A battery has two jobs. One is to supply electricity to the starter motor so the starter can turn the engine crankshaft rapidly enough for the engine to start. The other is to provide current to operate electric accessories, such as the radio, heater, and lights, when the engine is not running.

When the engine is running, the battery is dormant. The current needed to operate electric accessories is provided by the alternating cur­rent (A/C) generator. The alternator, as the A/C generator is frequently called, also restores to the battery current that was expelled in helping to start the engine.

Inside the battery is sulfuric acid, which is often referred to as elec­trolyte or battery acid. Without this acid, the battery couldn’t hold a charge. When your engine fails to crank or cranks too sluggishly to start, you can determine whether the battery is the cause of the problem by checking the strength of its sulfuric acid.

If your vehicle is equipped with a maintenance-free battery, look to see whether an indicator (sight glass) is present in the top of the battery. Peering into a sight glass lets you know the specific-gravity rating of the acid. Specific gravity is the weight of acid as compared to the weight of an equal volume of water.

Water has a specific-gravity rating of 1.000. Battery acid should have a specific-gravity rating of 1.230 to 1.300. If it doesn’t, a battery may not be able to store enough current to start the engine.

As a battery gets older, the strength of its acid diminishes. When the specific-gravity rating falls to between 1.200 and 1.230, the battery may still have sufficient energy to help thestarter motor crank the engine, ex­cept in cold weather when more power than the battery can supply is needed. But when the specific-gravity rating of electrolyte falls below 1.200, there won’t be enough energy left in the battery—winter or sum­mer—to start the engine.

What you see in the sight glass of a maintenance-free battery tells you one of three facts about the specific-gravity rating of the electrolyte:

  1. If there is a green dot inside a black border, the specific gravity rat­ing is between 1.230 and 1.300. The battery is probably not to blame for the engine failing to start. However, corrosion on battery cable terminals may be impeding the flow of current from the bat­tery to thestarter motor. Therefore, clean the cable terminals as de­scribed below to see whether this resolves the starting problem.
  2. If only black appears in the sight glass, the specific-gravity rating of electrolyte is between 1.200 and 1.230. The battery could be causing your starting problem, especially in cold weather. Charge the battery. There is another aspect to a “black” reading, especially if the battery is fairly new. Although the battery is probably okay, the al­ternator may not be providing it with a sufficient amount of cur­rent. Consequently, the battery may be forced to operate at a power level that cannot expel sufficient current to thestarter motor. Have a technician who specializes in automotive electrical systems test alternator output to rule out the possibility, but first check belt tension.
  3. If the sight glass of a maintenance-free battery is clear or light yel­low, replace the battery. Do not charge it. When sulfuric acid reaches this point (a specific-gravity rating of less than 1.200), hy­drogen trapped in the battery may ignite and cause the battery to explode if a charge is introduced. Suppose your battery doesn’t have a sight glass. Use a battery hydrometer, which you can buy from an auto partsstore, to determine the specific-gravity rating of electrolyte. The hy­drometer should have a built-in thermometer.

Follow these steps to obtain the specific-gravity reading:

  1. Unscrew the battery vent caps and check the level of the acid. If electrolyte in any cell is low, add distilled water, replace the vent caps, and drive the vehicle for a few hours before doing the test.
  2. Insert the hydrometer hose into the first cell and squeeze the bulb to draw enough electrolyte into the hydrometer tube so that the scale floats in the liquid. Be careful not to draw too much acid into the tube; otherwise, the scale will jam against the top of the hydrometer and prevent an accurate reading.
  3. Holding the scale at eye level, note where the numerals come to rest in relation to the top of the electrolyte. Jot down this reading. Then, check the temperature scale, which gives a correction factor relative to the temperature of the electrolyte. If the temperature of the electrolyte is 60°F, for example, the scale will tell you to sub­tract 0.008 from the hydrometer reading to get the true specific-gravity rating; if the temperature of the electrolyte is 100°F, you will have to add 0.008 to the reading.
  4. Return the electrolyte to the cell from which it was drawn. Repeat the procedure to obtain the specific-gravity ratings of the elec­trolyte in the other cells.
  5. When you have all the readings, and have made the necessary compensation for temperature, compare readings with one an­other. If the readings from any two cells differ by more than 0.050 specific-gravity points, replace the battery. The low-reading cell is dead.
  6. Add the readings together and divide by the number of cells (six) to get the overall specific-gravity rating of the electrolyte. As de­scribed above, a reading of 1.230 to 1.300 signifies a good battery. A reading of 1.200 to 1.230 indicates a battery that is borderline but that may be strengthened by charging. A reading below 1.200 tells you that the battery should be replaced.

If your vehicle needs a new battery, consider purchasing a main­tenance-free unit, which will eliminate frequent inspection to see that each cell is filled with electrolyte. The battery you buy should be rated at the cold cranking ampere (CCA) and hours reserve ratings recommended by the vehicle manufacturer. These ratings are inscribed on most original-equipment batteries. If your old battery is not original and you are uncertain as to what ratings your new battery should have, ask the service manager at a dealership that sells your make of vehicle.


When purchasing a new battery, make sure it has posts if your old battery has posts. If the cables of the old battery are attached to the side of the battery, your new battery should also have side-mountings.

The following procedure describes the proper way to replace a bat­tery. Steps 1, 2, 7, 8, and 9 should be followed if the engine cranks too sluggishly to start and the battery passes the sight-glass or hydrometer test. These five steps explain how to service battery cable connections (terminals).

  1. Remove the ground cable (marked – or NEG) . If the cable is attached to a post, loosen the clamp and twist the terminal from side to side by hand. If this fails to remove the terminal from the post, use a battery cable puller to do the job. This tool is available from auto parts stores. Using any other tool may damage the battery.
  2. Remove the positive cable (marked + or POS) in the same way.
  3. Remove the battery hold-down clamp.
  4. Use a battery carrier (strap) to lift the battery out of the vehicle.
  5. If the tray on which the battery sits is corroded, wash the tray with a mixture of baking soda and water. Baking soda eliminates corro­sion and neutralizes the acid that causes corrosion. Flush the tray with clear water.
  6. Using the battery strap, lift the new battery onto the tray and se­cure it with the hold-down clamp.
  7. Use a wire brush to clean dirt and corrosion from cable terminals.
  8. Connect the positive cable to the post or side-mounting marked + or POS. Tighten the fastener until it is snug. Then, connect the negative cable to the post or side-mounting marked – or NEG.
  9. Spread a light coating of petroleum jelly across the cable terminals. This will help prevent corrosion.
source:http://www.motorward.com/2010/06/how-to-maintain-a-cars-battery/