 4.15V after a charge is totally normal. Even new packs drop a little like that. A 2014 pack with 86–87% of its original range is still pretty decent, especially if it’s holding charge for weeks without draining fast.


Resting voltage alone isn’t a perfect measure of health. If the cells aren’t self-discharging quickly and they’re holding up under load, they’re likely fine.

 

[amberwolf]

I do IR test and capacity discharge test but sometimes a cell is testing ok for IR and capacity but once fully charged to 4.2v,
it rest at 4.10-15 after a while and some other (different brands/models) rest at 4.18-19.

Keep in mind that some chemistries don't charge to 4.2v in the first place. For instance, the NMC cells I have from EIG have a max allowable charge of 4.15v.

Curiously i experience this on older cells that could have more wear and i wonder if it is a sign of that wear or it is just a coincidence with cell construction seating slightly lower than another ?

Depends on the cells. If they are designed to charge to the voltage you're running them up to, then they may just be "a bit worn" and unable to retain all the charge they're given; it happens as cells age.

If they don't keep dropping in voltage just sitting there, after the initial drop right after charge stops, then it's probably fine.

I wonder if for those cells, we should charge them accordingly? I mean, should we lower the max voltage at which the charger stop ?

In my case, when I find cells don't retain a full charge, I stop trying to charge them that full. But I also don't charge my packs to full to begin with, to help them last longer. For instance, those EIG packs get charged to about 4v/cell, instead of 4.15v. (I did orignally charge them full, for a while, then decided to do what I do now, a few years ago).

When doing this you'd need to set your BMS or balancer up, if you need one****, to do it's thing below the normal point it would do so.

****those EIG cells I have are so well-matched that they stay the same, down to the hundredths of a volt, from full to empty; ideally every pack should be built that well, but most aren't, so a BMS with balancers is necessary to keep using the pack's available capacity.

ll keep gaining capacity above that rest voltage or just make heat? It was fine with older chemistry to overvolt them to fully charge (pB,nicd/nimh) but i was under the impression that lithium didn't need this? I'm i right

It's not reallly that Ni chemistries was "fine", it was generally required in order to fully charge them and balance them, without something that would do coulomb-counting to each cell. Ni dispenses with any excess charge once it reaches full by expending it as heat and then drops in voltage, so the typical chargers for them had thermal sensors and stopped charge based on Delta-T (sudden rise in temperature) and/or Delta-V (sudden drop in voltage)

But those are the only common types I've seen that worked that way.

Lead self-discharges, so a trickle charger can be helpful, as long as it doesn't produce a voltage or current that cuases outgassing of the electrolyte (which dries out the cell and makes it stop working, or causes the cell casing to expand and fracture, leaking the rest of the EL out... )


Lithium doesn't work these ways. If you keep trickle charging lithium, it could cause lithium plating inside, and that could damage the cell (and damaged cells *can* be a fire risk, dependng on what exactly went wrong inside). That's why a good charger (like the Grin Satiator I use vs an LED PSU like the Meanwell I also use) will turn off once charge current drops below some limit.

The inverter has annoying cooling fans and i look for ways to hear them less often.

That's relatively easy to fix. If it uses tiny high speed fans, remove them and use a duct / adapter from that size to a larger one that moves more air with a lower speed fan. Remove any grillework that is at the fans, like the casing cover, etc., as it will decrease the turbulence in the airflow, and decrease the noise.

I was told the pack is 2014 and the car was still displaying range in km around 86-87% of the original range.

Well, 11+ year old cells are...11+ years old. And being used, possibly heavily, means you don't know what remaining lifespan they have, or how they will perform, until you are using them in the application. The less demanding the application, the mroe likely they'll still do it.

If they all stay balanced with each other, they're still well-matched, and that's important. As long as they meet your performance needs, they'll still be good for however long they last.


My EIG cells are now at least a decade and a half old, and they're still going but not nearly as well as they did orignally. I would guess at this point they have about 3/4 the capacity they started with, and they sag more under load--most of this has been a relativley recent develompent, in the last year or so, so they are beginning to wear out; porbably be time to replace them soon.

