BATTERIES

A fully charged car battery should be 12.6 volts and you need atleast 12.4 volts to start a car .
the other purpose of the battery other than giving power to the starter to start the car are running the accessories when the cars off eg: alarms ,stereos ands keeping power to the ecu to maintain its memory .
If one cell in a battery has failed the whole battery needs replacing because you can not fix the cell at fault .
To get rid of the surface charge on the battery you can turn on the head lights for two minutes or you can disconnect the positive coil wire or the positive ignition plug and crank the engine over for 15 secounds .
when load is applied to a battery it must hold above 9.5 volts to be in  working order any lower is a failed battery also when testing only crank tester to half the cold cranking amps of the battery and do this for 15secs .when  cleaning a battery use warm water and baking soda to clean acid away  and coat terminals in grease or petroleum jelly .

a battery can be very explosive around naked flames because batterys give off hydrogen gas which is very flammable .