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

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

It even says "Conservative discharge that would not extract maximum performance". A lot of electronics built for li-ion will not work <3V so it's a perfectly reasonable benchmark, especially to show the difference between brands.

A lot of electronics need a 3.3v rail and produce that from a linear or buck regulator, so won't work below 3.4 volts.

I went to vapcell's website, looked at their charts and specifications. I disagree with your assessment.

They specify 3800mAh minimum discharge at 0.2C for their 4000mAh cell. They don't show a discharge curve at 0.2C in their charts (would be 0.8A) but they do have a 1A discharge. When the 1A discharge curve reaches 3V, the energy discharged is right around 3800mAh.

Lumafield discharged at 0.2C, and they saw only 3055mAh.

Vapcell's site mostly shows high current discharge curves, where yes there is more capacity below 3V available, but with Li-ion at lower currents, the curve is very steep past 3V, not much more capacity left after that. And when you're discharging at high current you won't expect to get the full capacity, anyway.

I'll also take this time to point out lygte-info which is a treasure trove of battery tests.

[0] https://www.vapcelltech.com/h-pd-193.html

[1] https://www.lygte-info.dk/

Battery operated devices, while nominally 3.3V, commonly work down to 3V or even 2.7V in line with the voltage curve of lithium (not li-ion) coin batteries.

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?

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

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.

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.

    > 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.

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.

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.

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.

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.