Battery
The primary purpose of a car battery is to START the engine.It also is used to filter or stabilize the power and provide extra power for the ignition, lighting and other accessories when their combined load EXCEEDS the capability of the charging system, i.e., when the engine is idling. In addition, a car battery provides power to the electrical system when the charging system is not operating.

An essential component of your vehicle is your battery, that lies hidden under the bonnet. It is essential that you regularly care for it, for the consequences can be quite embarrassing.Your car just would not move.Yes you can "push-start" but that would not be a long term solution.A well maintained battery can usually last for two years. But these days,with the technological improvements and testing, there are claims that some can last for more than five years.
 
What to look for while buying a new battery?
Battery buying strategy will vary based on climatic conditions.In the colder climates,higher CCA ratings are more important; whereas, in a hot climate, a higher RC ratings are of more importance. requirement.

A)Cold Cranking Amps (CCA)
The most important consideration is sizing the battery's CCA rating to MEET OR EXCEED, depending on the climate, the car's OEM cranking requirements. [CCAs] are the discharge load measured in amps that a fully charged battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds while maintaining the voltage above 7.2 volts. Batteries are sometimes advertised by their Cranking Performance Amps (CA) or Marine Cranking Amps (MCA) measured at 32 degrees F (0 degrees C) or Hot Cranking Amps (HCA) measured at 80 degrees F (26.7 degrees C), which are not the same as CCA.Do not be mislead by CA, MCA or HCA ratings.To convert CAs to CCAs, multiply the CAs by .8. To convert HCAs to CCAs, multiply HCAs by 0.69.]  

In hot climates, buying batteries with double or triple the CCA ratings that exceed the OEM requirement is a WASTE of money.  However, in colder climates the higher CCA rating the better, due to increased power required to crank a sluggish engine and the inefficiency of the cold battery. As batteries age, they are less capable of producing CCAs.

B)Reserve Capacity (RC)
The second most important consideration is the Reserve Capacity rating because of the effects of an increased parasitic (key off) load and in emergencies.  [RC is the number of minutes a fully charged battery at 80 degrees F (26.7 degrees C) can be discharged at 25 amps until the voltage falls below 10.5 volts.]  More RC is better in every case!  In a hot climate, e.g., if your car has a 360 OEM cranking amp requirement, then a 400 CCA rated battery with 120 minute RC with more electrolyte would be more desirable than one with 1000 CCA with 90 minutes of RC. [If more RC is required, two six volt batteries can be connected in series or two (or more) 12 volt batteries can be connected in parallel.  Within a BCI group size, generally the battery with larger RC will weigh more because it contains more lead.]

C)Type
The two most common types of CAR batteries are low maintenance (non-sealed) and maintenance free (non-sealed or sealed). [The low maintenance batteries have a lead-antimony/calcium (dual alloy or hybrid) plate formulation; whereas, the maintenance free batteries have a lead-calcium/calcium formulation.] The advantages of maintenance free batteries are less preventative maintenance, longer life, faster recharging, greater overcharge resistance, reduced terminal corrosion and longer shelf life, but are more prone to deep discharge (dead battery) failures due to increased shedding of active plate material.In hot climates, buying non-sealed batteries is recommended because a sealed
battery will NOT allow you to add water when required or to test the specific gravity with an external hydrometer.Some manufacturers introduced a third type of car battery, "dual", that Combined a standard battery with switch able emergency backup cells. For about the same cost a better approach was to buy two batteries and isolate them.In the future, you can expect to see more expensive valve regulated (gel cell and absorbent glass mat (AGM) .This is because car manufacturers want to extend their "bumper-to-bumper" warranty periods or as  the batteries are relocated from under the hood to avoid temperature extremes. For excessive vibration applications, it is best to buy a commercial or AMG battery.
Car batteries are specially designed for high initial cranking amps (usually for five to 15 seconds)  to start an engine; whereas, deep cycle (or marine) batteries are designed for prolonged discharges at lower amperage. A "dual marine" battery is a compromise between a car and deep cycle battery; however, a CAR battery will give you the best performance in a CAR.  For RVs, a car battery is used to start the engine and a deep  cycle battery is used to power the accessories.  [The batteries are  connected to a diode isolator and both are automatically recharged by the RV's charging system when engine is running.

D)Size
Manufacturers build their batteries to an internationally adopted Battery Council International (BCI) group number (24, 26, 70, 75, etc.)  specification, [which is based on the physical case size, terminal  placement and terminal polarity.]  The OEM battery group number is a good starting place to determine the replacement group.  Within a group, the CCA and RC ratings, warranty and battery type will vary in models of the same brand or from brand to brand.  Batteries are generally sold by model, so the group numbers will vary for the same price.  This means that for  the SAME price you can potentially buy a physically larger battery with more RC than the battery you are replacing, e.g. a 34/78 group might replace a smaller 26/70 group and give you an additional 30 minutes of RC.  If you do this, be sure that the replacement battery will fit, the cables will connect to the correct terminals, and that the terminals will NOT touch the hood when closed.
 
BCI and the battery manufacturers publish application guides that will contain the OEM cranking amperage requirements and group number replacement recommendations by make, model and year of car, and battery size, CCA and RC specifications.  Manufacturers might not build or the store might not carry all the BCI group numbers.  To reduce inventory costs, dual terminal "universal" batteries that will replace several group sizes are becoming more popular and fit 75% or more of cars on the road today. 
 
Battery manufacturers or distributors will often "private label" their batteries for large chain stores.  Below is a list in alphabetical order of the largest domestic battery manufacturers/distributors in North  America and my understanding of some of their brand names, trademarks and private labels.
 
E)Freshness 
Determining the "freshness" of a battery is sometimes difficult. NEVER buy a battery that is MORE than SIX months old because it is starting to sulfate.  [Sulfation occurs when lead sulfate can not be converted back to charged material and is created when discharged batteries stand for a long time or from excessive water loss.]  The date of manufacture is stamped on the case or printed on a sticker.  It is usually a combination of alpha and numeric characters with letters for the months starting with "A" for January (generally skipping the letter "I") and digit for the year, e.g., "J6" for September, 1996.  Like bread, fresher is definitely better.
 
F)Warranty 
As with tyre warranties, battery warranties are NOT necessarily indicative of the quality or cost over the life of the car.  Most manufacturers will prorate warranties based on the LIST price of the bad and replacement battery, so if a battery failed half way or more through its warranty period, buying a NEW battery outright might cost you less than paying the difference under a prorated warranty.  The exception to this is the free replacement warranty period.  This represents the risk that the manufacturer is willing to assume.  A longer free replacement warranty period is better and is usually an indication of the quality of the battery.

caution: Batteries contain a sulfuric acid electrolyte which is a highly corrosive poison, that will produce gasses when recharged and explode if ignited.When working with batteries, you need to have plenty of ventilation, remove jewelry, wear protective clothing and eye wear (safety glasses), and exercise caution. Whenever possible, please follow the manufacturer's instructions for testing, jumping, installing and charging.