Thanks Loup. You may have saved some a whole lot of pain.
I was told that puffy batteries are not good and to put them in a metal box until they can be properly disposed of. It wasn't explained what: "Properly disposed of" is. I imagine lithium batteries are considered hazmat? Any other signs of potential battery failure that should be monitored?
Failure or pre-fail evidence is not always visible. As Knoxville's Joker pointed out, the BMS should keep you from getting into the danger zone, it just won't let you use the battery or use it that much. The BMS is THE MOST IMPORTANT PART OF A LITHIUM PACK. And as he also said, the charge controller or battery charging system is the second most important thing to consider. I (ab)use the ISDT series of chargers for all my Lithium and AGM charging needs, including a Q6 Pro in all of my "power bags" and a couple of Q8 Max chargers for the bigger packs that I have that are still "portable". Either of them will charge any Lithium chemistry as well as any Lead Acid, Nickle Iron, or even any NiCd/NiMH. And they both have the flylead setup for balancing event the toughest of batteries. The Q8 Max can handle 8 cell Lithiums and balance close to 2A while charging at 30A, so you can really work on some big batteries with that one.
If you ever have had your laptop or other lithium battery powered device start getting "less and less capacity or runtime" it is because the onboard BMS is keeping ALL of the cells as a whole in that tight tolerance band of voltages. If one cell gets weak in your laptop's battery, the whole pack will act on that one cell's limits, so when that one cell gets too low, the pack shuts down rather than drag that cell lower than it's safety cutoff. Likewise, charging the pack back up suffers as well as that one weak cell gets charged quickly and the rest end up either slow charging through the BMS's equalization circuitry that tries to keep all of the cells at the same voltage, OR the rest of the pack never gets a full charge, which quickly ruins the rest of the pack.
BMS controllers do three things at MINIMUM, they keep the battery as a whole, and ANY individual cell from getting charged past their max voltage, or any cell getting discharged past their cutoff voltage. They also keep all of the cells "balanced" meaning they are at their same voltages EACH. The last main thing a BMS does is protect the pack against too much charge current or discharge current, especially at lower temperatures. BMS devices have different ways of doing each of these important roles, but at minimum they will open the circuit with one or both of the battery terminals to stop the charge or discharge of the battery if the voltage gets above or below safe setpoints, and they will try to equalize the voltage across each cell to keep the pack "balanced". Some BMS systems just throw a load (small resistor) across the higher voltage cells to pull them down to the voltage levels of the others, some use a charge pump scheme with inductors and capacitors to charge up a cap using the higher voltage cells and deliver it to the lower voltage cells. The ones using the resistor "load" are cheaper, and if you are charging daily work fine, but since they are a rather parasitic load on the pack, you don't want them for long term storage systems since they will be constantly draining down each cell to make them all match. For battery packs that you don't want to have to charge back up every few months, get the ones with the charge pumped BMS devices.
Most of my packs I have built myself, or at least rebuilt myself. I bought a BUNCH of the smaller 10-60AH batteries from companies like the better ones on Amazon since I use a LOT of 7AH to 65AH size batteries. Many of them I have torn apart, used the cells out of to put in other things, and put in better BMS systems as I did. I gutted a 20AH LiFePO4 back, to put in the bottom half of my Yaesu FT-897d radio since it was not only CHEAPER to go that route than Yaesu's NiMH packs, but also would give me 3 times the runtime AND be able to charge off of a normal 12V charger that could also be used on my AGM batteries. In the end, I had not only enough room in the radio's battery tray to put in a much better BMS, but also put in a solar charge controller so that I could plug in a 18V solar panel directly into the back of the radio and keep going.
Now, you do not need a BMS if you are talking about using a single cell in a device that only will discharge them and cuts off at around 2.0-2.4 volts, since you will be charging it somewhere else. That is why I pulled a LOT of old laptop batteries apart for their cells to be used in either radios or flashlights that only used one cell and did not try to charge that cell (or had a dedicated, smart voltage cutoff charger built in). The cells are safe in those devices without a BMS. Any other type of device, or one that uses multiple cells at one time, you will need a BMS in the system for safety and for long cell life. And there is nothing preventing anyone from making their own packs with a BMS using standard 18650 or 23650 cells in an appropriate cell holder so that you can swap the cells out if they get weak and keep going. This is all a "battery wall" is in most cases, your just making a smaller version.
