I have been researching li-ion batteries for some time and I am familiar with the precautions necessary when working with them... however, I am still learning and realized I made a bit of a rookie mistake that I wanted some opinions on:
(This post is admittedly verbose, but I feel that it is important to be as thorough as possible, especially when asking a safety related question...if you feel the info below is flipping your TLDR (too long, didn’t read) trigger, you can optionally just skip down to “The Issue” section).
Parts used: One old laptop battery: The original battery was only used for a few months (purchased brand new, direct from IBM, if that is any indication of original cell quality) and it worked perfect and always held a full charge. The problem was but one of the laptop ports broke. At that point, the battery was promptly removed (I do recall fully charging it before removal)...but then the battery sat on a shelf for several years.
Four powerbrick chargers: Exact charger used - Link Those small single PCB kind of charger that take micro-USB 5v input, have two Stardard USB 5v outputs, as well as a button, LED flashlight and a backlit percentage meter. These chargers also (claim to) have builtin over/under charge and discharge protections.
The project: I removed 6 lithium ion batteries from an old laptop battery and separated them such that I had 2 individual cells and 2 parallel sets (of just 2 batteries each)... to make a total of four small rechargable powerbanks. I hooked each of the four sets to individual usb powerbrick chargers (described abive)
I successfully charged each of the 4 groups of cells to 100% and tested their capacity by fully discharging them through 5v USB output (and recorded thier capacity) of which by my (admittedly rough) calculations was all in the same ballpark of 2000mAH per battery. Next I charged them all back up to 100%.
The issue: I forgot to test each cell prior to charging them for the first time. So I have no idea if any cell had a voltage under the 2.1v minimum required for a cell to be considered safely rechargeable. Granted, the charging circuits I bought charged them without issue (not to mention they also discharged them completely and recharged them back to 100% without any problems). I am none the less concerned that this mistake could have made the batteries unstable internally (ie: Potentially created dangerous internal shunts).
Should I really be concerned or am I being overly paranoid (given the fact that the cells all seem to have decent capacity)?
If there is cause fir concern is there anyway at this point to assuage those concerns by further testing the batteries? (For example fully discharging them through the charger circuitry down 0% and then letting them sit for a day/week/etc and then testing the voltage... <——- this example is just a guess on my part).
Additional general questions about li-ion battery creation:
This is a bit of a separate question, but when using these type of chargers do I need to add additional circuitry to add a thermistor to the mix? Reason I ask is that I have dissassembled store bought USB powerbanks of various well known name brands and there are no thermistors. (Note: I am specifically asking in terms of cells connected only in parallel, not in series/parallel groups).
Beyond the thermister question above are there any additional safety precautuons I should take in the powerbrick? Are fuses necessary (or even recommended) in parallel configurations with cells of matching capacities? Again, I don’t see them in the name brand, store bought powerbricks, so I am unsure.
And finally, a bit more advanced question: This is the only question that I have in regard to series/parallel battery configurations (right now anyway, still learning....always learning): If all cells are from the same (originally bundled top-tier name brand) laptop battery (that was used but verified to be fully functional), is it safe to assume the cells are sufficiently matched in terms of impedance? I only ask because I do not have the equipment to test impedance at this time. I know assumptions are in general a bad idea, but I have read that top laptop manufacturers always ensure their original parts suppliers do impedance matching for their laptops (baring any recalls of course).