SAFEST low discharge voltage for high drain unprotected 18650s?

[brightnorm]

Even though I'm a long-time CPF'r I know little about high drain 18650's, and I have always stayed away from them. I have unintentionally discharged protected Orbtronic 3500 Mah as low as 3.3 v with no apparent problems, but I'm afraid of doing that with unprotected high-drains.

I would appreciate any advice on this!

Thanks,
Brightnorm

[jimbo231]

You should use the search function on here..... 😉 J/K

[SilverFox]

Hello Brightnorm,

How are you doing?

The cell data sheets look at discharging down to 2.5 - 3.0 volts. That is probably a good target to shoot for.

There is some evidence that discharging down to 0 volts has minimal damage to the cell, but not much has been done on this. If you get into negative voltages you run the possibility of developing internal shorts and that can end up with "rapid disassembly sometimes accompanied with flame" and that is something we want to avoid.

Tom

[brightnorm]

Many thanks Tom. Wise advice as always!

brightnorm

[Gauss163]

[...] There is some evidence that discharging down to 0 volts has minimal damage to the cell, but not much has been done on this [...]

To clarify: there is no known data proving that it is safe to continue to use Li-ion cells that have been discharged down to 0V. Most likely it will be very risky to do so with most cells available at the consumer level. For further info see the discussion following this post.

[WalkIntoTheLight]

To clarify: there is no known data proving that it is safe to continue to use Li-ion cells that have been discharged down to 0V. Most likely it will be very risky to do so with most cells available at the consumer level. For further info see the discussion following this post.

To be fair, there's no data that proves a lithium-ion cell is unsafe if it drops to zero volts, either. It's probably not great for cell capacity, but I haven't seen any evidence that says the cell isn't safe to use. Charge slowly until it reaches 3v, etc. And, yes, I'd be a little worried about that cell, though that's because of anecdotal stories, not evidence.

[Gauss163]

To be fair, there's no data that proves a lithium-ion cell is unsafe if it drops to zero volts, either. It's probably not great for cell capacity, but I haven't seen any evidence that says the cell isn't safe to use. Charge slowly until it reaches 3v, etc. And, yes, I'd be a little worried about that cell, though that's because of anecdotal stories, not evidence.

No, there is both theory and experimental data proving that near zero-volt overdischarges are generally unsafe. The theory reveals the precise voltage where copper will start dissolving and the data shows the damage it can cause (dendrites, etc). Many of these studies are old (before many journals were online) so the papers are not so easy to find online by web searches. Probably the best way to locate them is in citations of recent literature on related topics, e.g. search for research on "near zero volt storage" - which has become a hot topic recently due to safety regulations for Li-ion battery shipping.

[WalkIntoTheLight]

No, there is both theory and experimental data proving that near zero-volt overdischarges are generally unsafe. The theory reveals the precise voltage where copper will start dissolving and the data shows the damage it can cause (dendrites, etc). Many of these studies are old (before many journals were online) so the papers are not so easy to find online by web searches. Probably the best way to locate them is in citations of recent literature on related topics, e.g. search for research on "near zero volt storage" - which has become a hot topic recently due to safety regulations for Li-ion battery shipping.

That's a start. The best paper I could find on the issue using that search is the following:

http://scholarworks.rit.edu/cgi/view...context=theses

While I certainly didn't read the whole thing, figure 9b shows an interesting graph where they worry about the copper dissolving into solution, which is the main worry about safety.

Eye-balling the graph, it looks like it becomes a concern when the battery voltage gets down below 0.5v. Above 0.5v, they say there is no dissolution occurring.

I'm not quite sure if this is the correct conclusion, but it seems to suggest that discharging down to 0.5v is okay, but any lower might pose a safety concern.

That doesn't really explain why manufacturers recommend 2.5v as a cut-off, but earlier the paper hints at this, stating that higher storage voltages are needed to allow for self-discharging that might eventually drain the batteries below a safe level.

[Gauss163]

That's a start. The best paper I could find on the issue using that search is the following:

http://scholarworks.rit.edu/cgi/view...context=theses

Yes, that 2017 RIT Thesis is among the first few matches in the search I suggested so it's a logical place to start, i.e. Enhancing Near Zero Volt Storage Tolerance of Lithium-ion Batteries, by Kyle R. Crompton.

