Automotive battery

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A typical 12 V, 40 Ah lead-acid car battery

An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile.[1] An automotive SLI battery (starting, lighting, ignition) powers the starter motor, the lights, and the ignition system of a vehicle's engine.

Automotive SLI batteries are usually lead-acid type, and are made of six galvanic cells in series to provide a 12-volt system. Each cell provides 2.1 volts for a total of 12.6 volts at full charge. Heavy vehicles, such as highway trucks or tractors, often equipped with diesel engines, may have two batteries in series for a 24-volt system or may have parallel strings of batteries.

Lead-acid batteries are made up of plates of lead and separate plates of lead dioxide, which are submerged into an electrolyte solution of about 38% sulfuric acid and 62% water.[2] This causes a chemical reaction that releases electrons, allowing them to flow through conductors to produce electricity. As the battery discharges, the acid of the electrolyte reacts with the materials of the plates, changing their surface to lead sulfate. When the battery is recharged, the chemical reaction is reversed: the lead sulfate reforms into lead dioxide and lead. With the plates restored to their original condition, the process may now be repeated.

Battery recycling of automotive batteries reduces the need for resources required for manufacture of new batteries, diverts toxic lead from landfills, and prevents risk of improper disposal.


Lead-acid batteries for automotive use are made with slightly different construction techniques, depending on the application of the battery. The "flooded cell" type, indicating liquid electrolyte, is typically inexpensive and long-lasting, but requires more maintenance and can spill or leak. Some flooded batteries have removable caps that allow for the electrolyte to be tested and maintained.

More costly alternatives to flooded batteries are valve regulated lead acid (VRLA) batteries, also called "sealed" batteries. The absorbed glass mat (AGM) type uses a glass mat separator, and a "gel cell" uses fine powder to absorb and immobilize the sulfuric acid electrolyte. These batteries are not serviceable: the cells are sealed so the degree of charge cannot be measured by hydrometer and the electrolyte cannot be replenished. They are typically termed "maintenance-free" by proponents, or "unable to be maintained" by skeptics. In particular, they are not suitable for older (pre-alternator) vehicles with unsophisticated charging control systems.[3][4][5] Both types of sealed batteries may be used in vehicular applications where leakage or ventilation for vented gasses is a concern.

The starting (cranking) or shallow cycle type is designed to deliver large bursts of power for a short time, as is needed to start an engine. Once the engine is started, the battery is recharged by the engine-driven charging system. Starting batteries are intended to have a low depth of discharge on each use. They are constructed of many thin plates with thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance.[1]

The deep cycle (or motive) type is designed to continuously provide power for long periods of time (for example in a trolling motor for a small boat, auxiliary power for a recreational vehicle, or traction power for a golf cart or other battery electric vehicle). They can also be used to store energy from a photovoltaic array or a small wind turbine. Deep-cycle batteries have fewer, thicker plates and are intended to have a greater depth of discharge on each cycle, but will not provide as high a current on heavy loads. The thicker plates survive a higher number of charge/discharge cycles. The specific energy is in the range of 30-40 watt-hours per kilogram.[2]

Some cars use more exotic starter batteries–the 2010 Porsche 911 GT3 RS offers a lithium-ion battery as an option to save weight over a conventional lead-acid battery.[6]

Use and maintenance[edit]

Fluid level[edit]

Filling a (flooded lead-acid type) car battery with distilled water

Car batteries using lead-antimony plates require regular watering to replace water lost due to electrolysis on each charging cycle. By changing the alloying element to calcium, more recent designs have lower water loss, unless overcharged. Modern car batteries have reduced maintenance requirements, and may not provide caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life. If the battery has easily detachable caps then a top-up with distilled water may be required from time to time. Prolonged overcharging or charging at excessively high voltage causes some of the water in the electrolyte to be broken up into hydrogen and oxygen gases, which escape from the cells; this is called gassing. If the electrolyte liquid level drops too low, the plates are exposed to air, lose capacity, and are damaged. The sulfuric acid in the battery normally does not require replacement since it is not consumed even on overcharging. Impurities or additives in the water will reduce the life and performance of the battery. Manufacturers usually recommend use of demineralized or distilled water, since even potable tap water can contain high levels of minerals.

