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Cơ bản Battery Service !

Thảo luận trong 'Điện - Điện tử' bắt đầu bởi Escape, 26/9/10.

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    #1 Escape, 26/9/10
    Last edited by a moderator: 26/6/14
    BATTERY SERVICE
    Battery services are routinely performed. These services include


    1. Testing
    2. Charging
    3. Cleaning
    4. Jumping a dead battery
    5. Adding water

    BATTERY TESTING

    Battery testing has changed in recent years; although the three areas are basically the same, the equipment has improved.

    1. Visual Inspection

    2. State of Charge
    a. Specific Gravity
    b. Open Circuit Voltage

    3. Capacity or Heavy Load Test

    Note: This does not include the Midtronics battery tester which has a different test procedure and will be discussed later in this module.

    VISUAL INSPECTION
    Battery service should begin with a thorough visual inspection. This inspection may reveal simple, easily corrected problems.

    1 . Check for cracks in the battery case and broken terminals. Either may allow electrolyte leakage, which requires battery replacement.

    2. Check for cracked or broken cables or connections. Replace, as needed.

    3. Check for corrosion on terminals and dirt or acid on the case top. Clean the terminals and case top with a mixture of water and baking soda. A battery wire brush tool is needed for heavy corrosion on the terminals.

    4. Check for a loose battery hold-down or loose cable connections. Clean and tighten, as needed.



    VISUAL INSPECTION - CONTINUED
    5. Check the electrolyte fluid level. The level can be viewed through the translucent plastic case or by removing the vent caps and looking directly into each cell. The proper level is 1/2" above the separators (about 1/8" below the fill ring shown below). Add distilled water if necessary. Do not overfill.

    6. Check for cloudy or discolored electrolyte caused by overcharging or vibration. This could cause high self discharge. Correct the cause and replace the battery.


    STATE OF CHARGE
    The state of charge of a battery can be easily check in one of two ways:

    1) Specific Gravity Test

    2) Open Circuit Voltage Test

    Note 1: A state of charge test is required to determine if there is sufficient charge in the battery to properly perform a capacity test (explained later).

    Note 2: The only exception to this is the MIDTRONICS Battery Tester. This new state of the art capacitance tester will be discussed later in this module.
    SPECIFIC GRAVITY
    Specific gravity means exact weight. A "Hydrometer" or a "Refractometer" compares the exact weight of electrolyte with that of water. Strong electrolyte in a charged battery is heavier than weak electrolyte in a discharged battery. By weight, the electrolyte in a fully charged battery is about 36% acid and 64% water. The specific gravity of water is 1.000. The acid is 1.835 times heavier than water, so its specific gravity is 1.835. The electrolyte mixture of water and acid has a specific gravity of 1.270, usually stated as "twelve and seventy."


    SPECIFIC GRAVITY READINGS
    By measuring the specific gravity of the electrolyte, you can tell if the battery is fully charged, requires charging, or must be replaced. It can tell you if the battery is sufficiently charged for a capacity (heavy-load) test. The battery must be at least 75% charged to perform a heavy load test. (The heavy load test will be discussed later). In other words, each cell must have a specific gravity of 1.230 or higher to proceed.


    If the battery is less than 75% charged, it must be fully recharged before proceeding. If the battery is 75% or higher proceed to a heavy load test. A battery not sufficiently charged will fail because it is discharged.
    SPECIFIC GRAVITY - EXCESSIVE CELL VARIATION READINGS
    Variation in specific gravity among cells cannot vary more than 0.050. The variance is the difference between the lowest cell and the highest cell. A battery must be condemned for excessive cell variation if more that 0.050. In the example below, the highest SG reading is cell #1 (shown in green) while the lowest SG reading is cell #5 (shown in blue); the difference is 0.070 which requires battery replacement. Cell #5 if failing.


    Many factors contribute to cell variation; for example, if water was just added to that cell, the cell is then diluted with water resulting is a lower specific gravity reading. Recharging the battery would correct this false reading. In some cases if a battery that has cell variation slightly over the specification and is only about 50% charge, charging the battery at a slow rate of charge (5A) may reduce the cell variation, thus saving the battery.
    SPECIFIC GRAVITY TEST PROCEDURE (HYDROMETER)

    1. Wear suitable eye protection.

    2. Remove vent caps or covers from the battery cells.

    3. Squeeze the hydrometer bulb and insert the pickup tube into the cell closest to the battery's positive (+) terminal.

    4. Slowly release the bulb to draw in only enough electrolyte to cause the float to rise. Do not remove the tube from the cell.

