How a Battery Functions

Battery Introduction

Automotive batteries produce electricity through chemical action. These batteries are rechargeable; consequently, they can also convert an electrical current into chemical energy and store that energy until it is needed.

The main tasks of the battery are:

• When the engine is being started, the battery must supply a large amount of power to turn the starter motor.
• The battery must provide a supply of power to the electrical system when the engine is not running or when the alternator is not producing enough electrical energy to meet the demands of the vehicle’s electrical components.
• The battery is required to act as a reservoir and stabilizer. In this role, it absorbs voltage spikes and surges that would damage sensitive electrical components.
  

Battery Construction

A typical car battery is constructed of six elements, and each element is made up of two groups of plates – namely, the positive group and the negative group. The plates of both of these groups are sandwiched together so that a sequence of alternating negative plates and positive plates is achieved (i.e., positive / negative / positive / negative, etc.). Insulating separators manufactured of a porous material are placed between each set of plates to prevent them from coming into contact with each other and short-circuiting.

Each element is fitted into the individual compartments – or cells – which are formed by the shape of the battery box. The battery is then filled with an electrolyte solution containing pure water and sulfuric acid so that all of the elements will be immersed. Furthermore, as each element produces between 2 and 2.2 volts, the total output of the battery will be between 12 and 13.2 volts.

The top cover of a standard battery box is usually fitted with six filler caps (i.e., one for each of the cells). In addition, the positive and negative terminal posts generally protrude from this top cover.

Battery Function

When the battery is fully charged, the electrolyte comprise 64% water and 36% sulfuric acid by weight. Chemical action of the electrolyte – working on active material in the plates–causes a transfer of electrons from the positive plates to negative plates, and when the battery is placed in a closed circuit, a surplus of electrons at the negative post flows to the positive post, producing an electric current. When in this state, the battery is converting chemical energy into electricity.

As the current continues flowing, the battery will begin to discharge. The sulfate in the electrolyte combines with the plate materials and the electrolyte becomes weaker and weaker, ultimately leading to complete discharge of the battery. The electrolyte of a discharged battery will be about 90% water and 10% sulfuric acid by weight.

Recharging the Battery

The battery can be recharged by passing an electric current back through it in the reverse direction. Usually, this current will be supplied by the vehicle’s charging system, which is powered by the alternator while the engine is running. Furthermore, if the battery were to be disconnected from the electrical system, it could be charged using a battery charger.

The previously-described chemical action is reversed during charging. That is to say,sulphate leaves the plates and returns to the electrolyte in the form of sulfuric acid. When this action has been performed for a sufficient time, the battery will have become fully charged.

Since the percentage of sulfuric acid in the electrolyte changes with the charge of the battery,the current condition of a battery can be checked by measuring the specific gravity of the electrolyte. When a battery is fully charged, the specific gravity of the electrolyte will be about 1.280. Furthermore, some batteries are provided with a built-in charge indicator which indicates the state of charge by testing the specific gravity.

Types of Batteries

Automotive batteries can be generally classified as either standard batteries or maintenancefree (MF) batteries. The following section will review the difference between them in addition to their advantages and disadvantages.

Standard Batteries

As mentioned above, standard 12-volt batteries have six filler caps, and these must be removed when checking the specific gravity of the battery fluid (electrolyte) or when adding water. Also, since water evaporates from the fluid, it may be necessary to add pure water from time to time. Many battery boxes are translucent and are provided with MAX and MIN markings which allow the fluid level to be checked without opening the top. Also, filler caps are often provided with vent holes in order that any build up of pressure inside the battery may be released safely.

Advantages of a standard battery:

• A standard battery can be repeatedly “deep-discharged.” This means that the battery can be used until it has discharged completely and can then be recharged to its original capacity.

