Prematurely aging cells while charging?

[svetz]

So I had my pack recharging and I use a kilowatt meter. I watch the watts and when it goes to 0 I know the BMS has finally cut it off.
But, I didn't unplug it last time, got busy with other stuff.

When I did get back to it, it was charging again. So, what I've figured out is that while charging each "s" row is at slightly different voltages descending across the pack. At some point the BMS stops the charge (first row hits 3.60V). But, then the resistors go to work equalizing the pack. Which means the first few rows drain. When it gets low enough, the BMS starts charging again.

So, I'm thinking those first rows are getting an unnecessary work-out (fill/drain, fill/drain). Mine is only a 4s, but if I went to 48V it would be 16s; which would be 16 sets of voltage difference at the final charging stage. Seems like the more "s" the more of this tiny cycling you'd have.

I suspect it's normal (can't eliminate internal resistance) but perhaps there's a better way? Let me know!

 

[electric]

BMS does not start charging again, it allows charging again. It's up to your charge source to know when to cycle the charge and when to float or disconnect. Repeated charging is a function of a charger, not the BMS.
In a normal healthy and balanced pack BMS should not even intervene at all. The fact that your BMS intervenes the charge points to some other possible issues, or maybe your pack is new and still doing initial balance, which should stop at some point.
Please describe your charge source(s) and their voltage settings for more details.

 

[svetz]

Been thinking about this, and in my application it's not an issue since I disconnect the charger when the BMS first cut's out (usually); and the cells all go to the same voltage after the BMS resistance does it's thing. I don't want them charging beyond 3.6V anyways, so not a problem in my case. Probably the worst that is happening is that the first row is working slightly harder as its charging to to 3.60 and then discharging to meet the rest of the pack. It would be interesting to hear what folks with a 16s (i.e., 48V string) are doing in a permanent setup as they must see a similar behavior and the voltage delta between 16 cells must be higher than 4.

It's up to your charge source to know ...

I use a 14.4V AC adapter; it's just a typical wall wart. It doesn't know anything.

 

[electric]

In most commercial applications chargers must have a float stage, which in terms of Lithium battery really just means to stop charging and hold the resting voltage, because Lithium has no real self-discharge. So, if your battery is exposed to charge source for longer than it needs to actually charge, then charger would drop to float and battery will not be aging prematurely.
In your case you are doing this part manually by unplugging the wall wart.
In general, if your charge source has no float and no stopping, i.e. it's just holding charge voltage like a power supply, then you are slowly cooking your cells and their life will be shortened. By how much shortened depends on voltage level and how much time it spends at that level.

 

[svetz]

No arguments there, but what about the folks with a 16sXp setup on a permanent charger?
Based on my observations I'd expect that when the charging first stops all the "s" rows will have slightly different voltages. The BMS's resistors will drain the high cells. If the float charge is voltage activated (as is my BMS) then once again current flows through the battery and the cycle should repeat some number of times before the last "s" row finally reaches the setpoint voltage; bring all the cells upto a full charge.

We're talking really small voltage changes so probably not a big impact, but I thought it was an interesting phenomena. An active balancer at the float stage might deal with it in one pass.

 

[electric]

float voltage is typically 3.4V per cell, while balancing is usually above 3.45V, so there is no balancing during float. BMS has nothing to do with float charge or any charge activation, it only intervenes if there is too much imbalance, causing highest cell to trip HVC, while overall charge voltage has not been reached yet. However, if your cells are healthy, then this balance process won't last forever, it will eventually finish over a number of cycles and won't happen anymore. So, what you are describing should be a temporary phenomenon, hence not affecting long term cell life. Active balancing is not cost effective on good quality cells, because you wouldn't pay premium for something that is only needed once in 10 years.

 

[svetz]

That makes great sense! Thanks!

 

[gehowi]

@electric ,

While testing my 200Ah cells I have this phenomenon:

I charged one cell with a supply delivering 18A @ 3.5V.
I started at 2.98V and this cell took more than 10h to reach 3.5V with almost constant max current.

