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Category: Maintenance

67. Why are replacement iBook and PowerBook batteries so expensive?

Part of the reason is that LiIon batteries can do bad things if they overheat (creation of Lithum metal which "burns" in water, chance of fire). So LiIon battery packs have an internal circuit to prevent overcharging (which would cause them to overheat). There can be several functions for the protection circuit, including shutting it down in case of over charging, when the voltage drops to a predefined level, or if it thinks the battery is otherwise damaged. Problems with the power circuit can cause a "good" battery to shut down. The recent post about fixing iPod batteries likely had to do with re-setting something in the battery protection circuit that caused it to shut the battery off early. The Buchmann site talks about methods some have used to try to re-set the protection circuit on batteries that seem to have died young. >>>>Trying to reset a LiIon protection circuit is dangerous to try yourself - you could end up with a nasty fire.<<<< Plus the electrolyte is flammable and caustic, so it needs to be well packaged to make a spill unlikely, even if you do manage to overheat it. All this costs. More detail below


http://www.buchmann.ca/chap2-page7.asp "Despite its overall advantages, Li-ion also has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the maximum charge and discharge current is limited and the cell temperature is monitored to prevent temperature extremes. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated." http://www.buchmann.ca/Chap4-page8.asp "What if a battery is inadvertently overcharged? Li-ion batteries are designed to operate safely within their normal operating voltage but become increasingly unstable if charged to higher voltages. On a charge voltage above 4.30V, the cell causes lithium metal plating on the anode. In addition, the cathode material becomes an oxidizing agent, loses stability and releases oxygen. Overcharging causes the cell to heat up. " (i.e. it could catch fire.) http://www.buchmann.ca/Chap10-page7.asp "An increasing number of modern batteries fall prey to the cut-off problem induced by a deep discharge. This is especially evident on Li-ion batteries for mobile phones. If discharged below 2.5V/cell, the internal protection circuit often opens. Many chargers cannot apply a recharge and the battery appears to be dead." http://www.buchmann.ca/Chap5-page3.asp "The Li-ion typically discharges to 3.0V/cell. The spinel and coke versions can be discharged to 2.5V/cell. The lower end-of-discharge voltage gains a few extra percentage points. Since the equipment manufacturers cannot specify which battery type may be used, most equipment is designed for a three-volt cut-off. Caution should be exercised not to discharge a lithium-based battery too low. Discharging a lithium-based battery below 2.5V may cut off the battery?s protection circuit. Not all chargers accommodate a recharge on batteries that have gone to sleep because of low voltage. Some Li-ion batteries feature an ultra-low voltage cut-off that permanently disconnects the pack if a cell dips below 1.5V. This precaution prohibits recharge if a battery has dwelled in an illegal voltage state. A very deep discharge may cause the formation of copper shunt, which can lead to a partial or total electrical short. The same occurs if the cell is driven into negative polarity and is kept in that state for a while. A fully discharged battery should be charged at 0.1C. Charging a battery with a copper shunt at the 1C rate would cause excessive heat. Such a battery should be removed from service." http://www.buchmann.ca/Chap15-page3.asp "The internal protection circuit of lithium-based batteries may be the cause of some problems. For safety reasons, many of these batteries do not allow a recharge if the battery has been discharged below 2.5V/cell. If discharged close to 2.5V and the battery is not recharged for a while, self-discharge further discharges the pack below the 2.5V level. If, at this time, the battery is put into the charger, nothing may happen. The battery appears to have an open circuit and the user consequently demands a replacement. "


Related Links:
http://www.buchmann.ca/
http://dealmac.com/forums/read.html?f=1&i=1092115&t=1092077



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