Charge and discharge[edit]

In normal automotive service the vehicle's charging system powers the vehicle's electrical systems and restores charge used from the battery during engine cranking. When installing a new battery or recharging a battery that has been accidentally discharged completely, one of several different methods can be used to charge it. The most gentle of these is called trickle charging. Other methods include slow-charging and quick-charging, the latter being the harshest.

The voltage regulator of the charge system does not measure the relative currents charging the battery and for powering the car's loads. The charge system essentially provides a fixed voltage of typically 13.8 to 14.4 V, adjusted to ambient temperature, unless the alternator is at its current limit. A discharged battery draws a high charge current of typically 20 to 40 A. As the battery becomes charged the charge current typically decreases to 2—5 amperes. A high load is when multiple high-power systems such as ignition, radiator fan, heater blowers, lights and entertainment system are running at the same time. In older (up to the 1980s) vehicles the battery may discharge unless the engine is running at a higher than idle rpm and the alternator/generator is delivering enough current to power the load. This is not an issue for modern vehicles where alternators provide enough current for all loads and a regulator keeps charging voltage in check. In such cars rpm has little influence on the battery voltage - tests show near normal voltage regardless of the AC / headlights / music / fan / defrosting / other electrical loads, even at idle.

Some manufacturers include a built-in hydrometer to show the state of charge of the battery, a transparent tube with a float immersed in the electrolyte visible through a window. When the battery is charged, the specific gravity of the electrolyte increases (since all the sulfate ions are in the electrolyte, not combined with the plates), and the colored top of the float is visible in the window. When the battery is discharged, or the electrolyte level is too low, the float sinks and the window appears yellow (or black). The built-in hydrometer only checks the state of charge of one cell and will not show faults in the other cells. In a non-sealed battery each of the cells can be checked with a portable or hand-held hydrometer.

A positive (red) jumper cable connected to battery post. An optional hydrometer window is visible by the single jumper clamp. (The black negative jumper clamp is not shown)

A vehicle can be jump started by the battery of another vehicle or by a portable battery booster. It is possible to charge a battery fully using solely the alternator, either by raising the engine's RPM while parked or by regular driving. Recharging can be enhanced by turning off accessories so the greatest amount of power is available to recharge the battery, as well as by moderately revving the engine, or driving in lower gears as that leads to higher RPMs, and producing higher alternator output than at idle speed.[7] A 12 volt car battery fully charged should output around 12.6 volts.

However, it is preferable to use a battery charger whenever possible because the above method will shorten the lifespan of the alternator and gasoline is much more expensive than wall outlet electricity. Simple chargers do not regulate the charge current, and the user needs to stop the process or lower the charge current to prevent excessive gassing of the battery. More elaborate chargers, in particular those implementing the 3-step charge profile, also referred to as IUoU, charge the battery fully and safely in a short time without requiring user intervention. Desulfating chargers are also commercially available for charging all types of lead-acid batteries.


Unlike lithium based batteries, automotive batteries last longer when stored in a charged state. Leaving an automotive battery discharged will shorten its life, or make it unusable if left for a long time (usually several years); sulfation eventually becomes irreversible with normal charging. Batteries in storage may be monitored and periodically charged, or attached to a "float" charger to retain their capacity. One practical method is to use an inexpensive 24 hour timer that turns a charger on for 30 minutes per day. Batteries are prepared for storage by charging and cleaning deposits from the posts. Batteries are stored in a cool, dry environment for best results since high temperatures increase the self discharge rate and plate corrosion.

In the past, storing lead-acid batteries on the ground, or on concrete or cement floors, was believed to cause batteries to discharge or be otherwise damaged, but this is no longer a concern.[8] In spite of this, the advice to never leave a battery on a concrete floor persists.[9][10] Modern batteries use tough polycarbonate cases that do not conduct current or allow moisture to pass, and maintenance free batteries are the norm, so large amounts of leaking acid are rarely seen, providing no route for current to flow.[11] One battery manufacturer even prefers storing new batteries on concrete in the summer to keep them cooler, decreasing the natural discharge rate.[11] Early batteries had wooden cases, and could absorb moisture from wet concrete, giving current a route to discharge.[12] Another explanation for the admonition to avoid concrete is that wooden cases in the earliest batteries encased a glass jar, which could be broken by swelling wood if the wood casing became damp.[13] Later hard rubber cases were porous and had a high carbon content, leaving another route for current leakage, but modern plastic cases are five or more times better insulators than rubber, and the terminal seals do not leak as they once did.[13]