 

[Jeff23spl]

Keep in mind that some chemistries don't charge to 4.2v in the first place. For instance, the NMC cells I have from EIG have a max allowable charge of 4.15v......

If the Tesla really have the Panasonic NCR18650B, they are advertised at 4.2v max and the A123 are also advertised at 3.65 so probably the age the more i think of it....
I will eventually try to find a brand new A123 just to see if it react the same. It was my first model that was so far from the advertised charged level (3.35 vs 3.65) and also my first experience with lifepo4....
I did capacity test each packs before making the 16s arrangement with an home made resistor load and a plc monitoring current/voltage over time to estimate wh and each of them was between 14.5-15.5Ah when advertised at 18Ah new. My test had some error margin but by testing fresh Milwaukee packs the same way, i was able to read very close. So my resistors bridge to feed 0-10v plc inputs and programmed scaling was not bad....But my A123 pack was around 80% remaining at time of testing it (2y ago) And still work...But i have an eye looking at prismatic cells to replace and get a bigger pack.

For the Tesla
I was looking for an easy answer but i'm now looking to re-use that bench to load the 6s74P tesla module to figure out it's real capacity. Capacity testing is probably the best way to know about state of a battery....
I would need to mod the bench to increase the load and calculate my stuff to avoid overheating the resistors...I was thinking of adding pairs of car headlight bulbs. A pair could make a single 24v load but i'm wondering if resistance would move much from colt to hot. I need to check that this weekend...

For the inverter fans thank for suggestions, I will investigate that option.

 

[BatteryMooch]

I did capacity test each packs before making the 16s arrangement with an home made resistor load…

You’re probably aware of this but for others who might not know…
A resistor load draws less current as the cell/pack voltage falls so any capacity measurements will be different than the manufacturer specs since they use a constant-current load.

 

[Jeff23spl]

You’re probably aware of this but for others who might not know…
A resistor load draws less current as the cell/pack voltage falls so any capacity measurements will be lower than the manufacturer specs since they use a constant-current load.

yes the current draw follow the voltage and decrease over time.
But with a smaller load than the factory measurement, you would normaly get more mah capacity since you take more time to discharge (lowered C value) or if you go very basic and only measure the time to discharge (Bad) and dont account for the reducing load, it will last way longer giving a false optimistic result...

But my test bench use 2 plc analog inputs reading consumed amps, and voltage at the load. The logic calculate instant watts result and sum the result divided by 3600 every second to make a cumulative reading. when the voltage input see the cut off voltage, it trigger the circuit to open with a contactor to avoid draining the batteries too low and this also freeze the result at the end. then i can divide the wh value by nominal voltage to estimate the Ah the pack is giving.

 

[BatteryMooch]

yes the current draw follow the voltage and decrease over time.
But with a smaller load than the factory measurement, you would normaly get more mah capacity since you take more time to discharge (lowered C value) or if you go very basic and only measure the time to discharge (Bad) and dont account for the reducing load, it will last way longer giving a false optimistic result...

But my test bench use 2 plc analog inputs reading consumed amps, and voltage at the load. The logic calculate instant watts result and sum the result divided by 3600 every second to make a cumulative reading. when the voltage input see the cut off voltage, it trigger the circuit to open with a contactor to avoid draining the batteries too low and this also freeze the result at the end. then i can divide the wh value by nominal voltage to estimate the Ah the pack is giving.

Ahh…my screwup on more vs less capacity…post modified,..thank you!

 

[Jeff23spl]

I did finally test the Tesla module with mixed loads of resistors and light bulbs to generate an initial 12a for 2xx pack so a very low rate and got average low results. It was around 4-4.1khw for a supposed underrated 5.3kwh...

I disassembled the pack since i'm going to reconfigure the best cells into a 20s13p for a Daymak 72v 28A. It would keep it below 1C so even if lowered capacity it should be ok for it.