From 3.4V the current dropped slowly to +-2A @ 3.5V.
I assumed the cell was at least 90% charged and expected I could not charge it more the next day.

I stopped overnight and disconnected the cell. After 10h resting I measured 3.31V.
I connected the charger again this morning and the current is again 16A for more than 3 hours already.
The cell does not get above 3.37V for now.

What does that say about the cell? Is that normal?

 

[electric]

@gehowi ,
can you estimate voltage drop over cables from supply to the cell? Gauge and length of the cables, connection quality (alligator clip vs bolted lug, etc).
Your test would have been much cleaner if you set your PS @ 3.65V, so entire charge would happen in one continuous cycle.
By stopping short and waiting before proceeding you added possibility of self-discharge, which you cannot measure because your charge was not full, so you don't have a good baseline.
From your numbers it seems that you put 228AH into 200AH cell ( 18A * 10h ) + (16A * 3h ) = 228AH , which is obviously not realistic.
So, you are either not measuring correctly or your cell has significant self discharge overnight.
I would recommend to discharge fully and then repeat the test @ 3.65V in one go, counting Amps and time to get accurate AH count ( or use a battery tester that has Coulomb counter ). Then, after 24hrs of resting, check voltage and then try to top off charge at the same 3.65V, again measuring AH. Whatever amount you have topped off after 24 hr after a previous full charge would be equal to 24hr self discharge.
All I can tell you is that a healthy LFP cell would have virtually no self discharge. As soon as you try to top it off, voltage would immediately jump back to 3.65V, not allowing any more AH into the cell.

 

[gehowi]

@electric, thanks again for your useful answer.

I used 3.5V because I don't have a power source that can deliver high current at 3.65V. I modified a 3.3V server power supply but the max I could get is 3.5V @ 18A. I will try to get a supply that can do the job.

2mm wires were soldered at the supply and bolted at the battery but I had to use 30cm of 1mm wire in series to limit the current to 18A. If I don't then the current is way too high and the PS shuts down... (yes the wire gets very hot..)

I measure the current through the wires with a DC amp clamp. I am sure it's correct to 0.2A.
I always measure the voltage directly on the battery poles. I don't know the cable loss during charge yesterday but the loss becomes low if the Amps go down so the battery reached certainly the 3.5V on the poles, I measured with a Fluke meter.

I guess the cell suffers large selfdischarge. It should not draw 16A for 3h again with the same supply without significant voltage rise.
Really something wrong with these cells.

I will make a datalogger to monitor current and voltage at the battery so you can see a complete cycle graph.
It will tell you more than everything else and I will need it to prove to the BLS seller..

 

[electric]

@gehowi, lithium self discharge is one of a few topics still surrounded by mystery and supported by a lot of nonsense in online communities. Most datasheets claim <3%/month and people take it literally to mean =3%/month, where it could be 0.1%, which is still technically <3%.
Actual self discharge varies widely based on cell manufacturing quality, where leakage can occur between anode and cathode for various reasons and good cells have virtually none, while crappy cells will have some. Really hard to nail down a good baseline, but acceptance also depends on application. If battery is cycled daily and you have much cheaper cells with higher than average self discharge, you can still be a happy customer. But you have to carefully watch your long term storage if you have to store such battery for a few months.

Also, people tend to mix naked cells vs. battery with BMS attached when speaking of self discharge. BMS will always add some discharge, but it's not always clear how much. Depends on BMS active features and if you can turn them on/off for storage. For example Bluetooth adds a lot ( relatively speaking ), but maybe it has a sleep mode. You really have to understand details of your BMS idle consumption. Good BMS supplier will publish this data. Usually it's easy to measure by inserting mA meter in series with highest cell positive tap wire, as that's where BMS draws it's power from ( subject to different BMS designs, not 100% set in stone ). BMS with on/off switch is always better in my book, allows to turn battery power off when needed and helps with storage.

Good battery/BMS will have negligible self discharge, you should be able to store for 6-8 months without any worries. This is why you don't see Lithium maintenance chargers on the market. A guy in another thread is looking for one, doesn't believe me when I said it's not needed.
Good quality naked cells can sit on the shelf for 3 years and not discharge much.