Changing a battery[edit]

When changing a battery, battery manufacturers recommend disconnecting the negative ground connection first to prevent accidental short-circuits between the battery terminal and the vehicle frame. Conversely the positive cable is connected first. Of course, this only applies to negative-earth vehicles - a better rule is to disconnect the earth or ground terminal first, this works whatever the polarity of the system. A study by the National Highway Traffic Safety Association estimated that in 1994 more than 2000 people were injured in the United States while working with automobile batteries. Another safety factor in the operation is to remove metal bracelets including watches.

The majority of automotive lead-acid batteries are filled with the appropriate electrolyte solution at the manufacturing plant, and shipped to the retailers ready to sell. Decades ago, this was not the case. The retailer filled the battery, usually at the time of purchase, and charged the battery. This was a time-consuming and potentially dangerous process. Care had to be taken when filling the battery with acid, as acids are highly corrosive and can damage eyes, skin and mucous membranes. Fortunately, this is less of a problem these days, and the need to fill a battery with acid usually only arises when purchasing a motorcycle or ATV battery.


Because of sulfation, lead-acid batteries stored with electrolyte slowly deteriorate. Car batteries are date coded to ensure installation within one year of manufacture. In the United States, the manufacturing date is printed on a sticker. The date can be written in plain text or using an alphanumerical code. The first character is a letter that specifies the month (A for January, B for February and so on).[14] The letter "I" is skipped due to its potential to be mistaken for the number 1. The second character is a single digit that indicates the year of manufacturing (for example, 6 for 2006). When first installing a newly purchased battery a "top up" charge at a low rate with an external battery charger (available at auto parts stores) may maximize battery life and minimize the load on the vehicle charging system.


Common battery faults include:

Corrosion at the battery terminals can prevent a car from starting due to electrical resistance. The white powder sometimes found around the battery terminals is usually lead sulfate which is toxic by inhalation, ingestion and skin contact. The corrosion is caused by an imperfect seal between the plastic battery case and lead battery post allowing sulfuric acid to react with the lead battery posts. The corrosion process is also expedited by over charging. Corrosion can also be caused by factors such as salt water, dirt, heat, humidity, cracks in the battery casing or loose battery terminals. Inspection, cleaning and protection with a light coating of dielectric grease are measures used to prevent corrosion of battery terminals.

Sulfation occurs when a battery is not fully charged. The longer it remains in a discharged state the harder it is to overcome sulfation. This may be overcome with slow, low-current (trickle) charging. Sulfation is the formation of large, non-conductive lead sulfate crystals on the plates; lead sulfate formation is part of each cycle, but in the discharged condition the crystals become large and block passage of current through the electrolyte.[15]

The primary wear-out mechanism is the shedding of active material from the battery plates, which accumulates at the bottom of the cells and which may eventually short-circuit the plates.

Early automotive batteries could sometimes be repaired by dismantling and replacing damaged separators, plates, intercell connectors and other repairs. Modern battery cases do not facilitate such repairs; an internal fault generally requires replacement of the entire unit.[1]

Exploding batteries[edit]

Car battery after explosion

Any lead-acid battery system when overcharged (>14.34 V) will produce hydrogen gas (gassing voltage) by electrolysis of water. If the rate of overcharge is small, the vents of each cell allow the dissipation of the gas. However, on severe overcharge or if ventilation is inadequate, or the battery is faulty, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. An internal spark can cause a hydrogen and oxygen explosion, which will damage the battery and its surroundings and which will disperse acid into the surroundings. Anyone close to the battery may be injured.

Sometimes the ends of a battery will be severely swollen, and when accompanied by the case being too hot to touch, this usually indicates a malfunction in the charging system of the car. Reversing the positive and negative leads will damage the battery. When severely overcharged, a lead-acid battery produces high levels of hydrogen and the venting system built into the battery cannot handle the high level of gas, so the pressure builds inside the battery, resulting in the swollen ends. An unregulated alternator can quickly ruin a battery by excessive voltage. A swollen, hot battery is dangerous.

