This involved disassembling a variety of old laptop batteries.
It was interesting how much/little safety each manufacturer put into their packs.
The bare cell itself already has some safety built in. Not so sure about extra cheap ones.
-Over current PTC
-Over pressure cut off
-Controlled venting. Still dangerous as this includes flames. But stops it from exploding.
External levels of safety includes, in order of activation
-Over charge/discharge cut off
-Over current PTC
A more detailed article
"Lithium-ion safety concerns"
Other good links
"Discharging at high and low temperature "
"Charging lithium-ion batteries"
How bad does it have to get before they go bang?
Pretty bad it seems. We will examine the lower threshold type.
Normal operating temperature is -20°C to 60°C (-4°F to 140°F)
At 130°C (265°F) you should stop using it.
Note, boiling water is 100°C (Depending on atmospheric pressure)
The thermal fuses in some packs are rated at 98°C.
At 150°C (302°F) get it off your lap and watch the light show.
Levels of failure required+
+Protection circuit fails to stop over charge/discharge.
+Thermal fuses fail to open circuit
+User does not notice the heat and smell emanating from the faulty device, assuming they are using it at the time.
+Vents are clogged.
Incidents of failure?
When it does it's big news, but considering the number of cells shipped (each laptop has at least 6), it is still very rare.
The failure is usually from a manufacturing defect rather from failure of the protecting systems.
Are cheap ones more susceptible?
They have to cut costs somewhere so a lower level of Quality Control might be it.
So you will be safe in the knowledge that if one fails on you you will get your 15 mins of fame, and most likely a new device.
From the packs I have seen, the worst one had no balancing system.
This can lead to premature capacity loss once the cells start getting unbalanced.
The best one had everything, but it doesn't help if the cell is faulty.
Once thing I did think was odd was that the cells were charged to 4.2v
Which contradicts what I had read before.
After more reading they can be charged to 4.2v(10% more capacity) but with a lower service life.
How many cycles do you want from your cell?
Even if there is a balancing circuit it may not be operating properly, leading to reduced service life.
A selection of protecting circuits from various packs.
Other related posts:
Mobility Scooter eCarts conversion
Samsung galaxy cutdown
How big a heatsink do you need to run a green laser pointer continuously?
Comment by CherryZhan, on 20-Jan-2010 22:12
The best one had everything, but it doesn't help if the cell is faulty.?
Got it,thank you
Comment by globallaptopbattery, on 30-Jan-2010 22:42
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