While I certainly didn't read the whole thing, figure 9b shows an interesting graph where they worry about the copper dissolving into solution, which is the main worry about safety.

Eye-balling the graph, it looks like it becomes a concern when the battery voltage gets down below 0.5v. Above 0.5v, they say there is no dissolution occurring.

I'm not quite sure if this is the correct conclusion, but it seems to suggest that discharging down to 0.5v is okay, but any lower might pose a safety concern.

That doesn't really explain why manufacturers recommend 2.5v as a cut-off, but earlier the paper hints at this, stating that higher storage voltages are needed to allow for self-discharging that might eventually drain the batteries below a safe level.

The lowest safe voltage will depend on many parameters. The conservative ballpark 2.5V figure you see in old datasheets presumably includes enough of a safety margin so that it applies generally for all normal parameters. This is mentioned briefly in the thesis in the text accompanying the figure, namely

It is important to note here that determined values of the onset potential for copper dissolution can vary since the choice of an oxidative current density threshold is arbitrary and non-faradaic process can contribute to the oxidative current. Additionally, the exact value of the onset of copper dissolution can be affected by factors such as ambient conditions and electrolyte composition. In the present work, 3.1 V vs. Li/Li+ will be assigned for interpretation purposes as below this value, no oxidative current was observed in the linear sweep voltammogram in Figure 9d, and therefore, no copper dissolution is expected to be occurring.

The adverse effects from copper ions being present in the electrolyte will lead to side reactions with the electrolyte, competitive reduction processes with lithium ions, and copper dendrite formation which can lead to internal shorting. Each of these effects can cause significant damage to the cell and reduce its recharge capacity and performance.[85–91]. Thus, for cells to be tolerant to near zero volt storage, cell design must be modified or different materials used to avoid the copper dissolution degradation mechanism.

Emphasis is original - not mine. In the next section he mentions some prior research on related matters, which may yield further details.

[WalkIntoTheLight]

The lowest safe voltage will depend on many parameters. The conservative ballpark 2.5V figure you see in old datasheets presumably includes enough of a safety margin so that it applies generally for all normal parameters.

I suspect the 2.5v standard is done for historical reasons. Manufacturers probably have their test equipment designed around this, and changing it would be costly for little benefit. There's almost no useful capacity left below 2.5v, so lowering that discharge level wouldn't inflate their capacity numbers.

It's probably easiest to just say "2.5 volts", rather than figuring out a new value every time they make a new design. If they stated 0.5v for a specific cell and 0.7v for another design, most buyers wouldn't care and they'd just continue to use 2.5v in all their stuff anyway.

If you drain a laptop battery pack down to 2.5v, and then leave it sitting for a week before you charge it up again, it might be down to 2.0v. That's still safe and won't get the cell manufacturer or laptop manufacturer in trouble. If the laptop maker makes the low cut-off 0.5v, he might get in trouble if users don't recharge right away.

[WalkIntoTheLight]

I was reading though the spec sheet of the Samsung ICR18650-30Q cell, and noticed something interesting on their recommendations for pack design.

Samsung specifies this: "Under 1.0V voltage, do not charge the cell". They list 1.0v - 3.0v as "Voltage range of cell which shall be charged by Pre-charging" (charge slowly, I presume). 2.5v is the recommended low-voltage cut-off, and 2.0v as "shut down your BMS".

They don't mention any length of time a cell can sit near 1.0v. I find it interesting that they say it's okay to charge a cell above 1.0v. Previously, I've only seen the 2.5v low voltage cut-off mentioned.

Cutting off at 2.5v may just be a way to make sure the cell stays above 1.0v, if it's left sitting awhile to self-discharge.

Anyway, at least for the 30Q, it appears that cells are dangerous only if discharged below 1.0v.

[Provo48]

I have an old, but never used laptop battery pack. I dismantled it and now I have 6 x 18650 batteries. The average charge on these are about 1.5v. Will it be safe to charge these and use them? Thanks for your advice.