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What's the difference between unprotected and protected LiIon battery

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Abdulbadii

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What's the difference between unprotected and protected Lithium Ion AA battery said in many advertisement
 

Protected ones (I have 18650 cells) have a small PCB mounted at the bottom (negative end) that prevents over charging and discharging. If you are using them stand alone mode, you should get the protected ones. If you are going to use them in a circuit that already has built in protection, you just need the unprotected ones.
 

A Lithium battery is very dangerous because if it is charged or discharged wrong it can explode or catch on fire. The fire is very hot and difficult to extinguish. Therefore it is not made in a "AA" size. An 18650 battery is larger and will not fit in the place of a AA size.
Battery packs made for a laptop computer have the protection circuit separate from the cells but in the pack. I have harvested and used many unprotected 18650 cells from old laptop battery packs. I am careful and never use protected cells.
 

... I am careful and never use protected cells.

It is not good to say that a lithium battery is very dangerous...

All batteries store energy and the danger level is just proportional to the stored energy. A big lead acid battery is perhaps far more dangerous. Just try to short a lead acid battery and see (please don't do that!)

Unprotected cells are supposed to be used with systems that have independent protection built in. I use 18650 cells in torches and toys and I have not a single problem: they are are all protected.

A Li-ion battery may catch fire if short circuited - but the amount of Li present in a single cell is too small to be of any danger. If they are really that dangerous, I wonder how passenger cars are using them?
 

A Li-ion battery may catch fire if short circuited - but the amount of Li present in a single cell is too small to be of any danger. If they are really that dangerous, I wonder how passenger cars are using them?

SOME passenger cars, anyway: most Priuses (Prii?) that you see are still NiMH
 

LithIUM, titanIUM, magnesIUM, potassIUM, sodIUM, aluminIUM, cadmIUM, selenIUM, germanIUM and thallIUM (and others) are highly reactive metals and some burn violently.
If a lithium battery charger detects a battery discharged to a voltage too low then it attempts a charge at a low current, if the voltage doesn't rise it stops the charging and gives an alarm. Because when discharged too low some of the lithium ions become a short circuit of pure lithium metal that could explode or catch on fire if a normal charging current is applied.
 

...LithIUM, titanIUM, magnesIUM, potassIUM, sodIUM, aluminIUM, cadmIUM, selenIUM, germanIUM and thallIUM (and others) are highly reactive metals....

By the way, selenium and germanium are not metals. Titanium has properties very similar to Aluminium and is often used in place of Al (it is stronger).

Most of the aircraft bodies are made of an alloy of magnesium and aluminium (it is stronger than Al and lighter than steel).

Metals, being solid, usually do not burn violently. carbon powder, soaked in liquid O2, burns beautifully. Hydrogen oxygen mixture burns violently.

It is rather tricky to set fire to Aluminium (it is used in fireworks; also magnesium), magnesium is easier. I tried to burn a Ti rod with a cigarette lighter (experiment unsuccessful).

Sodium metal is used in Napalm; it has sodium metal, petroleum jelly and white phosphorous. It is the phosphorous that burns. Petroleum jelly helps spread the fire. Sodium causes burns on the body (also danger if you put water to put out the fire)
 

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