Low-Voltage Battery Testing and Service

Battery Construction, Chemistries, and Rating Systems

A standard fully charged 12-volt lead-acid storage battery serves as the foundation for modern light-vehicle electrical systems. Internally, it contains six individual cells connected in series. Each cell produces approximately 2.1 volts, delivering an overall open-circuit voltage of about 12.6 volts when fully charged. Inside each cell, the positive plates consist of lead dioxide, the negative plates consist of sponge lead, and the surrounding electrolyte is a carefully formulated solution of sulfuric acid and water. When electrical loads draw power from the battery, a chemical reaction takes place: both the positive and negative plates convert into lead sulfate, while sulfuric acid is depleted and the concentration of water in the electrolyte increases.

Vehicle manufacturers utilize several lead-acid construction designs to meet vehicle demands. Conventional flooded batteries submerge the plates directly in liquid electrolyte. In contrast, Absorbed Glass Mat (AGM) batteries utilize glass-fiber separators that hold the electrolyte securely against the plates. This construction makes AGM designs spill-resistant and generally more vibration-resistant than conventional flooded units. Enhanced flooded batteries (EFB) incorporate internal improvements, such as reinforced plates or separators, allowing them to endure the frequent cycling demands of start-stop systems far better than conventional flooded designs. Some start-stop and micro-hybrid vehicles also utilize an auxiliary 12-volt battery or an ultracapacitor module dedicated to powering sensitive cabin electronics and preventing voltage dropouts during engine cranking events.

Battery cell and pack arrangements govern voltage and current output. Connecting two identical, equally charged 12-volt batteries in parallel maintains the circuit at 12 volts while increasing the total available cranking current, though the actual increase depends on battery condition and circuit wiring. Conversely, connecting two identical 12-volt batteries in series doubles the electrical potential to produce a nominal 24 volts while retaining the amp-hour rating of a single battery.

Standardized ratings define battery output capabilities. Cold Cranking Amperes (CCA) specifies the current that a fully charged battery can deliver at 0 degrees Fahrenheit for 30 seconds while maintaining a terminal voltage of at least 7.2 volts. Cranking Amperes (CA), sometimes referred to as Marine Cranking Amperes (MCA), measures output under the less severe temperature of 32 degrees Fahrenheit, resulting in a higher numerical rating than the same battery's CCA rating. Reserve Capacity (RC) measures endurance by stating the duration in minutes that a fully charged battery can supply a continuous 25-amp load at 80 degrees Fahrenheit before falling below 10.5 volts.