WORKING OF A BATTERY:
The potential or voltage difference created between two metals which are immersed in an electrolyte (acid), is called 'cell voltage'. A battery converts chemical energy of the metals submerged in electrolyte into electrical energy by means of an electrochemical oxidation-reduction process.

CONTENTS OF A CONVENTIONAL BATTERY:
When the battery terminals are connected to an external load, the sulphate ions in the electrolyte combine with the positive and negative plates forming lead sulphate and electrical energy is released. Oxygen in the active material of the positive plate combines with hydrogen from the sulphuric acid to form water, which reduces the concentration of the acid in the electrolyte. As the discharge continues, the active elements of both plates become more alike, ie, lead sulphate and the electrolyte become weaker. Therefore the voltage becomes lower since it depends on the concentration of the acid and the difference between the two plates. Eventually the battery loses its power of delivering electricity at an impressive intensity of voltage and is said to be discharged.

BATTERY CARE:
The health of a battery should be maintained or it will fail to serve you for a long time span. Servicing and testing of a battery should be done regularly in order to maintain prescribed specific gravity levels to attain good delivery of electricity to support the other circuit components. Periodic tests listed below should be carried out on a battery to ensure a smooth drive.
CHECK 1:
Hydrometer Test: Using a hydrometer, measure the specific gravity (corrected to 27 degrees) of the electrolyte in each cell.

CHECK 2:
Load Test: The load test is meant to gauge the battery's ability to withstand the load when the vehicle has to be cranked. It has to produce a surge in electricity to move the starter. Usually a fixed load tester is used for this purpose.
Connect the load tester to the battery terminals with the help of metal spring clamps. Ensure that the clamp-terminal contact is firm. Apply test load and note the voltage after 5 seconds. The voltage reading of a healthy battery after 5 seconds of discharge should exceed a required minimum voltage as specified by technical specs.

CHECK 3:
Retention of Charge Test: The battery should be charged and kept undisturbed for at least 7 days before checking the charge. Results obtained in the first two tests should not increase the specific gravity by 20 points. If the loss is not greater than 15 points, assume that the battery has passed the first two checks. In case the loss is greater than 20 points, battery replacement is recommended.


GENERAL TESTS:
Check the specific gravity of the battery.
Specific gravity is the weight of the fluid in comparison to the weight of water (ie, water has a SG of 1.0). Battery acid changes the SG depending on the charge-level of the battery and also on the type of battery. Fully charged batteries have SG of 1.255 to 1.280. Fully discharged batteries have SG of 1.110 to 1.130. The specific gravity test is done with the help of a hydrometer. A graduated float which is enclosed in the meter shows SG readings. Before testing, ensure that the hydrometer is an accurate one.

Check Open Circuit Voltage (OCV) of the battery.
If the OCV is right, conduct the High Rate Discharge (HRD) test (voltage should be 10V or above after 5 seconds). If it is low, check individual cell for necessary action. If HRD test is successful proceed to Step 3. If the battery fails the HRD test, the failure has to be diagnosed. If the vehicle has run 10km or more, switch on headlights for about 5 minutes before checking to avoid polarisation.

Check earth leakage in the vehicle.
Switch off all the on-off switches, cassette player, clock, etc before earth testing your battery. Leakage upto 2V is within limits. If there is no leakage proceed to Step 4. If there is earth leakage, identify the leak and take corrective action.

Check for surface leakages.
If there is surface leakage, remove the battery from the tray, wash it well with water, dry it and then fit it back into the metal tray.

Check battery terminal clamps for corrosion.
If corroded replace them. Check whether the solderings on the wire are intact, or else re-solder them. Clean the terminal clamp well before fixing it back on the battery pole.

Check fan belt tension.
If loose, tighten it so that the alternator/dynamo runs consistently without breaks and slips. If worn out replace the belt.

Check starter current draw, it should be within the following limits :
a) Cars -> 80 - 150 Amps
b) Amb Dsl/Matadors -> 200 - 225 Amps
c) Jeeps/Tractors -> 250 - 280 Amps
d) LCVs -> 225 - 250 Amps
e) HCVs -> 350 - 400 Amps
If the starter motor draws more current than the specified limits, rectify the starter assembly.

Check charging systems.
The charging voltage for a 12V electrical system is 14.2 +/- 0.1 Volts. If the charging voltage is low then the battery suffers sulphation and if it is high, the active metal in the battery is shed. In both cases, alternator/dynamo should be checked and rectified if necessary.

Check charging current for normalcy to complete the checking procedure.

FACTORS AFFECTING BATTERY LIFE:
Overcharging:
It severely corrodes positive plate grids which weaken and finally disintegrate. Overcharging decomposes electrolyte into hydrogen and oxygen which causes excessive concentration of the electrolyte because of loss of water from the mixture of the battery fluid. This damages the plates and separators. High temperatures also show adverse effects on plates, separators and container. Excessive gassing creates the possibility of the active metal blowing away from the plate surface and fine acid spray escaping from the battery.

Undercharging:
Perpetual undercharging results in sulphation of plates and running down of cells. Sulphate deposits are also seen on cell separators, which leak through and create short circuits between positive and negative plates. Undercharging also leads to buckling of plates.

Idle battery:
If left idle and self-discharge takes place, sulphation is observed on battery terminals. Sulphation of the battery kills battery life sooner than normal.

High Specific Gravity:
High SG destroys positive and negative plates and also reduces battery life.

Impure water top-up:
Impure water introduces impurities in the battery every time it is added. Iron and chlorine attack the plates causing to shorten their overall life. Chlorine bleaches separators which is harmful.

Negligence in water top-up:
The concentration of the acid increases due to less amount of water and so damages are borne by the plates and separators. The plate areas above the electrolyte level get hard and lose capacity.