Battery Preventive Maintenance

While working with lead-acid batteries (and corrosion), please wear glasses to protect your eyes in the unlike even of an explosion.

The plates need to be covered at all times to prevent an internal battery explosion or sulfation. For non-sealed wet batteries (with filler caps), if the electrolyte levels are above the plates but low, allow the battery to cool to room temperature, and add only distilled, deionized or demineralized water to the level indicated by the battery manufacturer or to within 1/4 to 3/8 inch (6 to 10 mm) below the bottom of the filler tubes (vent wells or splash barrels). Avoid overfilling, especially in hot weather, because the heat will cause the electrolyte to expand and overflow. If the electrolyte levels are below the tops of the plates, add enough water to cover the plates, recharge the battery, and top off the levels when the battery is at room temperature. In an emergency, use rain water rather than reverse osmosis or tap water because rain water does not contain calcium or magnesium. Using tap water to refill batteries can produce calcium sulfate crystals that can fill the pores and coat the plates. State-of-Charge (SoC) readings will be inaccurate immediately after the addition of water, recharges or discharges.

Tighten loose hold-down clamps, battery terminals and connectors.

 If required, remove any corrosion, lead oxidation, paint or rust with a brass wire battery brush (by brushing the corrosion away from you) or "ScotchBrite" pad from the mating surfaces of both ends of each battery cable terminals, battery posts or terminals, and engine grounding strap connections. (A stiff steel wire brush may damage protective lead plating on copper connectors or terminals.) Heavy corrosion can be neutralized with a mixture of one pound of baking soda (bicarbonate of soda) to one gallon of warm water. A good bare metal to metal mating surface is required for good conductivity. To prevent corrosion on terminals, thinly coat the terminals, terminal clamps and exposed metal around the Car battery with high temperature grease or silicone. Do not use the felt or metal washers between the mating conductive surfaces with General Motors-type side terminals. For Deep Cycle batteries, use "No Oxide A" (or the battery manufacturer's recommended grease) on the terminals and connectors. Do not use the felt or metal washers between the mating conductive surfaces with side, stud or "L" terminal batteries. Use of some stainless steel alloys and other metal washers, nuts and bolts have also been known to cause problems with electrolysis and high resistance. Corrosion is caused by one or more the following:

Dirty or wet battery tops normally caused from expansion of electrolyte from overfilled cells.

Acid fumes leaking through the vent caps, which could be a sign of overcharging.

Electrolysis due to the mismatch of metal alloys used in the battery posts and terminals.

Clean the battery top to eliminate conductive paths created by dried or wet electrolyte and to prevent corrosion.

Clean the alternator or charging system to allow better heat transfer and check the alternator belts for cracks and correct tension.

Replace any battery cables that are corroding, swelling or damaged with equal or larger diameter cable. Larger cable is better because there is less voltage drop.

Headlight Test In the dark, pull your car up to a building wall or a garage door. Turn off the engine and leave on the headlights. If they're bright, then your battery is probably fine. If the lights are dim, but get brighter when the engine is started, this tells you the battery could be bad and requires further testing. If the lights are dim and stay dim when the engine is started, this tells you that your charging system requires further testing.
Open Circuit Voltage (OCV) Test An OCV test may be performed with a voltmeter.
  1. To determine if the battery is experiencing a problem, turn off all electrical loads and the charging source. 

  2. For an accurate reading, allow the battery to sit with no electrical loads applied for at least one hour. 

  3. Connect a voltmeter to the positive and negative terminals and measure the terminal post voltage with no loads or chargers connected to the battery. 

  4. To determine the battery's state of charge, compare the OCV reading on the voltmeter to the Open Circuit Voltage Chart.
    Open Circuit Voltage VS. State of Charge
    12.66V    100%    
    12.45V    75%    
    12.24V    50%    
    12.06V    25%    
    11.89V    0%

We do not recommend that you substitute the OCV test for a specific gravity test, which is more accurate. The OCV test, however, is the only way to determine the state of charge of a sealed no-maintenance battery.

Specific Gravity Test This test is performed with a hydrometer, which is the most accurate hand held tool for determining the state of charge of a lead acid battery.

  1. Draw electrolyte into the hydrometer a few times so that the float reaches the same temperature as the electrolyte. This will increase the accuracy of your readings. 

  2. Hold the hydrometer vertically so that the float is free and does not touch the inner walls of the barrel. 

  3. Hold the hydrometer so that the liquid is level in the barrel and at eye level. 

  4. When you draw the electrolyte, make sure that the hydrometer is full. 

  5. Check each individual battery cell. If the specific gravity varies more than .050 or "50 points" among the cells while the battery is at a 75% state of charge or above, then the battery is bad and should be replaced. The cells that have a specific gravity of 50 points less than the highest cell are bad cells. A hydrometer reading of 1.265 or greater at 80°F indicates a full charge for Interstate batteries. To determine the battery's state of charge, compare the hydrometer reading to the Specific Gravity Chart.
    Specific Gravity VS. State of Charge
    1.265    100%    
    1.225    75%    
    1.190    50%    
    1.155    25%    
    1.120    0%    

  6. To get the most accurate hydrometer reading, you should adjust your hydrometer reading according to the temperature. If the electrolyte temperature is ABOVE 80°F , ADD .004 (called "four points") to the hydrometer reading for each 10 degrees above 80°F. If the electrolyte temperature is BELOW 80°F, SUBTRACT four points from the hydrometer reading for each 10 degrees below 80°F.

Load TestThis test is a 15-second discharge of the battery at a 1/2 cold cranking amp level. A more accurate testing method than a voltmeter or a hydrometer, the load test is often required to determine whether a battery is good or bad. It is used by professional technicians.