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How a Battery Works

 

Voltage      Chemical      Cranking Power

The standard automotive battery in today's vehicles is 12 volts. Each battery has six cells with 2.1 volts. A car battery is considered fully charged at 12.6 volts.

When a battery drops voltage, even a small amount, it makes a big difference. For instance, when a battery drops from 12.6 to 12.0 volts, its power drops from 100% to 25%. At 12.4 volts, a car battery is 75% charged. At 12.2 volts, it's 50% charged.*

A car battery is considered charged at 12.4 volts or higher. It is considered discharged when it's at 12.39 volts or less.

*A fully charged specific gravity of 1.265 corrected to 80F
  is assumed.

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How a Battery Works | How to Jumpstart a Battery | Definitions
Safety and Handling | Maintenance & Storage Tips | Charging a battery
Selecting the Right Battery | Recycling