Monday, March 10, 2008

Car Battery Myths (Part 1 of 4)

This FAQ assumes 12-volt, six cell, negative grounded, lead-acid SLI batteries used to start gasoline or diesel engines found in most cars, light trucks and vans. The information contained within the FAQ would also apply to starting batteries found in trunks, motorcycles, boats, snowmobiles, jet skis, farm tractors, lawn and garden tractors, SUVs, etc. Aircraft, buses and larger vehicles use higher volt systems. Some aircraft use special purpose AGM and Ni-Cad batteries for starting because of the extreme cold. For 6-volt batteries, divide the voltage by two; for 8-volt batteries, divide by 1.5; for 24-volt batteries, double the voltage; and for 36-volt batteries, triple the voltage.

1.1. At the first sign of slow starting, dim headlights at low RPM, ammeter indicating discharge at high RPM, or if your battery seems to be loosing performance, recharge, remove the surface charge, and load test it. Weak or bad batteries can also cause stress or premature failures of charging systems and starters. (Please see Section 4. )

1.2. Perform regular preventive maintenance, especially during hot weather and before cold weather. (Please see Section 3.)

1.3. In hot climates use AGM batteries or non-sealed batteries, so you can replace the lost water. (Please see Section 7.)

1.4. Keep the battery charged at 100% to prevent sulfation, but avoid under and over charging. (Please see Section 9.)

1.5. Replace with the freshest battery with the largest Reserve Capacity (RC) that will physically fit in your vehicle, compatible with your charging system, and the Cold Cranking Amp (CCA) rating for your climate or that meets or just exceeds the vehicle's Original Equipment Manufacturer's (OEM) CCA requirement. (Please see Section 7.)

1.6. After deep discharges or jump-starts, recharge your battery, remove the surface charge, and load test it for latent damage. (Please see Section 4.)

1.7. Temperature matters! Heat kills car batteries and cold reduces the available capacity.

Because only the rich can afford cheap batteries...

A car battery is a rechargeable electrochemical device that stores chemical energy and releases it as electrical energy upon demand. When a car battery is connected to an external device, such as a starter motor, chemical energy is converted to electrical energy and direct current flows through the circuit. A good quality lead-acid car battery will cost between $50 and $150 and, if properly maintained, it should last five years or more. The four major purposes of a car, or SLI (Starting, Lighting and Ignition) as it is known in the battery industry, battery are:

To start the engine.

To filter or stabilize the power.

To provide extra power for the lighting, two-way radios, audio system and other accessories when their combined load exceeds the capability of the charging system.

To supply a source of power to the vehicle's electrical system when the charging system is not operating.

With a 5% compounded annual growth rate, worldwide sales of SLI batteries represent roughly 63% of the estimated $27 billion annually spent on lead-acid batteries.

2.1. How is a battery made?

A 12-volt lead-acid battery is made up of six cells, each producing 2.1 volts and that are connected in series from positive to negative. Each cell is made up of an element containing positive plates that are all connected together and negative plates, which are also all connected together. They are individually separated with thin sheets of electrically insulating, porous material “envelopes” [labeled #3 in the diagram below] that are used as spacers between the positive (usually light orange) and negative (usually slate gray) plates to keep them from electrically shorting to each other. The plates [#2 in the diagram below], within a cell, alternate with a positive plate, a negative plate and so on.? A plate is made up of a metal grid that serves as the supporting framework for the active porous material that is “pasted” on it.

[Source: BCI]

After the “curing” of the plates, they are made up into cells, and the cells are inserted into a high-density tough polypropylene or hard rubber case [#1 in the diagram above]. The cells are connected to the terminals [#5 in the diagram above], and the case is covered and then filled with a dilute sulfuric acid electrolyte [#4 in the diagram above]. The battery is initially charged or “formed” to convert yellow Lead Oxide (PbO or Litharge) into Lead Peroxide (PbO2), which is usually dark brown or black. The electrolyte is replaced and the battery is given a finishing charge.? Some batteries are “dry charged” meaning that the batteries are shipped without electrolyte and it is added and charged when they are put into service.

