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Road-Ready Power: A Guide to Car Batteries

By R&D
Published on March 13th, 2024

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The battery stores electrical energy used for ignition, lighting, and accessory power in an automobile or truck.



The battery is most commonly located in the engine compartment. In some automobiles, the battery is located underneath the rear seat or in the vehicle’s trunk.


A battery consists of series of plates covered with lead dioxide alternating with plates covered with lead, all surrounded by an “electrolyte” of 36% sulfuric acid and 64% distilled water. Each plate separated from the next by a porous mat. The lead dioxide plates are negative and the lead plates are positive. 

All negative plates are connected to the each other, and all positive plates are also connected to each other. Groups of these plates create a battery “cell.” The negative plates of each cell are then connected to the positive plates of the adjacent cell, with the positive plates connected to the negative plates of the adjacent cell, cumulating the voltage of the cells. 

Six cells are used in a 12 volt battery. (“Voltage” measures the amount of stored electrical energy.) When a load is applied to the battery, acid in the electrolyte reacts with the plates to create lead sulfate, in the process releasing electrical energy. Charging the battery reverses the process to store electrical energy.

Typical lead acid batteries employ liquid electrolyte. In “gel batteries,” the electrolyte is mixed with silica. “Absorption glass mat” batteries have plates formed as cylinders, with the electrolyte stored in fiberglass mats wound around those cylinders to make the battery resistant to vibration.


Drivability Symptoms

A weak or discharged battery either will not spin the starter rapidly enough to start the engine or will not hold the starter solenoid closed, producing a repeated clicking sound as the solenoid closes and then immediately reopens, interrupting current flow to the starter. 

A battery can have sufficient voltage to illuminate the headlights, but not enough to start the vehicle. The first signs of battery failure often occur when weather turns cold because battery voltage decreases as ambient temperature drops – a decrease from 70º F. to 0º F. reduces the output of a fully charged battery 35 percent.

Modern motor vehicles depend on adequate battery voltage for proper functioning of computer controlled systems. Even though a weak battery may be able to start the car, low battery voltage can diminish the strength of electronic signals to and from the vehicle’s computers. 

Consequently, diagnosis of any computer-related vehicle system always begins with assuring that the battery is fully charged.

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Inspection, Test, and Diagnosis

Visually inspect the battery case. Any damage, such as a crack in the case or a loose terminal, requires battery replacement. Dirt on the battery case, especially on the battery top, can create “parasitic drain” constantly discharging the battery. Wash away dirt with a solution of baking soda and distilled water. 

Though many batteries are sealed, others have removable caps over the cells that can be carefully pried off to verify that electrolyte level is above the plates. If so, check electrolyte level and add distilled water as necessary to bring its level above the tops of the plates.

Next, remove the battery cables and check for corrosion. Post-type battery terminals protrude from the top of the battery, with the cables clamped to the posts. “Side-mount” batteries, common on General Motors vehicles, have terminals recessed into the side of the battery case, with the cable bolted onto the terminal. 

Corrosion - usually a white, powdery deposit – creates electrical “resistance” that reduces current flow to and from the battery. Remove corrosion with a wire brush, an inexpensive battery terminal cleaning tool available at any auto parts store, or chemically by brushing on a solution of backing soda and distilled water. 

To avoid creating sparks, always disconnect the negative cable first. Hydrochloric gases can accumulate inside a discharged battery and a spark can explode them.

A car mechanic holding the battery

To test battery voltage, connect a Digital Multimeter (DMM) across the battery terminals with the engine off. Set the DMM to “V” to measure voltage, connect the positive lead to the positive battery terminal, and the negative lead (inserted in the DMM’s COM port) to the negative terminal. 

The reading is the battery’s actual stored voltage. Assuming 70º F. temperature, 12.6 volts or above indicates fully charged. 12 volts or less is a ‘dead’ battery. (Each cell in a ’12 volt’ battery actually holds 2.1 volts, so a ’12 volt’ battery should hold 12.6 volts.)

The last step is verifying alternator/generator function by checking battery voltage with a DMM exactly as described above, but with the engine running. This test requires a fully charged battery, so recharge the battery with an external battery charger, if necessary. 

At fast idle, a fully charged battery should read between 13.2 and 15.2 volts, dropping by no more than 0.5 volts when the headlights and all accessories are turned on. A reading above those levels indicates the alternator/generator is overcharging the battery. If below, alternator/generator output is low or there is excessive electrical resistance in the charging circuit.

If the battery connections are clean and the alternator is functioning within specifications, but the battery either cannot be fully recharged or fails again soon after recharging, it should be replaced.


Replacement of a battery does not require reprogramming.


Each time the battery discharges and recharges, its ability to convert lead sulfate to lead and back again diminishes, until eventually the battery cannot store adequate voltage. Until then, however, a battery can be recharged. This should be done in a well-ventilated area using an external battery charger, with the battery cables disconnected.

When there is not time to recharge a dead battery, a vehicle can be “jump-started” using the charged battery in another vehicle. Connect the positive cable of the dead battery to the positive of the good battery. 

Then connect the negative cable of the good battery to a good ground on the vehicle with the dead battery, such as the frame or other heavy gauge steel or cast iron part. Start the vehicle with the good battery, and then start the vehicle with the dead battery. 

A car mechanic replaces a battery during maintenance.

After starting, remove the cables in the reverse order. Electrolyte in a discharged battery can freeze. If so, thaw the battery before attempting a jump start or charging it.

A replacement battery should be the same physical size as the original equipment battery. Size is designated by battery “group number,” such as 75 (common on General Motors vehicles) or 35 (many Hondas, Nissans, and Toyotas). 

The replacement battery should equal or exceed the vehicle manufacturer’s specification for “cold cranking amps,” which measures current flow from the battery at 0º F. Batteries are also rated by “reserve capacity,” which serves as a guide to how long the battery can generate starting power before it is discharged. 

Replacement batteries located inside the passenger compartment or trunk should have the same provisions for external ventilation from the battery case as the original equipment battery.

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