By Jim Kerr
New cars perform vastly better than vehicles built only a couple decades ago. A major factor in this has been the advancement of electronics and their incorporation into almost every automobile system. One drawback to this advancement though, has been the extra load placed on the automobile electrical system and battery. Recently, a friend was enjoying a few quality days at the lake. The car hadn’t moved for several days, but when they went to start it, the battery was dead. It needed a boost.
The car doors had been closed, the stereo off, and the lights were off so they were not the culprit. My friend asked me if a 12-volt trickle charger would be helpful to prevent the electronics from draining the battery again when the vehicle is in storage or stopped for days at a time. He was correct in thinking a trickle charger would prevent the battery from going dead, but this should only be necessary for those vehicles that sit for long periods (months) of time.
Modern electronics can cause the battery to go dead faster than on vehicles from the 1970’s and older. Each electronic device on the vehicle uses a “little” electricity even though everything may be turned off. On-board computers are the biggest users of the electricity. Fuel injection computers, climate control computers, keyless entry modules, lighting computers, digital radios, clocks, memory seats, and trip computers all have memories in them, with many of them programmed by the driver. The drain on the battery to keep these computers operational can kill the battery over time.
Electronics have also helped us reduce this electrical drain compared to vehicles in the 1980’s and 90’s. Now, computers “go to sleep”, an operational mode where they are using almost no electrical power. They monitor input signals and data, and if there is a signal that requires them to wake up, they do. It’s just like a sleeping person listening for the alarm clock. Otherwise, the computers just stay in low power mode. A computer that stays “awake” all the time however, can drain a battery.
Other electronic devices also put a drain on the battery. Alternators, voltage regulators, and ignition systems use electricity even when not operating. Diodes, one-way electrical gates located inside these devices, are supposed to prevent the flow of electricity through these parts when the systems are turned off but diodes can fail and cause a larger flow of electricity. Fortunately, this is rare.
Testing current flow from the battery has changed along with the automotive electronics. It used to be sufficient to place a test light between the battery post and the disconnected battery cable. If the test light came on, there was a battery drain on that vehicle. That test no longer is valid for smaller computer drains. The only correct method of testing for a battery drain is to use an ammeter connected between the battery post and the disconnected cable.
The ammeter measures the actual current flowing from the battery into the electronics systems. A typical engine computer will use between five and eight milliamps (thousandths of an amp) to keep the memories working. Most vehicles should show a maximum reading of 25 to 28 milliamps or less when the test is performed, however a few luxury cars loaded with all the toys may be a little higher. In comparison, a trunk light uses about 900 milliamps or .9 amps and would drain the battery overnight.
If the measured current flow is too high, then fuses are disconnected one at a time until the faulty circuit or module is identified. Some vehicles will require a waiting period of up to 1/2 hour before an accurate measurement can be taken. During this time the computers are “awake” and use more power. After sitting for several minutes without any switches being operated the computers go back into “sleep mode”. A note of caution – opening a door (which operated the interior lights) or turning on the ignition switch or headlights will damage many ammeters. The testing must be done carefully to prevent high current flows from passing through the meter.
Finally, other factors can cause a battery to go dead faster than normal. Warm temperatures cause the battery chemical reactions to occur faster. This causes the battery to go dead quicker. Dirt or dust on top of the battery and high humidity levels also cause a problem. The electricity leaks through the damp dirt directly from one battery post to the other causing the battery to discharge. Keeping a battery clean, cool and dry will ensure it retains its charge longer.