[Source: BCI]

Two important considerations in battery construction are porosity and diffusion. Porosity is the pits and tunnels in the plate that allows the sulphuric acid to get to the interior of the plate. Diffusion is the spreading, intermingling and mixing of one fluid with another. When you are using your battery, the fresh acid needs to be in contact with the plate material and the water generated needs to be carried away from the plate. The larger the pores or warmer the electrolyte, the better the diffusion.

There is an excellent detailed description of how battery is made on the BCI (Battery Council International) web site at

2.2. How does a battery work?

discharging processcharging process
[Source: BCI]

A battery is created by alternating two different metals such as Lead Dioxide (PbO2), the positive plates, and Sponger lead (Pb), the negative plates. Then the plates are immersed in diluted Sulfuric Acid (H2SO4), the electrolyte. The types of metals and the electrolyte used will determine the output of a cell. A typical fully charged lead-acid battery produces approximately 2.11 volts per cell. The chemical action between the metals and the electrolyte (battery acid) creates the electrical energy. Energy flows from the battery as soon as there is an electrical load, for example, a starter motor, that completes a circuit between the positive and negative terminals. Electrical current flows as charged portions of acid (ions) between the battery plates and as electrons through the external circuit.? The action of the lead-acid storage battery is determined by chemicals used, State-of-Charge, temperature, porosity, diffusion, and load.

A more detailed description of how a battery works can be found on the BCI web site at

2.3. Why do batteries die?

When the active material in the plates can no longer sustain a discharge current, the battery “dies”. Normally a battery “ages” as the active positive plate material sheds (or flakes off) due to the normal expansion and contraction that occurs during the discharge and charge cycles. This causes a loss of plate capacity and a brown sediment, called sludge or “mud,” that builds up in the bottom of the case and can short the plates of a cell out. In hot climates, additional major causes of failure are positive grid growth, positive grid metal corrosion in the electrolyte, negative grid shrinkage, buckling of plates, or loss of water.? Deep discharges, heat, vibration, and over charging accelerate the “aging” process.

Another major cause of premature battery failure is sulfation. It is caused when a battery's State-of-Charge drops below 100% for long periods or undercharging. Hard lead sulfate fills the pours and coats the plates. Please see Section 16 for more information on sulfation. Recharging a sulfated battery is like trying to wash your hands with gloves on. Using tap water to refill batteries can produce calcium sulfate that can also fill the pores and coat the plates.

In a hot climate, the harshest environment for a battery, a Johnson Controls survey of junk batteries revealed that the average life of a car battery was 37 months. If your car battery is more than three years old, then it is living on borrowed time. Abnormally slow cranking, especially on a cold day, is another good indication that your battery is going bad. It should be externally recharged, surface charge removed, and load tested. Dead batteries almost always occur at the most inopportune times, for example, after you have jump-started your car, late at night in a dark airport parking lot after returning from a long trip, during bad weather, or when you are late for an appointment. You can easily spend the cost of a new battery or more for an emergency jump start or a tow or for a taxi ride.

According to the manufacturers and distributors, most of the “defective” batteries returned during free replacement warranty periods are good. This suggests that some dealers of new batteries do not know how or take the time to properly recharge or test them. This situation is improving with the widespread use of easy to use and more accurate conductance type battery testers made by Midtronics, for example.

Preforming preventive maintenance on car batteries is easy and should occur the lessor of once a month when the climate is hot or every time the oil is charged. Here are six simple steps:

3.1. If the electrolyte levels are low in a non-sealed battery, allow the battery to cool to room temperature, and add only distilled water to the level indicated by the battery manufacturer. If there is no recommendation, add to within 1/8 to 1/4 inch (3 to 7 mm) below the bottom of the filler tubes (vent wells or splash barrels). The plates need to be covered at all times. Avoid overfilling, especially in hot climates, because heat causes the electrolyte to expand and overflow. In an emergency, use rain water rather than reverse osmosis or tap water because rain water does not contain calcium or magnesium.

