Battery Testing Basics By Ken Buie Previously we’ve looked at different parasitic loads that cause a battery to go dead when the car sits. This month we will take a closer look at battery testing. After all, the battery is the heart of the electrical system. Therefore, it must be in peak condition to provide power to start the engine and help operate the accessories. Safety is of the utmost importance when handling and testing batteries. Always wear safety glasses when testing a battery. Hydrogen gas and oxygen are produced by batteries during normal operation. Sparks or flames can ignite this gas, causing an explosion which may shatter the battery. Always observe all safety precautions to keep sparks or flames away from the battery. Battery acid will damage the car’s paint and your personal body parts. Batteries have come a long way since the days of rubber cases sealed with tar. Ah, the good old days! Today’s batteries are smaller, lighter, and much more pow- erful than the older ones. But it doesn’t matter if it’s 1953 or 1993. You still need a battery to start the car. The basics of battery testing apply to all types of bat- teries. All you need are the right tools and some basic test procedures. Battery Testing. The three main bat- tery tests are: * Open circuit voltage test ¢ Hydrometer * Battery load test An open circuit voltage test is espe- cially helpful on batteries with sealed vent caps. Since you cannot perform a hydrometer test on a sealed battery, an open-circuit voltage test indicates the battery’s state of charge. To perform an open-circuit voltage test, the battery should be disconnected from the vehicle and allowed to stabilize for 10 minutes before testing. A fully charged 12-volt battery will show a voltage of 12.6 volts. Each cell in a battery produces 2.1 volts. It takes six cells to make up a 12-volt battery (6 x 2.1 = 12.6). When you take a voltage reading on a battery that has just been charged, you find the voltage is higher than 12.6 volts. This extra voltage is called a surface charge. Remove the surface charge by applying a 150-amp load to the battery for 10 to 15 seconds. Allow the battery to stabilize, then compare your voltmeter reading to the following chart to determine the state of charge. 12.6 or higher 100% charge 12.4 75% charge 12.2 50% charge 12.0 25% charge 11.9 or lower discharged For the most accurate open-circuit voltage test use a digital voltmeter. Hydrometer Test. A battery contains approximately one gallon of electrolyte. Electrolyte is a solution of 64 percent water and 36 percent sulfuric acid. A hydrometer is used to draw out a sample of the electrolyte for testing. A hydrometer determines the sulfuric acid content of the electrolyte by measuring the specific gravity of the electrolyte. A specific gravity test is another indicator of a battery’s state of charge. The chart below indicates the state of charge for a battery at 80 degrees F. Specific Gravity Chart 1.265 100% charge 1.225 75% charge 1.190 50% charge 1.155 25% charge 1.120 discharged As shown, a fully charged battery has a specific gravity reading of 1.265 at 80 degrees. A 1.265 reading means the electrolyte has 36 percent sulfuric acid by weight. Pure sulfuric acid has a specific gravity of 1.835. Water has been assigned a number of 1.000 at 80 degrees. Therefore, a battery with a specific gravity reading of 1.265 means the solution is 1.265 times heavier than pure water. A glass tube type hydrometer has a calibrated float that “floats” at different levels depending on the state of charge of the battery. A specific gravity reading 1.265 means the electrolyte is more dense than a reading of 1.120. The more dense the solution is, the higher the float will rise in the tube resulting in a higher reading. Temperature affects the accuracy of your hydrometer. Most hydrometers are calibrated for temperatures between 60 and 80 degrees F. As the electrolyte heats up, it expands. It then becomes less dense, causing the float to sink lower in the tube and show a lower specific gravity reading. As the elec- trolyte cools, it shrinks in volume and becomes more dense. The float will now rise higher and read too high. A hydrometer with a thermometer ensures a more accurate reading. Add .004 (called four points) to your reading for each 10-degree increment above 80 degrees. You should then subtract .004 (four points) from your reading for each 10-degree increment below 80 degrees. Under-hood temperatures in the summer will cause the electrolyte temperature to exceed 100 degrees. Use the graph in Figure | as a guide for temperature correction. The follow- ing chart gives some examples of how to apply temperature correction. Fig. 1. Temperature Correction Example Example 1 Temperature 100 degrees Specific gravity 1.235 Temp. correction + .008 True specific gravity 1.243 Example 2 Temperature 80 degrees Specific gravity 1.250 Temp. correction .000 True specific gravity 1.250 Example 3 Temperature 20 degrees Specif