Battery basics

Motor,  Apr 2002  by Layne, Ken

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MASTERING THE BASICS

Early automobiles didn't have batteries. In fact, they didn't have much in the way of electrical systems. Ignition was provided by some sort of magneto, and the engine was cranked by muscle power. Headlights-if a car had them-were fired by acetylene gas, and early horseless carriages had no electrical accessories.

With respect to electrical systems, modern cars would be similar to those of 90 or 100 years ago if it weren't for storage batteries. If today's cars didn't have batteries, they wouldn't have inductive ignition systems, self-starters, safe lighting, radios, air conditioners or their myriad digital electronic systems. This installment of Mastering the Basics looks at the fundamentals of battery construction, operation and testing.

Battery Design & Construction

Battery chemistry and design haven't changed radically since automotive batteries were introduced in the second decade of the last century. Materials and construction methods have evolved steadily, however, making modern batteries far more powerful and reliable than their ancestors.

Almost all automobile batteries are 12-volt, lead-acid, wet-cell batteries. We call them "storage" batteries, but they do more than store electricity. A battery changes chemical energy to electrical energy as it provides power to the electrical loads on a car. It then changes that electrical energy back to chemical energy as it's recharged by the alternator.

The battery performs its magic through the chemical action of two dissimilar metals in the presence of a liquid electrolyte. The metals are applied in paste form to conductive plates, usually made of a calcium alloy. Electrically negative (-) plates are lead, positive (+) plates are lead dioxide. Alternating - and + plates have insulating separators between them to form a battery element. Each element is then placed in the battery's plastic case to form a cell. The + and - plates of each cell are connected to each other and to the equivalent plates of the other cells. When the electrolyte mixture of water and sulfuric acid is added, the battery cell is complete.

The lead and sulfuric acid compounds cause each cell to develop about 2.1 volts when fully charged. Therefore, a 12-volt battery with six cells actually provides 12.6 volts. The number and size of the plates determine the current capacity of the battery. In short, chemistry determines voltage; total plate area determines the amperage rating. Compounds used in other kinds of batteries develop different voltages, but size usually has a direct relationship to current capacity.

Discharging & Charging

When a battery supplies power to a circuit, current flows out of the battery, through the circuit and back to the battery. Because we stick with the tried-and-true conventional current theory for automobile electrical systems, we think of current flowing from the positive terminal and returning to the negative terminal. During discharge, an electrochemical reaction takes place that reduces the acid percentage in the electrolyte. We won't dwell on the chemistry, but sulfur ions released from the electrolyte combine with lead on both the positive and negative plates to form lead sulfate. That's why we say a badly discharged battery is "sulfated." If it's sulfated too badly, no amount of charging current will restore it to like-new condition.

When an alternator recharges a battery, charging current flows into the battery positive terminal, through the battery to the negative terminal, then back to the alternator. Recharging reverses the chemical condition of the battery. Sulfate ions recombine with the electrolyte, and the plates return to their original lead and lead dioxide states. The process where a battery is continually discharged and recharged is called cycling.

Hydrogen gas is produced during charging, which increases the acid percentage in the electrolyte. It also creates a hazard because hydrogen is very explosive. That's why sparks and open flame must be kept away from a battery, particularly during charging.

Specific Gravity, Temperature & Battery Performance

Battery performance and battery testing relate directly to temperature and the specific gravity of the electrolyte. Specific gravity is the measurement of any material compared to that of water. The specific gravity of water is always 1.000. The specific gravity of battery electrolyte is 1.260 to 1.280 when the battery is fully charged with the electrolyte temperature at about 80 deg F.

Electrolyte specific gravity changes during the chemical reactions of discharging and charging. That makes specific gravity measurement an important indication of a battery's state of charge. The following table lists the approximate state of charge and the corresponding specific gravity:

If a battery has removable cell caps, you can use a hydrometer to measure the specific gravity of the electrolyte. Draw in enough electrolyte to float the indicator without letting it touch the top or sides of the tube. Then, with the indicator at eye level, read the specific gravity on the float. Return the electrolyte to the cell from which it was taken and repeat the test for the other cells in turn.