Li-ion Battery experiences

[Gionnirocket]

So I have some Atlas garden tools..a lawn mower, string trimmer, edger, hedge trimmer and blower and all have worked very well and battery life has been great. This will now be the 6th season using them. 2 batteries are a 5 full years old and the 3rd is 2 years old.
I was able to get these battery specs from the manufacturer.
Battery #57014 - 80v 2.5 Ah / 40v 5.0Ah Lithium-Ion Battery.
Cell Type: SAMSUNG SDI INR18650-25R (lithium-ion rechargeable cell)
Voltage: 3.6V
Capacity: 2500 mAh
80V 1P battery: 3.6 V x 20 cells = 72 V / 2500 mAh. 10 battery cells in series and two sets in series.

Every winter I put them in storage at 70°f in my indoor basement work area and at 35.5v/ series string, which is the voltage of the battery on the store shelf when purchased and approximately at 40% SOC. Doing this I've had no noticeable diminishing capacity over the first 5 years and the batteries stayed at this level... with no more than -0.2v drop over the 6 month winter storage.
Typically i would check them a few times over the 6 month storage but this past year I became both distracted and complacent. Of course at the start of the 6th season something was different. When I went to do my initial charge to get some work done one of the 5 year old batteries was rejected by the charger. When I check the voltage it had dropped to 12V. The other 2 still above 35v
So in an attempt to salvage it I broke out the bench top power supply and set it up outside setting it to CC 0.25A and then 12v. Progressively I raised the voltage 0.25v as I saw the current drop to near 0.. then I would raise it another 0.25V. I did this alternating between the series strings every 3v until I had each raised to 25v. All the while monitoring temperature and it never went above ambient. At this point it was such a beautiful day that I decided to go for a ride and see if it would remain stable in the meantime. When I returned one string remained at 25v but the other had dropped to 12v again. So I worked that string back to 25v which it did very quickly. Then I raised the CC to 1a and brought both strings to 36V alternating again every 3v in the process. At this point I again waited to see if things stabilized and shut down for the day leaving the battery outside. Next morning both strings were still at 36v so I put the battery in the 2a factory charger and it was working. It took a little longer than usual, about 3 hours but the battery reached typical full charge voltage of 41.5v/series string. All the time the temp staying pretty near ambient though the charger did get typically warm. All my normal experience with them over the years. I let it rest for 30 minutes and it stayed at full charge so I plopped it in the lawnmower for a quick yard trim and then the hedge trimmer and then to the blower and it worked as good as new. Ran it down to 36v and that's where it still was the next morning. The battery looks and performs as well as the other 2 as far as my initial test reveals.

So was any damage done?
I know that batteries left at to low a SOC can internally short. But how long was it at that state?
Is it still safe to use?
Should I continue to use it but store/charge it outdoors? Or am I just being paranoid.
My initial thought is to at least store/charge it outdoors for at least the rest of this season.

Battery geek opinions and experiences welcomed.

 

[Chazmo]

Gionni, yes it’s true, as you know, that storing lithium-ion batteries near 50% of their capacity is best for longevity. But, I’m trying to understand how you store them. Are you saying you connected the three batteries together while in storage? Why wouldn’t you store them individually (not connected, that is)?

 

[Gionnirocket]

No. Each battery is dual voltage use. It's 40v/5ah or 80v/2.5ah nominal depending on the tool you use it with.
Each battery has 2- 40v series sets. The voltages measured at the output is for each 40v string.

https://www.harborfreight.com/80v-25-ah-and-40v-50-ah-lithium-ion-battery-57014.html?utm_source=google&utm_medium=cpc&utm_campaign=21901739192&campaignid=21901739192&utm_content=171677808222&adsetid=171677808222&product=57014&store=3287&gad_source=1&gbraid=0AAAAADAHb4diufZJG2u0Sb9WGYCcdXhvb&gclid=Cj0KCQjwtpLABhC7ARIsALBOCVrH2kyur78oQJuNc68JKdLniEvQOLHqQXC_s3gYGky5cq2wfjm5alcaAo6uEALw_wcB

The batteries are stored with no connections to anything... Just sitting on a shelf.

 

[Chazmo]

Thanks for bearing with me. OK, I understand. But, how are you able to charge only half the pack at a time? I think that’s what you were describing, right? Is there a switch that allows you to access each 40V “half”?

I guess the real concern is why did your battery discharge while it was in storage? Are you certain that you had it partially charged before you stored it? In any case, it sounds to me like it’s properly taken a charge and it should be OK to use normally. I doubt these batteries have cell isolation fuses that would allow it to work even if you had a cell failure...

Good luck!!!!

 

[Gionnirocket]

The battery outputs each 40v string individually and then the tool connects them in series or parallel.
The charger is rated 40v/2a so it's basically charging each string individually but simultaneously.
Wanting to be careful and know if the issue was with one string or both... I charged them individually at a lower current and in 3v steps

 

[Chazmo]

The battery outputs each 40v string individually and then the tool connects them in series or parallel.
The charger is rated 40v/2a so it's basically charging each string individually but simultaneously.
Wanting to be careful and know if the issue was with one string or both... I charged them individually at a lower current and in 3v steps
View attachment 192404

Ah, great. Thanks for helping me visualize this, Gionni!!!! It’s really two independent batteries in one package then.