I started testing them and it confirm that those a really used cells. Going along my expectation with the lowered resting voltage...

at 1100mah rate of discharge it return between 2150-2400mah...
At 500mah, 2450-2500mah for the 10-15 i did test (the majority will go thru the 1100mah to sort them faster)
(I did test them at 2000mah but results where low and the cell was heating)

IM using a new 8slots charger/discharger that was cheap price on AliExpress and it seem to work very nicely. I can select load rate and cut voltage. I did set it at 4.2 and 2.8v for the above results.


My plans was to use the Opus BT3100 4slots and an EBL electronic tester beside the 8 slots to get 16 at the time but
Despite i like the EBL for it's graphic display it is limited to 500mAh so it takes forever to test a set of cells and get higher rating because of that 500mah rate...
And i did figured out that the tests coming from the Opus at 1000mah are recharge current capacity instead of discharge. Giving higher results. (I will investigate further for that one since i bought it because of its good reviews...) The opus was returning 2500-2600mah for cells that would test 2250-2350 on the 8ch



Impedance around 40mohm tested with a no name high frequency tester that seem quite accurate.
This is not bad impedance for slow rate but it seem to sag a lot more than similar cell of similar impedance with less wear.



Conclusion:
5.3kwh rated for 6s74p means about 3226mah per cells but it is told to use NCR18650B and the general ones are rated 3400 and some other place mention 3050mah for a Tesla S cells...
General tests are 500mAh rate and mine get around 2450-2500 at that rate so at minimun a 20% loss and if it was originally 3400 that would be 28% lost for a 11y old battery probably used a lot.

I wil still use them but i was expecting a little better when buying the module. (Was told about a 15% km range loss but it is more around 25%...

 

[BlueSwordM]

Oh yeah, I wouldn't recommend buying any Model S modules at this point, even the Model S Plaid; they all use the same average maximally abused NCR18650X whatever.

Just buy Model 3 packs or bulk EVE 50E if you want to get your money's worth.

 

[Jeff23spl]

Oh yeah, I wouldn't recommend buying any Model S modules at this point, even the Model S Plaid; they all use the same average abuse NCR18650X whatever.

Just buy Model 3 packs or bulk EVE 50E if you want to get your money's worth.

i had to try that S one to know

I bought just 1 module from the 16 for sale and i was expecting good results and the need to buy more... but i probably won't....I think those could rather be used ''as is'' For a 12S Powerwall. (2-4 modules would still make 8-16kw 48v but i don't how how it would react over time once already reached 25-30% wear. I did read that it should wear out faster once the first 20% is lost but i will know...

If you account for the pita to harvest them and clean the glue individually from each cell, testing, and the need for top rings and individual skins...it don't worth it for 2300mah cells.


Eve50e? do you have links ? I'm slowly looking at 21700 but in my area it is still not popular. Even a model3 pack, i would need to buy a wrecked car myself and resale the other parts to keep the cells as a bargain. Junk yard are afraid to play with batteries and if they do, they will try to resale more than half the price of a new one from Tesla like they do with other used car parts...

 

[BlueSwordM]

i had to try that S one to know

I bought just 1 module from the 16 for sale and i was expecting good results and the need to buy more... but i probably won't....I think those could rather be used ''as is'' For a 12S Powerwall. (2-4 modules would still make 8-16kw 48v but i don't how how it would react over time once already reached 25-30% wear. I did read that it should wear out faster once the first 20% is lost but i will know...

If you account for the pita to harvest them and clean the glue individually from each cell, testing, and the need for top rings and individual skins...it don't worth it for 2300mah cells.


Eve50e? do you have links ? I'm slowly looking at 21700 but in my area it is still not popular. Even a model3 pack, i would need to buy a wrecked car myself and resale the other parts to keep the cells as a bargain. Junk yard are afraid to play with batteries and if they do, they will try to resale more than half the price of a new one from Tesla like they do with other used car parts...

EVE 50E can be ordered in bulk for 1.50$USD/cell + shipping from say, Vapcell or other similar distributors.