All high-power rechargeable cells are potentially dangerous if mistreated either mechanically or electrically, and it's as well to realize what the dangers are....
These cells - and I include all technologies here, including lead-acid - have a low internal resistance which means if you short them by accident huge currents can flow. More than 20 x C can be expected in many cases - that is twenty times the charge capacity in amps. If you have a 14 amp hour battery and you short it out you could see around 280 amps until something breaks - and a very rapid build up in temperature within the battery case could easily cause an explosion and the spread of very hot, corrosive and/or burning contents far and wide.
Of course, you do (hopefully) have fuses in the leads to protect against such destructive currents, but they won't save you if a cell develops an internal short, which can happen if the cell is mechanically or electrically damaged or overheats.
Under normal conditions the chances of that happening are very small, but the possibilty does exist, and it has been said that large rechargeable batteries are potential bombs because of the stored energy when they are charged which can be released catastrophically quickly if something goes wrong. Imagine half a kilowatt hour of energy being released in about three minutes inside a closed battery case contaning ten or twenty individual metal cased cells......
Hopefully the latest lithium technology is intrinsically safe, but it wasn't always so - some of the early lithium cells were prone to internal shorts and the results were often spectacular and expensive. You will have surely heard of exploding laptop and mobile phone batteries - a year or so ago Sony recalled a huge number of laptop batteries for free replacement because there was an 'issue' and some laptops had caught fire.
In order for modern lithium cells to really be safe in use, sophisticated built-in charge control circuitry has become the norm - usually referred to as the battery's BMS (Battery Management System). This is essential to ensure proper charging and balancing of cells, and it's usual also to protect against excessive discharge. In an e-bike battery this last is often a function of the controller rather than the battery itself. The BMS is matched to the cells used, so 'roll your own' batteries could be potentially dangerous unless you know what you're doing.
Rog.
