I have been using 18650 Lithium Ion batteries for several years now building battery packs and power walls. My power wall has 8KW of energy stored in 1120 batteries soldered together in packs They are charged through my solar panels and discharged into an inverter when power is needed. As long as I don’t draw down the batteries below 2.6 volts, or charge them greater than 4.2 volts, the batteries will last years. Now if I charge above the 4.2 volts, the battery begins to overheat and in that instance, they will more likely become dangerous. That’s why I use a charge controller specifically designed for Lithium-Ion batteries.
Now, how dangerous are they, well I have seen videos on YouTube, although I haven’t experienced anything like it, where a 18650 battery was short-circuited, and within several minutes, exploded in a ray of sparks.
I also know the Lithium-Cobalt Oxide Battery, which is the flat packs, expands when they are ending their life cycle and these batteries will explode more quickly than 18650 or other Lithium batteries when shortedor after they have started expanding.
Here’s something I found on the web
The 5 types of Lithium-ion batteries along with their properties are given below.
Lithium-Cobalt Oxide Battery
Used mostly in handheld electronics (Cell phones, Laptops and Cameras)
Risky especially when damaged
Cobalt is scarce and expensive
Low discharge rates
Highest energy density (110-190) Wh/kg
Lithium-Titanate Battery
Can operate at very low temp (-40°C)
Rapid charge and discharge
Used in Mitsubishi i-MiEV
Lower inherent voltage 2.4 V (compared to 3.7 V)
Lower energy density (30-110) Wh/kg
Lithium-Iron Phosphate Battery
Dramatically reduces the risks of overheating and fire.
Offers much less volumetric capacity
Used in power tools and medical equipment
Longer-life and inherently safe
Lower Energy Density (95-140) Wh/kg
Lithium-Nickel Manganese Cobalt Oxide Battery
Longer life and inherent safety
Cobalt is scarce and expensive
Less prone to heating
Used in Power tools, e-bikes and electric power trains
Lower energy density (95-130) Wh/kg
Lithium-Manganese Oxide Battery
Lower cost
Longer life and inherently safe
Used in Hybrid Vehicles, Cell phones, Laptops
High discharge rates
Lower energy density (110-120) Wh/kg
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