Disadvantages of a standard battery:

• As the battery is being charged, the water in the electrolyte tends to evaporate; and since the cells are vented to the atmosphere, the battery loses water.
• When charging, the explosive gas hydrogen is produced.
• Because of plate composition, a standard battery has a relatively short life span in normal use (2 to 5 years).
• A standard battery performs poorly at high temperatures.
• Terminals corrode more easily on a standard battery.

Maintenance-free Batteries

Maintenance-free (MF) batteries are completely sealed and have no filler caps. These are modified batteries and have the following advantages:

• Since they are sealed, they do not loose water through evaporation.
• Maintenance-free batteries have extremely-long shelf lives.

However, maintenance-free batteries do have several disadvantages:

• They may perform poorly after being repeatedly deep-discharged.
• They may be difficult to charge as a slightly higher voltage is required for successful charging.
• The specific gravity of the electrolyte in a maintenance-free battery cannot be tested.

Battery Codes

Each car battery with six cells will produce a total voltage of 12 volts. However, battery performances will vary with respect to model – in general, larger batteries can supply and store more electricity. In this respect, several standards or ratings have been set up to identify battery performance, the most important of these being the “ampere-hour capacity.”

Ampere-Hour Capacity

The ampere-hour capacity is used to determine how much current a new, fully-charged battery (at a temperature of 26.7 degrees C) can deliver for 20 hours before the voltage drops below 1.75 volts per cell. For example, a 12-volt, 120-ampere-hour battery can deliver 6 amperes for 20 hours. This is the most common method of rating batteries and is sometimes referred to as the “20-hour rating”.

JIS Battery Codes

A combination of numbers and letters can be found on the majority of batteries. These will indicate a battery code which specifies some information regarding the battery.

The JIS battery code indicates the classification to which the battery belongs. The first number indicates the performance rank, which gives a rough idea of the ampere-hour capacity. The next letter represents the size ranking (in width and height). The next number indicates the approximate length of the battery case in centimeters. Finally, the remaining letters determine the position and the shape of the battery posts.

There are other codes specified by DIN, etc.

Precautions for Handling Batteries

A battery is a power source – it stores a large amount of energy. Consequently, a battery may pose a number of different threats to safety if not handled properly.

Battery Fluid

Battery fluid (or electrolyte) contains sulfuric acid – a substance that may cause severe burns to the skin or may damage many kinds of material if contacted. It is important to remember that if battery fluid should be left in an open area, only the water will evaporate and the acid will become ever more concentrated. Undiluted sulfuric acid is highly corrosive and can dissolve metals such as  luminum, copper, and even iron. Battery fluid must, therefore, be washed off will large amounts of cold water if it should come into contact with skin, clothing, or any item other that the inside of its designated container. Note that this precaution applies equally to even the smallest amounts of this liquid.

Danger of Explosion

A battery will generate a highly-explosive combination of hydrogen and oxygen gasses when it is being charged. This gas generally fills the space inside the battery box; however, since batteries are often provided with vent holes, these gasses may also be found in the area immediately surrounding the battery. An exploding battery poses a very serious treat to safety, not least because of the large amounts of acid that will be thrown in all directions. Never smoke, use naked flames, or allow sparks to be generated near a battery which is being charged.

Short Circuiting and Sparks

Short circuiting of a battery should be avoided as it may be harmful to both the vehicle’s electrical system and to the battery itself. Also, avoid generating sparks at the batteries terminals and other similar locations as these could ignite either the hydrogen which is released from the battery or vaporized engine fuel. You should follow the designated procedures when connecting or disconnecting a battery: Always connect (or disconnect) battery leads or booster cables to (or from) the battery posts in the correct sequence.

Overcharging

Overcharging should be avoided as it will raise the temperature of the electrolyte, possibly causing the electrolyte to spray out from the battery vents. The battery must be charged at a proper rate and the electrolyte must not be allowed to become too hot.

In addition, a battery must not be discharged too much either, as an electrolyte which is too dilute (or weak) can freeze and cause the battery case to burst.