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Is a car battery that drops to 11.9 volts considered dead?

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How to find out the remaining life of lead acid battery? 

Lead acid battery

 
Judging a battery's remaining life from its terminal open circuit voltage wont give you a clear picture. Have you heard the story of ‘Blind Men and the Elephant’?

You need to know the battery's ‘internal resistance’.

As the battery becomes old, its internal resistance increases. If it crosses the maximum allowable value, the battery is due for replacement. Find the maximum allowable internal resistance for your battery type from its manufacturer.

A telltale sign of a weak battery is that it heats up under high load.

Battery internal resistance: 

Battery internal resistance: It is formed due to the chemical composition of the battery. It is very easy to calculate internal resistance by applying a predetermined known (resistive) load on the battery. Measure load current and load voltage to calculate the internal resistance of the battery by applying simple Ohm’s law.

Circuit to find out the internal resistance of a battery: 

Circuit to find out internal resistance of a battery

Battery Tester: 

Or we can use an automated battery testing meter to do the same.

Battery tester

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