Dispelling Electrical Test Myths Parts Il, Il & IV By Dan Marinucci In Part I, I dis- cussed misperceptions about battery rat- ings and how these Sa things affect electri- ~ cal diagnosis and repair. Now, Il look at one of the biggest myths of the en- tire auto service industry: The belief that we all speak the same technical lan- guage. Coincidentally, Bob Newman, equipment manager at JIMCO, Inc., touched on one aspect of this myth in his excellent article on the expression “Joad dump” in last month’s Exchange (see February 1994). But the language I find causing confusion is the expres- sion “battery voltage.” Open Circuit Voltage. Voltage is electrical pressure, current (amps) is electrical volume. Obviously then, bat- tery voltage is the battery’s electrical pressure. Confusion – not to mention wasted time and misdiagnosis – often occur when people fail to distinguish between open-circuit battery voltage and operating voltage. Open-circuit battery voltage is the battery’s electrical pressure when no current is flowing out of it. In other words, when there’s no substantial load on the battery. I emphasize the word substantial because in the real world, a wee bit of current is flowing out of the battery when the ignition switch is off. Components such as the clock, computer memory, voltage regulator, etc. are “hot” all the time and normally may draw as much as 50 milliamps (0.050 amp) from the bat- tery when the ignition switch is off. The reason for an open circuit may be anormal condition such as an igni- tion switch that is turned off. The cause being an abnormal situation such as a broken wire or connection. But the result is the same: no current flows through an open circuit. Open-circuit voltage on a fully charged battery should be 12.5 to 12.6 volts. Operating Voltage. Operating voltage is battery pressure when a load device is operating and a substantial amount of current is flowing out of the battery. Typical load devices are lights, motors, solenoids, relays, etc. Many technicians remember operating volt- age as “loaded” or “under load” volt- age because the battery’s operating a load. The greater the electrical load on the battery, the greater the current flow out of the battery. The greater the current flow, the lower the battery voltage drops. When there’s a load on the battery and the engine is shut off, op- erating voltage eventually drops to the point that the battery discharges and can’t provide any more current. In other words, the battery goes dead. You can liken this situation to a wa- ter well storage tank. If you turn on one faucet after another in the house, water flows out of the storage tank until the tank’s empty. The water system’s pressure switch should sense the empty storage tank and turn on the well pump to replenish the tank. If it doesn’t, the house’s water system runs dry. The pressure switch equivalent in an automotive electrical system is the volt- age regulator; the well pump equiva- lent is the alternator. Of course, the alternator doesn’t produce unless the engine’s running. Test Tips. Always check battery MARCH, 2001 voltage with an accurate digital volt- meter. When the vehicle has a crank- ing problem and open-circuit voltage is below about 12.3 to 12.4 at room temperature, you really should re- charge the battery before you continue testing. When open-circuit voltage is 10.4 to 10.5 volts, it’s usually a telltale sign of a shorted cell inside the battery. But above and beyond these two readings, the only meaningful battery voltage measurements are operating voltage. Unfortunately, not everyone realizes this. For example, an installer calls an auto electric shop because an engine with a fresh starter won’t crank or cranks sluggishly. The auto electric specialist instinctively asks the installer for a battery voltage reading. All too often, the installer reports open-circuit voltage instead of operating voltage because he just doesn’t know any bet- ter. Because the auto electric person doesn’t realize the installer measured open-circuit voltage, he assumes the battery is good and embarks on a di- agnostic wild goose chase. This chase often culminates in a perfectly good starter coming back as aneedless war- ranty return. (Forget for the moment that the vehicle probably didn’t needa starter in the first place!) When you disable the ignition and crank the engine for 15 seconds, the battery’s operating voltage must never drop below 9.6 volts at room tempera- ture. Experience shows that a healthy battery usually won’t drop below 10.0 volts during this test. The moral of the story is that open- circuit voltage may look fine. But the moment you load the battery by crank- ing the engine, the battery’s operating voltage plummets. The result is one of four symptoms: Page 1 of 4