..::How to service laptop batteries::..

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    Post by mehedi on Sun 07 Nov 2010, 6:36 pm

    How to service typical laptop batteries

    Most laptops batteries are 'smart', meaning thatsome form of communications occurs between the battery and user. Thedefinition of 'smart' varies among manufacturers and regulatoryauthorities. Some manufacturers call their batteries 'smart' by simplyadding a chip that sets the charger to the correct charge algorithm.The Smart Battery System (SBS) forum states that a 'smart' battery mustprovide state-of-charge (SoC) indications.

    There are two common architectures of 'smart' batteries, consisting ofthe single wire system found on high-end cameras and radiocommunications devices, and the two-wire system typically used onlaptops. The two-wire system is usually configured to the SystemManagement Bus (SMBus). Because of its common use in laptops, we willfocus on the SMBus system. Figure 1 shows the layout.

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    Figure 1: Two-wire SMBus system.
    The SMBus is based on a two-wire system using a standardizedcommunications protocol. This system lends itself to standardizedstate-of-charge and state-of-health measurements.

    Battery connection

    The SMBus battery has five or more batteryconnections consisting of positive and negative battery terminals,thermistor, clock and data. The connections are commonly unmarked andattempting to test this type of battery appears complicated. Figure 2describes the functions of a battery with 6 connections.

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    Figure 2: Connections of a typical laptop battery.
    The positive and negative terminals are usually placed on the outside; no norm exists on the arrangement of the contacts.

    The positive and negative battery terminalsare commonly located at the outer edges of the connector. The innercontacts accommodate the clock and data. (On a one-wire system, clockand date are combined.) For safety reasons, a separate thermistor wireis brought to the outside. This allows temperature protection if thedigital communication is disabled.

    Some batteries are equipped with a solid-state switch that is normallyin the off position. In such a case, no voltage is present. Connectingthe switch terminal to ground will turn the battery on. If this doesnot work, a proprietary code may be needed to activate the battery.

    How can I find the correct terminals? To beginwith, use a voltmeter to locate the positive and negative batteryterminals. Establish the polarity. If no voltage is available, asolid-state switch may need to be activated. With the voltmeterconnected on the outer terminals, take a 100-Ohm resistor (other valuesmay also work), connect one end of the resistor to ground, and with theother end touch each terminal while observing the voltmeter. If novoltage appears, the battery may be dead or the pack requires a digitalcode to activate. The resistor protects the battery against a possibleelectrical short.

    Once the connection to the battery terminals is established, chargingshould be possible. If the charge current stops after 30 seconds, anactivation code may be required. This code is often difficult, if notimpossible to obtain.

    Some battery manufacturers even add anend-of-battery-life switch. At a preset age, cycle count or capacitylevel, the battery stops functioning. Manufacturers explain thatcustomer satisfaction and safety can only be guaranteed if the batteryis regularly replaced. Such policy tends to satisfy the manufacturermore than the user. Newer batteries generally do not have this feature.

    It is recommended to utilize the thermistor during charge and dischargeto protect the battery against over heating. The thermistor can bemeasured with the Ohmmeter. The most common thermistors are 10 Kilo OhmNTC or 10kOhm at 20°C (68°F). NTC stands for negative temperaturecoefficient, meaning that the resistance decreases with risingtemperature. A positive temperature coefficient (PTC) will increase theresistance. Warming the battery with your hand may be sufficient todetect a small change in resistor value.

    An SMBus battery contains permanent andtemporary data. The permanent data is programmed into the battery attime of manufacturing and includes battery ID number, battery type,serial number, manufacturer and date of manufacture. The temporary datais acquired during use and consists of cycle count, user pattern andmaintenance requirements. Some of this information is renewed duringthe life of the battery.

    Repairing a 'smart' battery

    Laptop batteries can be repaired but the workis often time consuming. The success rate varies with battery type. Onemust remember that the 'smart' battery consists of two parts, thechemical cells and the digital circuit. In some cases, the chemicalbattery can be fully restored but the fuel gauge may be inaccurate orits data is corrupt.

    Anyone attempting to repair SMBus battery mustbe aware of some non-compliance. Unlike other tightly regulatedstandards, the SMBus allows some variations. This may cause problemswith existing chargers and the SMBus battery should be checked forcompatibility before use.

    If the cells are weak, cell replacement makes economic sense. Whilenickel-based cells are readily available, lithium-ion cells are notsold on the open market. This precaution is understandable whenconsidering the danger of explosion and fire if the cells are assembledin a careless way. Always replace the pack with the same chemistrycells.

    During cell replacement, the circuit of the'smart' battery may need to be kept alive with a supply voltage.Disconnecting the circuit, if only for a fraction of a second, canerase vital data and render the circuit unusable. To assure continuedoperation when changing the cells, connect a secondary voltage througha 100-Ohm resistor before disconnecting the cells. Remove the secondarysupply only after the circuit is fed from the new cells.

    The open terminal voltages of the replacement cells should be within10% of each other. Welding the cells is the only reliable way to getdependable service. Attention must be paid to limiting the amount ofheat transferred to the cells during welding. Excess heat can damagethe cells.

    During storage, each cell may haveself-discharged to a different charge level. This is especially evidenton nickel-based batteries. To assure proper charge of all cells withoutovercharging some, trickle charge the newly repaired pack for about 14hours, then discharge and recharge normally. Such a cycle is alsoneeded to reset the battery's fuel gauge circuit. Lithium-ion canaccept a normal charge in about 3 hours.

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