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Automotive Technology

 

The Purpose of the Automotive Battery


The automotive battery is designed primarily to start the vehicle. The battery's ability to 'dump' power is the CCA (Cold Crank Amps) measurement. Ability to produce power at a given temperature (minus 18deg C)

The determinant is the size of the starter motor which is again determined by the size of the engine it is starting. A typical six cylinder petrol engine needs up to 600CCA. A diesel engine often needs a little more.

However, the capacity to operate auxiliary consumers such as park lights, electric windows, radio, DVD etc. while the motor is not running is also a consideration. This is measured in RC (Reserve capacity). The RC is the number of minutes the battery will maintain a usable voltage (above 10.5v) while delivering a 25 amp current draw. Therefore a vehicle with many electrical accessories requires a battery with a larger RC rating.

In some circumstances, it is desirable to have a high performance battery with a high CCA rating such as 750, 1200 or even 1400 CCA but low reserve capacity. This application may be starting a large motor only with no need for running accessories. Agriculture, earth moving equipment and some marine applications would be typical.

Read about Chargers.



Automotive Battery failure

 

Recharge only / still serviceable:

A recent battery study has shown that up to 15% of batteries replaced are simply flat or partially discharged. Batteries which can be fully restored after recharging can be re-fitted to the vehicle. It is important to test all batteries being replaced, as there may be a problem with the vehicle's electrical system.

Usage related failure:

This occurs when a battery has premature failure due to extreme conditions of use commonly found in harsh climates.
Low electrolyte levels expose the bus bar and ultimately the upper portion of the plates that can cause irreversible sulphation and accelerated corrosion. The low electrolyte levels can be the result of lack of battery maintenance, or as a result of overcharging where fluid is lost through gassing. As an additional problem, the acid concentration in the battery from lower electrolyte levels can cause increased corrosion of the remaining 'wet' part of plates. Batteries with higher electrolyte volume above the plates assist in minimising these problems, and ensuring batteries are regularly maintained when operating in harsh conditions will also prolong battery life.

High under bonnet operating temperature. Harsh operating conditions, and increasing vehicle under bonnet operating temperatures, can cause early battery failure through a number of areas. These conditions contribute to corrosion of the positive plate, grid growth which can result in a short circuit, and loss of plate active material.

Vibration effects. This can cause physical damage to battery components and often sudden failure of the battery.

Vibration damage can include:

  • Loss of active material from the plates, resulting in a loss of battery charge and possible short circuits.
  • broken or cracked grid frames, causing short circuits; usually causing separator damage.
  • perforation of the separator envelopes at the bottom where the plates sit in the battery case. This can result in short circuits.

Overcharging. Where the vehicle charging system has been operating at a higher than normal voltage, the battery is subjected to virtually continuous charging. This can result in faster grid corrosion, loss of plate active material, loss of electrolyte, plate growth and eventually disintegration of the positive plate.

Undercharging. This can occur when the vehicle voltage charging system is too low to fully recharge the battery. The result is a loss of charge and irreversible sulphation of the battery.

Plate or Grid Related Corrosion:

Grid corrosion of the Positive plates within a battery is a normal 'end of battery life' condition that is commonly caused due to higher operating temperatures, overcharging or loss of electrolyte fluid. Of the positive and negative plates within a battery, it is the positive plates where the grid metal can completely oxidise and disintegrate due to these operating conditions. Different alloys added to the positive grid lead can have an affect on the corrosion rate. Antimonial lead grids generally have a higher corrosion resistance than calcium lead grids, and along with higher volumes of electrolyte above the plates, can help overcome this problem, allowing batteries to operate more successfully in harsh conditions.

Soft Positive Plate Material ("mushy plates"). This is a condition usually resulting from high operating temperatures or overcharging. Often, both conditions may have occurred.

Sulphation. This occurs when a battery stands in a partially or fully discharged state for long periods of time, or is continually undercharged. As a result of these conditions the active lead material on the plates becomes lead sulphate which also hardens the plates. Depending on the length of time the battery has been in this condition, the sulphation may be irreversible. In addition, if the electrolyte level in the battery is low, the exposed part of the plates will become inactive and sulphated. Therefore batteries with higher electrolyte levels will go a long way to reducing problems due to sulphation and assisting longer battery life.

Open Circuit:

Broken cell to cell connection. This is where there has been a complete failure of the intercell weld. Weld quality is critical for reliable battery performance and good working life. This problem is largely minimised by leading manufacturers, due to accredited quality assured manufacturing processes.

A broken bus bar. This type of failure can be caused by excessive corrosion of the bus bar due to low electrolyte level in the battery. Extensive overcharging and/or elevated operating temperatures can also lead to accelerated corrosion of the bus bars leading to breakage and open circuit. Batteries with the capacity for higher electrolyte levels will help to ensure a longer battery life.

Short Circuit:

Plate to bus bar short circuits can result from bent plates contacting the bus bar, or corroded positive plates which have grown upwards, contacting the bus bar and causing a short circuit. This fault can be age related and can be a normal 'end of life' condition. It can also be a result of overcharging, cycling or elevated operating temperatures.

Plate to plate short circuits occur when positive and negative plates make contact causing short circuit, resulting in battery failure. Vibration can cause the plates to wear or pierce the separator material leading to a short circuit. .

Worn out or 'End of Life' conditions:

The normal 'end of life' condition in a battery is when one or more cells cease functioning due to the positive plate grid having corroded and finally collapsing. Operating temperatures have a definite effect on battery life, and high temperatures will accelerate these 'end of life' conditions.

(The above is general industry knowledge and these notes where taken from reports from several battery manufactures such as Exide, Yuasa, East Penn, Trojan etc.)

 
 
 
 

 
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