Automotive/SLI Batteries

An automotive battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery (starting, lighting, ignition) to power the starter motor, the lights, and the ignition system of a vehicle’s engine.

Automotive 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 automotive batteries in series for a 24-volt system or may have parallel strings of automotive batteries.

Lead-acid automotive 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. 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. 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.


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.

Fluid level

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 car batteries include extra electrolyte above the plates to allow for losses during the battery life. If the car 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 car 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 car battery. Manufacturers usually recommend use of demineralized or distilled water, since even potable tap water can contain high levels of minerals.

Charge and discharge

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 car battery or recharging a car 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 car battery, a transparent tube with a float immersed in the electrolyte visible through a window. When the car 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 automotive 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 automotive battery each of the cells can be checked with a portable or hand-held hydrometer. In emergencies a vehicle can be jump started by the battery of another vehicle (but not recommended with newer vehicles) or by a portable battery booster. It is possible to charge a car battery fully using solely the alternator, either by raising the engine’s RPM while parked or by regular driving. It will typically take one and a half hours of driving overall to charge the car battery, plus another minute or two for every time the car is started. This process can be enhanced by using lower gears as that leads to higher RPM’s and therefore higher alternator output, which can also preserve battery life when lots of electricity is being used by the air conditioner or heater, the radio, the headlights, etc., although that has the drawback of lower gas mileage. 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 like units manufactured by Pro Charging Systems; charge the car 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. In spite of this, the advice to never leave a battery on a concrete floor persists. 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. One battery manufacturer even prefers storing new batteries on concrete in the summer to keep them cooler, decreasing the natural discharge rate. Early batteries had wooden cases, and could absorb moisture from wet concrete, giving current a route to discharge. 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. 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.

Changing a battery

When changing a car battery, battery manufacturers recommend disconnecting the negative ground connection first to prevent accidental short-circuits between the car 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 2010 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. Batteries by Fisher would suggest only a trained professional change any battery. You should always wear proper safety equipment when changing an automotive battery.