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  • What Stops Laptop Batteries From Exploding

    Lithium is highly reactive and flammable. To tame the beast within, laptop batteries should contain multiple safety circuits and mechanisms. Read below to find out what these are and how many generic batteries do away with some or all of them to make their batteries cheaper.

    Fig. 1. Inside a Laptop Battery

    battery fig

    1 Controller IC monitors each cell voltage for overcharge or overdischarge conditions and engages the cutoff switches if required. Also monitors the voltage across the switches to prevent over current.

    2 Control switches cut off the charge or discharge depending on the control signals of the controller IC.

    3 Temperature fuse that cuts off the current if the control switches experience abnormal heating. This fuse is not recoverable.

    4Thermistor measures the battery temperature inside the pack. It's terminals are connected to the charger so it can sense the temperature of the pack and control the charge current until the battery it's fully charged.

    Cell Mechanical Safety Devices

    battery fig

    In addition to the safety circuit the cell should contain mechanical safety devices such as an interrupt device (CID) which opens the electrical path if an excessively high charge voltage raises the internal cell pressure to 10 Bar (150 psi); and a safety vent which allows a controlled release of gas in the event of a rapid increase in cell pressure.

    Lithium-Ion Safety Concerns

    Long battery runtimes have always been the wish of many consumers. Battery manufacturers have responded by packing more active material into a cell. To make room for the extra active material manufacturers have had to make the separator between the anode and cathode thinner and thinner. While this has enabled a doubling of energy density since lithium-ion was introduced in 1991 it comes at a price. The thinner separator means the small metal particles present in the Li-Ion mixture are more likely to pierce the separator causing a short circuit. If this happens usually only a mild short will occur causing an elevated self-discharge. Little heat is generated because the discharging energy is low and providing the cell is equipped with a safety vent any gas pressure is vented preventing a dangerous build up. If the cell temperature becomes high enough the controller ic will open the temperature fuse (if present) cutting all current supply to the cell. All going well the cell will become inert but the laptop battery will be rendered useless and need replacing. If, however, enough metal particles converge on one spot, a major electrical short can develop and a sizable current will flow between the positive and negative plates. This causes the temperature to rapidly rise, leading to an unstoppable thermal runaway, also referred to 'venting with flame'. If a cell reaches this condition not even the safety circuit board or mechanical safety devices built into cell can stop this massive reaction. This type of failure triggered a recall of almost six million Sony built Li-Ion battery packs in 2008. Sony confirmed the cause to be oversized metal particles piercing the separator. This along with factory fires at LG & Panasonic Li-Ion manufacturing plants illustrates that producing Li-Ion cells is an incredibly difficult and dangerous practice. It also raises the question that if the vastly experienced and resourced Japanese and Korean manufacturers with huge R & D budgets can suffer production difficulties how much trust can be placed with the relatively new smaller Chinese manufacturers. Time will tell if these Chinese manufacturers are able to improve their safety and quality. In the meantime it is best to follow the lead of laptop manufacturers and stick with laptop batteries containing only Japanese or Korean cells.

    Why You Should Only Buy A Laptop Battery With Well Known Brand Name Cells Such As Panasonic or Sanyo

    1. To prevent the infiltration of unsafe packs onto the market, well known manufacturers only sell lithium-ion cells to approved battery pack assemblers. The inclusion of an approved safety circuit is part of the purchasing requirement. This forces price driven battery pack assemblers who are willing to fore go important components of the safety circuit in order to save costs to buy cheaper unbranded ungraded cells.
    2. The complex assembly techniques required to make Li-Ion cells make the elimination of all metal particles nearly impossible. However improved production methods by the well known brands ensure that these particles are very small thus reducing the likelihood of them piercing the separator and short circuiting the battery.
    3. Despite a laptop battery representing a major component cost in the production of new laptops all major laptop manufacturers use only Japanese or Korean made cells. This raises the question why would a major manufacturer such as Dell or Toshiba choose not to use cheaper Chinese cells when even a saving of only one dollar per battery would result in millions of dollars extra profit? The answer is simple, at the moment only Japanese or Korean made cells offer the necessary safety and performance.

    The non-brand replacement batteries that are found on the plethora of internet sites are popular with users because of low price. The vast majority of these batteries do not provide the same high safety standard and performance as the main brand equivalent. A wise shopper spends a little more and replaces the battery either with the original manufacturer's battery pack or a quality generic battery brand such as 2-Power who use licensed battery assemblers and guarantee the use of only Japanese or Korean brand name cells such as Panasonic or Sanyo in the manufacture of their laptop batteries.

     

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