    5. Read the specific gravity indicated on the float. Be sure the float is drifting free, not in contact with the sides of top of the barrel. Bend down to read the hydrometer at eye level. Disregard the slight curvature of liquid on the float.

    6. Record your readings and repeat the procedure for the remaining cells.


    ADJUSTED SPECIFIC GRAVITY READINGS
    Temperature correction is needed because specific gravity changes with temperature. Cold thickens the electrolyte and raises the specific gravity. Heat thins the electrolyte and lowers the specific gravity. Hydrometers are calibrated at 80'F (26.7'C). Electrolyte temperatures above or below 80'F must be adjusted. For every 10'F increment below 80'F, subtract 0.004 to the hydrometer readings, and for each 10'F increment above 80'F, add 0.004 to the readings. See the examples below.


    SPECIFIC GRAVITY TEST PROCEDURE
    (AC Delco Battery with built- in Hydrometer)

    1. Wear suitable eye protection.
    2. Observe the built-in hydrometer.

    Green Dot is visible: the battery is sufficiently charged for further testing (Heavy Load Test).

    Dark Green Dot is visible: the battery needs to be recharged before further testing.

    Light or Yellow Dot is visible: replace the battery


    OPEN CIRCUIT VOLTAGE
    A digital voltmeter must be used to check the battery's open-circuit voltage. Analog meters are not accurate and cannot be used.

    1 . Turn on the headlamps' high beam for several minutes to remove any surface charge.

    2. Turn headlamps off, and connect the digital voltmeter across the battery terminals.

    3. Read the voltmeter. A fully charged battery will have an open-circuit voltage of 12.6 volts. On the other hand, a totally dead battery will have an open-circuit voltage of less than 12.0 volts.

    Note: If the battery is 12.4v or higher, proceed to heavy load test. If the battery is less than 12.4v, the battery must be fully recharged before testing. Be sure to remove the surface charge completely; this is the number one mistake technicians make. If need be, place a load tester on the battery and load the battery for 10 seconds at approximately 200 amps. Allow a few minutes for the battery to recover then measure the open circuit voltage. This should remove the surface charge and allow an accurate open circuit voltage measurement. (Remember: a reading of 12.4 volts or higher load test the battery, 12.3 volts or less, recharge the battery.)


    HEAVY LOAD TEST
    While a State of Charge test determines the battery's state of charge, it does not measure the battery's ability to deliver adequate cranking power. A capacity, or heavy-load test measures the battery's ability to deliver current. A battery load tester such as a Sun VAT-40 is used. (Note: the battery must be at least 75% charged before a heavy test can be performed.)


    DETERMINE CAPACITY RATING
    The capacity rating is located on the battery label. Ratings can be expressed in CCA (Cold Cranking Amps), AH (Amp-Hour), or JIS (Japanese Industrial Standard.) JIS uses a six digit code (not shown). A conversion table is offered below that can be printed. If no rating is found on the battery, then use the OEM battery rating found in most repair manuals.



    HEAVY LOAD TEST PROCEDURE

    1. Install the load tester as shown in an earlier slide.

    2. Load the battery by turning the Load Increase control until the ammeter reads 3 times the amp-hour (AH) rating or one-half the cold-cranking ampere (CCA) rating. 3. Maintain the load for no more than 15 seconds, and note the voltmeter reading.

    4. If the voltmeter reading during the test is


    Note: Results will vary with temperature. Low temperatures will reduce the voltage reading, so the electrolyte should be at 70'F or above. If not, use the following conversion table:

    CURRENT DRAINS
    Parasitic drains are the small current drains required to operate various electrical systems, such as the clock, computer memory, or alarms, that continue to work when the car is parked and the ignition is off. All vehicles today have parasitic drains and over time will drain all batteries if not driven or charged periodically. The problem is when the parasitic drain becomes excessive, usually over 35 milliamps.

    Unwanted battery drain can also be the reason why a battery keeps discharging. Unwanted battery drain can be a result of excessive parasitic drain, or if the top of the battery is wet or has excessive corrosion, it could create a path between the two battery posts, causing a current drain; usually 0.5 volt potential or higher will result in a battery discharge. This is called Case Drain.


    Continue....
     

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