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CT scans of 1k lithium-ion batteries show quality risks in inexpensive cells

(www.lumafield.com)
287 points jonbruner | 9 comments | 24 Sep 25 19:29 UTC | HN request time: 0.001s | source | bottom
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Neywiny ◴[24 Sep 25 22:58 UTC] No.45366921[source]▶
I find their table of advertised vs actual capacity to be misleadingly negative. They only discharged to 3v. 2.7 could be viewed as more standard. 2.5 is not unheard of. For example, the vapvell 4000 they said was around 3000. They even have a note that says that isn't a reasonable capacity estimate. And yet they still put the percentage and the number. As if they've falsely advertised. However, if you go to vapcell's graphs on their website, it all tracks. Feels out of scope for the report and shouldn't have been done
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1. quickthrowman ◴[26 Sep 25 22:02 UTC] No.45391469[source]▶
How do you even drain a 3.7V lithium ion battery below 3.3V? My devices that use 18650s will not let them go below that. Is it 3.3V nominal and the actual voltage is lower, like how they’re 4.2V fully charged?
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2. joecool1029 ◴[26 Sep 25 22:49 UTC] No.45391820[source]▶
I guess leave them sitting in a laptop in a barn for a few years. I was given some old shit gateway the other day, tried to charge it, no dice. Ripped the pack apart to find samsung 18650's with approx 0.7V voltage.

Against ALL recommendations I put these cells into a MC3000 and they charged up just fine to 4.2V. It does a 0.15A charge per cell until it gets above 3V. Then I had it set to 1.92A bulk charge. Fire extinguisher nearby lol

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3. privatelypublic ◴[27 Sep 25 02:11 UTC] No.45392775[source]▶
4.2v for 100% SoC (State of Charge, charging), settles down closer to 4v once charging stops (likely not exactly correct, just my rote memory). 80-90% of capacity/SoC is between 3.7-3.8v.

The remaining 10-20% is above 3.8 and below 3.70v. 4.2v is the max to ever intentionally apply to a cell, and 2.5v is the minimum anybody specs as end of discharge.

As such- the "nominal" voltage is 3.7v or 3.8v.

Possibilities for 3.3v cut off include: LEDs' combined forward Voltage, a BMS set to that voltage, high drain applications pulling the voltage below a lower (potentially much lower at 10C to 50C discharge) cutoff voltage.

These are, of course, for NMC lithium secondary/lithium ion cells. NOT LiFePO4/LFP/LTO/Na-ion.

Theres lots of FUD, but most lithium stories are Li-Po cells (cell phones, RAdio-Control, laptops). Of the Li-Po's- most of them are for RC usage without any inbuilt protection.

LiPo's are beat treated like they're ziplock baggies full of 100mL of gasoline. If you handle them, I suggest buying an Ash Pot- their double walls give you a chance of containing a flaming pack. Best to just do it outside though.

4. privatelypublic ◴[27 Sep 25 02:16 UTC] No.45392791[source]▶
Can't recommend charging cells that have fallen below their official stop voltage- the liability and risk is too high. However, numerous papers have shown that the serious risks happen if they're reverse charged.

I wouldn't use anything but a bench supply stuck outside though.

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5. avar ◴[27 Sep 25 09:25 UTC] No.45394350[source]▶

    > How do you even drain a 3.7V lithium
    > ion battery below 3.3V?
Connect the + and - terminals with an appropriately sized resistor, it'll drain all the way to 0V.

    > My devices that use 18650s will
    > not let them go below that.
Because you're not using the + and - terminals, you're using the + and - supply of a BMS, which is connected to those terminals. For this sort of testing you need to bypass the BMS, which'll have its own voltage cutoffs.
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6. quickthrowman ◴[27 Sep 25 13:00 UTC] No.45395352[source]▶
I understand you can discharge a battery completely with a resistor, just like any other capacitor. I also know there’s a battery controller in my device.

My actual question should’ve been ‘Do people really use lithium ion batteries in devices without battery managers?’ I absolutely would not.

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7. avar ◴[27 Sep 25 14:54 UTC] No.45396220{3}[source]▶
Neywiny's comment upthread isn't that you should use these without a BMS, but that the review is relatively less useful because it's stopping testing at a relatively high voltage. E.g. if you search for "panasonic_ncr18650b.pdf" you'll find that Panasonic's own datasheets use a cutoff of 2.5v.
8. Liftyee ◴[27 Sep 25 15:05 UTC] No.45396309{3}[source]▶
Some do. Anecdotally, some (uncommon, enthusiast-grade) flashlights I use don't have battery managers/over discharge protection, though most of mine do. If you can bear the responsibility of maintaining and storing the batteries properly, IMO there's no significant problems.
9. joecool1029 ◴[27 Sep 25 18:11 UTC] No.45398113{3}[source]▶
As I said, it's against all recommendations. I've done this a few times over the years though and haven't hit the copper shunting problem described in literature. If they did have that problem my charger would pick up the short and stop charging them.

I did have a failure once and it was on a new molicel. I damaged the outside wrap of a cell while building a pack and it had a short and self-discharged the cell, likely reverse charging it in use. A week or so later the charger rejected it. When I pulled it out it was a fully shorted cell that would accept no charge, but it did not catch fire.