Originally Posted by stazi34
I agree with what DrFredPhD said, LIB are very dangerous if tampered with or altered physically.
Yeah, running/charging multiple lithium batteries typically isn't a good idea. But the videos that DrFredPhD posted aren't what you'll get with a cell phone battery.
Li-ion and Li-po batteries in commercial devices have built-in circuitry to prevent overcharging, overdischarging, reverse polarity, and short circuits. If you short it, the circuitry kicks in and kills everything. Putting it back in the device/charger resets the kill switch. Same if the charge gets too high, or the discharge gets to low. Circuity switches the battery off, and prevents overcharging and runaway discharging.
Those videos show batteries that don't have that circuitry. Either it was removed, or they're cells used by hobbiests and were purchased unprotected. Unprotected cells are available for hobbiests (R/C stuff, high-powered flashlights, etc) and some use them because the circuitry in protected cells leeches a bit of your juice. In an R/C car, that could mean a few more seconds and one more completed lap, in an R/C plane or helicopter, a few more seconds of air time before you crash. But there are obvious dangers.
Those dangers don't exist with commercial protected batteries. Well, they do, but that's when you buy a cheap knock-off from China, or the cell is defective, like those Sony and Dell laptops that were catching fire several years ago.
But even with protected commercial batteries, problems can occur (just not explosively dangerous ones). If the cells are mismatched in capacity, or one has a higher charge than the other, the higher capacity one will try to charge the other one, and the battery life of both will suffer, plus since they're also trying to power the device or receive a charge, they'll get really hot and can be damaged. Running identical cells is one thing. Charging should be done separately so that they start off the same. It's not the same as with a laptop or electric car, because they have additional circuitry to make things safe. It's also not the same as using a battery bank to power/charge your device, because effectively, that's just the same as plugging into a USB jack.