If I understand you and you found both halves of the battery discharged down to 12V, it certainly wasn’t a cell failure or anything like that which caused it. You would’ve observed voltage differences between the two sides. Something discharged it; my guess is that you just made a mistake when you put it away, and it was already discharged. But, regardless of how it got discharged, it sounds like you did exactly the right thing to revive it and observe its behavior without stressing it.

I’m too novice to answer your questions with any certainty, but I’d probably do what you suggest and charge it outside a couple of times, as needed, before trusting it as “back to perfect” without paranoia.

Oh, the only other thing I know with lithium-ion is that discharging it that deeply usually means that when you go to recharge it it can heat up and shorten its life. But, again, having said that, the way you charged it back up probably mitigated that!

 

[harryS]

There must be some fault in the battery circuitry that drained both sides equally to 12V. I am surprised that the battery let you recharge too. An ebike BMS wouldn't let you even measure the cells if it was below 30V, My thoughts are if you let the battery sit for 4-6 weeks to see if they hold their charge, they will be flat again,

From what I've read/remember, it's having cells sit at zero volts can cause them to be bad actors if recharged. They develop metallic deposits (dendrites) that cause internal short circuits. Might be OK at 1.2V, but who knows.

 

[Gionnirocket]

The batteries let me charge... But the charger at that low of a voltage did not.
On tool batteries the charge and discharge connections are one/same so I guess it's more practical to build the safety into the charger/tool. The 2- 5yr old batteries are of the same lot and were used and stored pretty much the same.
I installed V meters in the lawn mower so I can discharge them equally.

I run one down 3v or so... Stop in the shade for something to drink and then run the other. Both batteries can be in the mower at once but it will run the first to LV cut off 31.5v(3.15v/cell) then swith to the other. I side step this by unlocking battery 1 so it no longer makes connection when I see it down about 3v... Then push it back in when battery 2 is down 3v.
Not sure what happened. Very confident I stored it at 35.5v as I've always done. Even between use during the summer as I try not to run them to LV cut off when practical. LV cutoff is 3.15 (+30%) a generous margin above the 2.5v noted in the spec sheet
I'll probably won't need it for another 2 weeks so I'll see what I have then.

 

[6zfshdb]

Interesting thread. Thanks for posting.

I've had similar issues with my DeWalt tool batteries. It seems that battery makers uses different charging & monitoring schemes. With DeWalt for example, the low voltage cutoff is in the tool, not the battery BMS. I usually don't have issues when using them in DeWalt tools, but they are easy to over discharge when used in other applications.

I frequently use their 20/60V batteries in my 52V e-bikes and occasionally get careless. It's easy to do when you have 3 - 20V battery strings in series to get 60V. It is difficult to monitor them individually. I like your idea of putting individual voltmeters on each string. I may do something similar on the bikes.

I've "revived" several over discharged tool batteries using the same bench power supply technique, and all have performed normally afterward.

In your case, I suspect it was a cell balance issue caused by an unknown fault that caused one string to go below minimum. The only sure way to prevent these problems would be to monitor each cell in a string individually. Problematic at best.

I usually do the cell balance routine on my e-bike batteries by fully charging, letting them sit for an hour or so and then fully charging again. I sometimes do this several times. This usually prevents extreme voltage disparities between strings. I've never had to do this with cordless tool batteries though. I wish my e-bike batteries were built to the same standards as tool batteries. They seem to last far longer before diminished capacity sets in.

 

[Chazmo]

In your case, I suspect it was a cell balance issue caused by an unknown fault that caused one string to go below minimum. The only sure way to prevent these problems would be to monitor each cell in a string individually. Problematic at best.

Hey, 6, I was guessing he had a cell problem as well, but I think he said both strings were discharged (equally). That doesn’t sound like a cell issue to me (not sure!), and I think Harry’s theory is the right one. If Gionni’s battery is working properly now, I’ll bet that some microcontroller in the BMS circuit went runaway and finally shut down after voltage dropped too low. That might explain how this could come back to life with no issues.

As a former software engineer who worked with hardware products, I can assure you all that there’s no shortage of bad programming in the microcontroller world.

 

[Gionnirocket]

Interesting thread. Thanks for posting.

I've had similar issues with my DeWalt tool batteries. It seems that battery makers uses different charging & monitoring schemes. With DeWalt for example, the low voltage cutoff is in the tool, not the battery BMS. I usually don't have issues when using them in DeWalt tools, but they are easy to over discharge when used in other applications.

I frequently use their 20/60V batteries in my 52V e-bikes and occasionally get careless. It's easy to do when you have 3 - 20V battery strings in series to get 60V. It is difficult to monitor them individually. I like your idea of putting individual voltmeters on each string. I may do something similar on the bikes.

I've "revived" several over discharged tool batteries using the same bench power supply technique, and all have performed normally afterward.

In your case, I suspect it was a cell balance issue caused by an unknown fault that caused one string to go below minimum. The only sure way to prevent these problems would be to monitor each cell in a string individually. Problematic at best.

I usually do the cell balance routine on my e-bike batteries by fully charging, letting them sit for an hour or so and then fully charging again. I sometimes do this several times. This usually prevents extreme voltage disparities between strings. I've never had to do this with cordless tool batteries though. I wish my e-bike batteries were built to the same standards as tool batteries. They seem to last far longer before diminished capacity sets in.

I believe with this brand the LV cutoff is in the tool as well. That's been my experience with all tools and I believe it makes sense from a practical stand point.

That said... The battery is holding steady at 36v so I'm led to believe it was this guy at fault.