3.2. Tighten loose hold-down clamps and battery terminals.

3.3. Check and remove any corrosion or oxidation from both ends of each battery cable terminals and both battery posts or terminals. Red and green felt battery terminal washers will help prevent corrosion. They are available at any store selling car batteries.

3.4. Clean the battery top and alternator.

3.5. Check the alternator belt for cracks and retention, if slipping.

3.6. Check the battery cables for corrosion or swelling and replace if necessary with equal or larger diameter wire size.

Below are seven simple steps in testing a car battery. If you have a non-sealed battery, it is highly recommended that you use a good quality, hydrometer, which can be purchased at an auto parts store for between $5 and $20. A hydrometer is a float-type device used to determine the State-of-Charge by measuring the specific gravity of the electrolyte in each cell. It is a very accurate way of determining a battery's State-of-Charge and weak or dead cells.

If you have a sealed battery or to troubleshoot charging or electrical system, you will need a digital voltmeter with 0.5% (or better) accuracy. A digital voltmeter can be purchased at an electronics store, such as Radio Shack, Jameco, Frys, etc., for between $50 and $400. Analog voltmeters are not accurate enough to measure the millivolt differences of a battery's State-of-Charge or measure the output of the charging system. A battery load tester is optional. A more accurate way of testing the CCA (Cold Cranking Amp) and capacity of lead-acid car batteries is by using a conductance tester, such as a Midtronics, costing between $100 and $600.


Visually inspect for obvious problems such as a loose or broken alternator belt, low electrolyte levels, dirty or wet battery top, corroded or swollen cables, corroded terminals or battery posts, loose hold-down clamps, loose cable terminals, or a leaking or damaged battery case.

If the electrolyte levels are low in non-sealed batteries, allow the battery to cool and add only distilled water to the level indicated by the battery manufacturer or to between 1/8 to 1/4 inch (3 to 7 mm) below the bottom of the plastic filler tube (vent wells). The plates need to be covered at all times. Avoid overfilling, especially in hot climates, because heat will cause the electrolyte to expand and overflow.


Recharge the battery to 100% State-of-Charge. If non-sealed battery has a difference of 0.03 (or more) specific gravity reading between the lowest and highest cell, then you should equalize the battery. (Please see Section 9.)


Surface charge is the uneven mixture of sulfuric acid and water along the surface of the plates as a result of charging or discharging. It will make a weak battery appear good or a good battery appear bad. You need to eliminate the surface charge by one of the following methods after recharging a lead-acid car battery:

4.3.1. Allow the battery to sit for between four to twelve hours to allow for the surface charge to dissipate.

4.3.2. Turn the headlights on high beam for five minutes, shut them off, and wait five to ten minutes.

4.3.3. With a battery load tester, apply a load at one-half the battery's CCA rating for 15 seconds and then wait five to ten minutes.

4.3.4. Disable the ignition, turn the engine over for 15 seconds with the starter motor, and wait five to ten minutes.


If the battery's electrolyte is above 110° F (43.3° C), allow it to cool.? To determine the battery's state-of-charge with the battery's electrolyte temperature at 80° F (26.7° C), use the following table.? The table assumes that a 1.265 specific gravity reading is a fully charged, wet, lead-acid battery. For other electrolyte temperatures, use the Temperature Compensation table below to adjust the Open Circuit Voltage or Specific Gravity readings. The Open Circuit Voltage will vary for gel cell and AGM type batteries, so check the manufacturer's specifications.

