Few Queries about 18650 Lithium batteries - Page 1
Forum section: electronics / beginners

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 06:31:38 am
content: Hello,I have few doubts about 18650 batteries (Lithium batteries in general). These questions came to me when collecting 18650s from old laptop batteries.When I checked the voltages of these batteries after a full charge, they gradually come down to 3.8v within a day and stays there. (checked that its stable for more than a week). So, are these cells usable? I checked their capacities (cut off at 3v) and it comes around 500 to 800mAH (they are 2000mah Sony and Sanyo cells). How long can we use these cells before discarding them? I mean my question is, if you are okay with the capacity can we still use them?Thanks

author: bob91343
author_rank: Super Contributor
timestamp: February 02, 2022, 06:54:01 am
content: I don't see a problem with using them, as long as you can tolerate the reduced capacity.

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 07:00:22 am
content: Quote from: bob91343 on February 02, 2022, 06:54:01 amThanks for your reply.So, people discard them only due to the reduced capacity then (due to the hassle involved in charging fequently). Because in many places I see it mentioned that if the cell cannot hold 4v for more than a week, then it should be recycled.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 08:09:26 am
content: You probably are talking about lithium ion batteries, not lithium batteries.If they really come down to 3.8V after a full charge, they are broken and useless, with so much internal resistance they barely accept any charge. But the explanation can also be: you are not fully charging them. That would also explain the very low capacity you get.Are you sure you are fully charging them? What is the charge voltage, charging current, and the charge ending current? Have you verified with a multimeter for example that they sit at 4.2V while in charger?If the capacity is really down to <40% of original, I wouldn't use them. Usual end-of-life condition is 70%.

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 08:37:39 am
content: Yes you are correct. Lithium ion batteries.I am charging them upto 4.16v with a constant current of 580mA. I am not sure about the charge ending current as It was being charged by a TP4056 battery charging module.I was just curious about the rationale behind not using them at reduced capacities.Thanks.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 09:12:33 am
content: Does the charger module keep the cell at 4.16V or just cut charging immediately after reaching 4.16V?If we assume that the cells are aged, with increased ESR, then:* Charging at 580mA and neglecting to have a long enough CV phase already cuts out maybe 20% of capacity;* Discharging at a significant current (btw, what was it?) to 3.0V, which is higher than datasheet discharge end condition, leaves maybe another 20% of unused capacity.With good-condition cells, the percentages are significantly smaller than this.It's well possible the actual capacity is much higher.Yet, the typical end-of-life conditions are capacity drop of -30% or DC ESR rise of +100%. Using badly aged cells, theoretically, could have safety implications, but as they are proper brands, it's not a huge risk. In any case, if they perform so poorly, they are not very useful, reason alone to just buy new cells.

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 09:20:12 am
content: Thanks for your thoughts.The charging module keeps the cell at 4.16v. I am cutting it as its spending too much time (around 15 mins) in that voltage.Discharge current is 500mA.But I agree that it will not say the actual capacity as the discharge current sways between 530mA(in the beginning) and 470mA(near the end) and I don't completely trust the battery capacity module (one of the cheap ones).Thanks again for the input and suggestions.!

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 10:01:08 am
content: 15 mins in CV is normal even for good cells. For aged, it takes even longer. You are terminating it too early, so they end up a bit undercharged.Though, dropping to 3.8V indicates they are only around 60-65% or so. They shouldn't dropthislow, even with just 15 minutes of CV after C/4 charge. I guess they are just very badly aged. I woudln't use them.For reference, if you take a brand new 2000mAh cell, charge it at 500mA until 4.20V, and then let the current taper down keeping at 4.20V for 15 minutes, they end up at like 95% SoC and 4.10V open-circuit voltage (which can be measured after a few days). Aged cells have higher equivalent DC resistance, meaning that you need to apply 4.20V for longer to push the charge in (due to Ohm's law).

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 10:16:19 am
content: To be correct, it does not drop to 3.8 very fast.For example, I charged a cell to 4.15v yesterday. Now, its almost 24 hours and now it is at 4vWhen you tell that it ends up undercharged, what difference does a 0.05v make in capacity ?I understand now about the ESR you were talking about. The charging module has two LEDs to indicate "charging" and "complete". I was removing the cell before the "complete" light comes on. I was doing this because, the battery was charging for longer time and I did not have the knowledge about the ESR.Now, let me wait till it finishes charging and estimate the capacity again. But how much time should I wait before terminating it manually if its not getting fully charged? In other words, what if the cell never reaches 4.2v no matter how much ever time you charge it?

