Sealed lead-acid batteries became widely used in the 1980s by General Motors. They are still popular today and marketed as maintenance-free. A small valve is installed in the battery to allow gases to escape. Being unable to service each of the six battery cells is a disadvantage, especially when it comes to battery testing. By far, the most accurate condition test is performed with a hydrometer. This quick and easy procedure tests the specific gravity of each cell. Electrolyte is drawn into a glass tube that floats a calibrated ball. The higher it floats, the greater the charge. This is impossible with a sealed battery; other procedures are needed.
Test the battery voltage. Set the voltmeter dial to DC (direct current) volts. Place the voltmeter leads on the positive and negative battery posts. Read the voltage. The voltage should read greater than 12.6 volts and less than 14.0 volts.
Charge the battery. Perform a high-rate charge test. Hook positive and negative battery cables onto the battery posts and set the charger to fast charge or boost. Take a voltage reading with the voltmeter. If the voltage rate quickly climbs and exceeds 14.0 volts, reading as high as 17.0 volts, the battery cannot contain the voltage and is defective. Replace the battery.
Perform a slow charge test. Slow charge the battery for several hours. Periodically inspect the battery. If it is bubbling, spewing and has spattered battery acid on the cover, the battery is defective and must be replaced.
Perform a load test. Slow charge the battery for several hours, overnight if possible. If the battery has passed all the other tests, is clean and dry and the voltage reads correctly, attach a load tester and read the scale. The needle should be in the green zone. Turn the load test knob. This allows current to flow into a carbon pile where it is turned to heat. Do this for an approximate time it would take to start a vehicle. The needle should stay in the green zone. Repeat the test two or three more times. Each time the needle should remain in the green zone. If it drops into the red, replace the battery.
If a battery fails after several years of service, for example, a five year rated battery is in its sixth year of service, odds are it needs to be replaced. The only exception would be a parasitic drain, such as headlights left on overnight, being the cause. Clean the battery periodically with a mild solution of baking soda and water. Rinse thoroughly with fresh water.
Batteries can explode. Take safety precautions by wearing gloves and safety glasses.