Digital Voltmeter Open Circuit Voltage

Approximate State-of-Charge

Hydrometer Average Cell Specific Gravity

Electrolyte Freeze Point




-75° F (-59.4° C)




-55° F (-48.3° C)




-34° F (-36.7° C)




-16° F (-26.7° C)




-10° F (-23.3° C)


Electrolyte Temperature Fahrenheit

Electrolyte Temperature Celsius

Add or Subtract to Hydrometer's SG Reading

Add or Subtract to Digital Voltmeter's Reading




































































[Source: BCI]


Electrolyte temperature compensation, depending on the battery manufacturer's recommendations, will vary. If you are using a non-temperature compensated HYDROMETER, make the adjustments indicated in the table above. For example, at 30° F (-1.1° C), the specific gravity reading would be 1.245 for a 100% State-of-Charge. At 100° F (37.8° C), the specific gravity would be 1.273 for 100% State-of- Charge. This is why using a temperature compensated hydrometer is highly recommended and more accurate. If you are using a DIGITAL VOLTMETER, make the adjustments indicated in the table above. For example, at 30° F (-1.1° C), the voltage reading would be 12.53 for a 100% State-of-Charge. At 100° F (37.8° C), the voltage would be 12.698 for 100% State-of-Charge.

For non-sealed batteries, check the specific gravity in each cell with a hydrometer and average cells readings. For sealed batteries, measure the Open Circuit Voltage across the battery terminals with an accurate .5% (or better) digital voltmeter. This is the only way you can determine the State-of-Charge. Some batteries have a built-in hydrometer, which only measures the State-of-Charge in ONE of its six cells. If the built-in indicator is clear, light yellow, or red, then the battery has a low electrolyte level and if non-sealed, should be refilled and recharged before proceeding. If sealed, the battery is bad and should be replaced. If the State-of-Charge is BELOW 75% using either the specific gravity or voltage test or the built-in hydrometer indicates “bad” (usually dark or white), then the battery needs to be recharged BEFORE proceeding. You should replace the battery, if one or more of the following conditions occur:

4.4.1. If there is a .050 (sometimes expressed as 50 “points”) or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). Applying an EQUALIZING charge may correct this condition. (Please see Section 9.)

4.4.2. If the battery will not recharge to a 75% or more State-of-Charge level or if the built-in hydrometer still does not indicate “good” (usually green or blue, which indicates a 65% State-of-Charge or better). If you know that a battery has spilled or “bubbled over” and the electrolyte has been partially replaced with water, you can replace this old electrolyte with new electrolyte and go back to Step 4.2 above. Battery electrolyte (battery acid) is a mixture of 25% to 35% sulfuric acid and distilled water when fully charged. It less expensive to replace the electrolyte than to buy a new battery.

4.4.3. If a digital voltmeter indicates 0 volts, there is an open cell.

4.4.4. If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell. A shorted cell is caused by plates touching, sediment (“mud”) build-up or “treeing” between the plates.


If the battery's State-of-Charge is at 75% or higher or has a “good” built-in hydrometer indication, then you can load test the battery by one of the following methods:

4.5.1. Disable the ignition and turn the engine over for 15 seconds with the starter motor.

4.5.2. With a battery load tester, apply a load equal to one half of the CCA rating of the battery for 15 seconds.

4.5.3. With a battery load tester, apply a load equal to one half the OEM cold cranking amp specification for 15 seconds.

DURING the load test, the voltage on a good battery will NOT drop below the following table's indicated voltage for the electrolyte at the temperatures shown:

Electrolyte Temperature F

Electrolyte Temperature C

Minimum Voltage Under LOAD


































[Source: Interstate Batteries]


If the battery has passed the load test, please go to Section 4.7 below.? If not, remove the load, wait ten minutes, and measure the State-of-Charge.? If the battery bounces back to less than 75% State-of-Charge (1.225 specific gravity or 12.45 VDC), then recharge the battery (please see Section 9.) and load test again.? If the battery fails the load test a second time or bounces back to less than 75% State-of-Charge, then replace the battery because it lacks the necessary CCA capacity.


If the battery passes the load test, you should recharge it as soon as possible to prevent lead sulfation and to restore it to peak performance.

1 comment:

Anonymous said...

If your car battery is over charging approximately how long does it take for a cell to die??