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 11:11:54 am
content: Charger ICs usually have some time-out built in. If not, an hour or two is good (after reaching the CV voltage), IMHO.Lengthy CV phase is an indication of really high ESR, or even worse, high leakage current (self-discharge). Both can lead to excessive heating.If it's dropping from 4.0V @ 24hrs to 3.8V after a few more days, it really sounds bad. Just get some new cells.

author: tunk
author_rank: Super Contributor
timestamp: February 02, 2022, 11:31:36 am
content: What are you going to use them for?The TP4056 should do CC/CV/cut-off charging.After discharging them, what's the voltage e.g. 5-10 minutesafter discharge? A high increase indicates high internalresistance.Falling to 3.8 or 4V in day is pretty bad; in my experiencethey do not stop there, but continue to discharge (sometimesbelow 3V).The laptop batteries usually are 3S2P, and in my experienceoften one parallell pair goes bad and the four others are ok.Edit: Do you know what the initial voltage was?

author: 4kruby
author_rank: Contributor
timestamp: February 02, 2022, 02:12:30 pm
content: Quote from: tunk on February 02, 2022, 11:31:36 amI don't have a application in mind yet. Just charging them up and measuring their capacitiesQuote from: tunk on February 02, 2022, 11:31:36 amI don't understand. You want to discharge them and measure their voltage? and it will have increased voltage after discharge?Quote from: tunk on February 02, 2022, 11:31:36 amfor 2 it was 3.5v and for 2 it was 2.8v and for 2 it was 1.8vQuote from: Siwastaja on February 02, 2022, 11:11:54 amThanks. I see that they have degraded more. But I don't see any heating from them till the charger IC finishes the charging. They are as cool as it started. I am just experimenting with them.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 02, 2022, 02:16:43 pm
content: Heating is more of a problem at higher currents. But large voltage variations as a function of current work as an indicator even at lower currents.Really high leakage current is a warning sign about an internal short circuit which can get worse. Cells have safety mechanism against internal shorts, but nothing's perfect, so you should dispose of cells that leak charge, instead of trying to use them.

author: rpiloverbd
author_rank: Regular Contributor
timestamp: February 04, 2022, 12:25:40 pm
content: I agree with Siwastaja. It's always better to be in safe side.

author: wizard69
author_rank: Super Contributor
timestamp: February 07, 2022, 12:15:04 am
content: Quote from: 4kruby on February 02, 2022, 08:37:39 amIf the battery is significantly reduced in capacity and don't allow the user to get the expected run time then they are useless. At least in the context of that users.At some point though the batteries will be past their useful life to anybody and may become unsafe. frankly it sounds like you have tapped out batteries though you may want to monitor them through a complete recharge cycle.Look at it this way, if these were alkaline batteries would you even be bothering? Also consider this; Tesla should be moving to new battery tech over the year. In no time at all these 18650 batteries should be cheap.

author: ThermallyFrigid
author_rank: Frequent Contributor
timestamp: February 07, 2022, 12:20:05 am
content: The only way you can really tell is to do a discharge capacity test.I test them on a DL24P load tester. It also lets me accurately measure how warm (or hot) the cell get at pre-set loads.If the capacity is within specs and the temperature during discharge is within specs, use it.It's perfectly fine to use like capacity cells together for certain applications....even if their capacity is lower than original.

author: kjr18
author_rank: Regular Contributor
timestamp: February 07, 2022, 09:50:21 am
content: Quote from: 4kruby on February 02, 2022, 09:20:12 amYou end charging those way too early. Just look at TP4056 datasheet: after reaching about 4.12V charge current should slowly fall, and around 4.2V should stay until charge current falls below one tenth of set charge current. In your case if you set charge current at 580mA, charge should end when charge current is below 58mA.Just connect battery and charge it until red LED goes off and blue or green led turn on. You could touch cell from time to time to check it's temperature, if it rises above ambient temperature too much, you should get rid of it.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 07, 2022, 10:07:03 am
content: Quote from: kjr18 on February 07, 2022, 09:50:21 amTo be precise, the "correct" ending current would be specified in the datasheet of the cell, not the charger datasheet; usually C/20, so for a 2000mAh cell, 100mA.But this is of course just what the manufacturer uses to define what is 100% SoC. If the ending current is lower, charging goes on for longer, and results very slightly over 100%, this isn't usually of a concern. If the ending current is higher, charging is cut earlier, and the cell remains slightly below 100%.With aged cells, if you want to have full possible capacity, the last thing you want to do is to use too high cutoff current, because you are leaving even more unused capacity than on a less aged cell. That 58mA cutoff is fine, just let the charger do its job until finished. It takes longer than on a fresh cell.