Another potential cause of explosion is when the battery terminals are short-circuited via a very low resistance path (like a wrench or other tool dropped or lying across the terminals). Apart from the sparks which usually occur in a short circuit, heating due to the internal resistance of the battery can cause the electrolyte to boil, also leading to explosion due to buildup of water vapor pressure (unrelated to electrolysis).

Persons handling car batteries should wear protective equipment (goggles, overalls, gloves) to avoid injury by acid spills. Any open flame or electric spark including lit tobacco products like cigarettes, cigars, or pipes in the area also present a danger of igniting any hydrogen gas escaping from a battery. Also for this reason, the final connection (or disconnection) of a jumper cable should be to the car's frame rather than the grounded terminal of the battery; doing this keeps any potential sparks further from the battery.

Terms and ratings[edit]

Terminal voltage[edit]

The open-circuit voltage is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery:

Open-circuit voltageApproximate
acid density
12 V6 V
12.66 V6.32 V100%1.265 g/cm3
12.35 V6.22 V75%1.225 g/cm3
12.10 V6.12 V50%1.190 g/cm3
11.95 V6.03 V25%1.155 g/cm3
11.70 V6.00 V0%1.120 g/cm3

Open-circuit voltage is also affected by temperature and the specific gravity of the electrolyte at full charge.

The following is common for a six-cell automotive lead-acid battery at room temperature:

See also[edit]


  1. ^ a b c Horst Bauer Bosch Automotive Handbook 4th Edition Robert Bosch GmbH, Stuttgart 1996 ISBN 0-8376-0333-1, pages 803-807
  2. ^ a b David Linden (ed), Handbook of Batteries Third Edition,McGraw Hill, New York,2002 ISBN 0-07-135978-8, chapter 23
  3. ^ Clark, M.S. (Steve) (2008). "Lead-Antimony, Lead-Calcium, Lead-Selenium, VRLA, NI-CD. What's In A Name?". Retrieved 1 February 2012. (noting sealed batteries are unreliable, "susceptible to thermal runaway, open circuit failure which renders the battery inoperable, and typically have much shorter lifetimes than their flooded brothers.")
  4. ^ HandyBob (2010) [2004]. "The RV Battery Charging Puzzle". Retrieved 1 February 2012.  (saying that with sealed batteries, you can not check the electrolyte to monitor their condition or diagnose if they are being improperly charged, and they give you "less power in the same amount of space and weight.")
  5. ^ Sterling, Charles (2009). "FAQ: What Is The Best Battery System to Use for an Auxiliary Charging System". Retrieved 2 February 2012.  (discussing excessive cost and poor performance of sealed gel or AGM batteries versus regular lead-acid flooded batteries.)
  6. ^ Wert, Ray (2009-08-19). "2010 Porsche 911 GT3 RS: Track-Ready, Street-Legal And More Power". Archived from the original on 21 October 2009. Retrieved 2009-09-18. 
  7. ^ Magliozzi, Tom; Magliozzi, Ray (Apr 1, 2007), "Is revving the engine a good idea during a jump-start? Find out", Car Talk (Tappet Brothers) 
  8. ^ "Battery Parked",, February 8, 2011, retrieved June 2, 2013 
  9. ^ Popular Mechanics Complete Car Care Manual; Popular Mechanics Series, Hearst Books, 2005, p. 289, ISBN 9781588164391, retrieved June 2, 2013 
  10. ^ Balfour, John; Shaw, Michael; Bremer Nash, Nicole (2011), Advanced photovoltaic installations, Jones & Bartlett Publishers, p. 30, ISBN 9781449624712, retrieved June 2, 2013 
  11. ^ a b Magliozzi, Tom; Magliozzi, Ray (2008), Ask Click and Clack: Answers from Car Talk, Chronicle Books, pp. 68–69, ISBN 9780811864770, retrieved June 2, 2013 
  12. ^ "Car Care Auto Clinic", Popular Mechanics 177 (11), November 2000: 136, ISSN 0032-4558, retrieved June 2, 2013 
  13. ^ a b "Storing Batteries on Concrete?", (Interstate Battery System of Detroit), retrieved June 2, 2013 
  14. ^ From the Mopar Mailing List archives. "Battery Date Codes". Retrieved 2009-09-18. 
  15. ^ "Description and treatment of sulphated batteries". 
  16. ^ Battery Council International (1996), BCI Battery Replacement Data Book

External links[edit]