Sometimes cells can develop self discharge over time due to abuse, which is more likely in your case, assuming the factory wasn't pure garbage to begin with. Exceeding temperature limits, over charge/discharge, etc. can lead to increased self discharge. Swelling is often an indication that self discharge will be present, as mechanical pressure inside the cell creates pockets of current leakage.

 

[gehowi]

@electric,
You are (always) right..
I get every year some defective portables from a hospital. I am surprised sometimes to find 18650 cells inside of 8 y old that has been on a shelf somewhere for more than a year and still have more than 3.2V. There are usually dead cells (in parallel) but the others can mostly be reused.
I don't spend time to check capacity. I charge unknown 18650 cells with 2A. If they get warm in 20 minutes I put them in the trash. If not then I charge further to 4.2V with 0.5C. After one year in my drawer they are mostly more than 3.5V.
My self made 3 point doorlocks with motor uses 3 old 18650 cells in series. I charge them once a year and some are used daily.
Sure, good cells do not need a maintenance charger.

But these 200A cells don't get warm. I just finished discharging another cell with 20A from 3.5 to 2.48V: 101Ah / 294Wh in 5h33. That's double than the one before but still very bad. Even they are not charged to 3.65 and there may be errors, it still far from 200A.

I don't think they suffer from my abuse. I never exceeded 2.5 or 3.7V during the use and I never used without BMS. Only now I got too low by testing the degraded cells which are not in use for one month.
I will see what my datalogger will show me for the next tests. I have to make one first.. Will be ready in 2-3 days.

 

[electric]

@electric,

But these 200A cells don't get warm. I just finished discharging another cell with 20A from 3.5 to 2.48V: 101Ah / 294Wh in 5h33. That's double than the one before but still very bad. Even they are not charged to 3.65 and there may be errors, it still far from 200A.

I don't think they suffer from my abuse. I never exceeded 2.5 or 3.7V during the use and I never used without BMS.

Sounds like you bought garbage cells from China. I would not be surprised if you bought used or defective cells marketed as "new".
Finding reputable cell supplier is the most critical part, it takes a lot of effort, trial and error, etc.

Large cells don't get noticeably warm because your C rate is too low to generate enough heat losses. There is much mass and surface area to shed small amounts of heat quickly, so you don't notice it without precise caloric measurements. Same for self discharge, not enough leakage current to raise temperature.

 

[gehowi]

Yes, I bought the second set 1 month later and I wanted cells with 6mm bolt/nut connection again but they could not deliver anymore. Only cells with 4mm screw.
Can you imagine: they deliver 200A cells with a 4 mm RVS screw in an soft aluminium pole. A little too much force and the thread is damaged or the screw breaks inside the pole. Any hard cable comes lose if you just push it a little.
I experienced while charging now that fasten the 6mm nut more can make a difference of several Amps at 20A. So what would be the difference at 200A.

When I asked why such a small screw, they said: to protect the poles.. But now I think it's a cheap solution for another problem:

I saw now, by accident, that such cell expands rapidly more than 5 mm (middle) in half an hour if discharged lower than 2 volt. The link sheets between the batteries have 6mm holes and no play around the 6mm bolt. The batteries had to fit very close together.
So when a group of 8-16 expands a few mm per cell, the forces on the poles must he huge. May be they made thousands of such copper linksheets already, so they used a smaller screw that has more play and cannot hold these forces. So the poles do not damage.
A serious safety concern if the poles lose contact with the linksheets at high currents. If it catch fire then they will probably tell that you did not fasten the screw enough.

I contacted the manufacturer of the cells a few days ago to ask about the production date of the M6 cells and sent pics of the labels. They answered:

From the current information, we think that you purchased un-qualified batteries from China. Can you please let me know where you purchased those batteries?
Cheers
Collin
韩爽(Collin) +86-13698825949
Wuhu ETC Battery Limited.

Yes, very sure it's (used?) garbage.