![]() The normal key-off current drain on a battery may be as high as 300 to 400 milliamps right after the engine is shut off. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. Leaking or shorted diodes can often cause a visible fluctuation in the output voltage of the charging system. If you get a voltage reading, it means one or more diodes are leaking and the alternator needs to be replaced. If all of the diodes are doing their job, there should be no AC voltage reading at the battery. Switch your DVOM to AC and check the voltage again. Here’s another check few technicians do, but it’s one that can easily detect bad diodes in an alternator. Then, take your reading with the engine idling. If the DVOM is not auto ranging, set the scale to 20 volts and select DC. You can check the charging voltage by touching the test leads of a DVOM to the positive and negative battery terminals. At idle, most charging systems produce 13.8 to 15.3 volts with no lights or accessories on. The actual output voltage produced by the alternator will vary depending on temperature and load, and will usually be about 1.5 to 2 volts higher than battery voltage. Using a voltmeter to check for voltage drops across connections can reveal bad connections that may reduce the charging voltage. In cases where the powertrain control module (PCM) controls voltage regulation, a problem in the voltage regulation circuit means the PCM will have to be replaced. Since most late-model import alternators are internally regulated, a failure of the regulator means the alternator must also be replaced. Charging problems can be caused by electrical faults in the alternator or voltage regulator, poor wiring connections at the battery or alternator, or a slipping or broken drive belt. A low battery also may not have enough reserve power to crank the engine, so the vehicle can be stranded until the charging problem can be diagnosed and repaired. Once battery voltage drops below a certain threshold, the onboard electronics, ignition and fuel systems may stop working normally or cause the engine to stall. If the charging system fails for any reason or does not produce enough power to meet all of the vehicle’s electrical demands, the battery quickly runs down. These parts can be ruined if the alternator is mounted low on the engine and is subjected to road splash that contains salt water or debris. Even so, the shaft bushings and brushes are wear items that don’t last forever. Most alternators fail electronically long before they wear out mechanically. The circuit board measures only 0.5” x 1.5”.Normal charging voltage can vary from 13.5 to 14.5 volts or even higher depending on the battery’s state of charge, temperature and electrical load on the alternator. Note that surprises abound when constructing prototypes. The layout has not yet been carefully checked or built. These components are about as small as I like to work with. I did a SMT layout using 0805 size components, 1N753 zener and SOIC-14 IC. Related Products: Batteries | Batteries | Battery Contacts, Clips and Holders For energy conservation, connect to battery via a pushbutton (Push to Test). The overall current drain as shown is about 25mA which tends to be wasteful for continuous operation. This current can be adjusted simply by varying the series resistors (R9 through R13). The LEDs are biased to operate at about 4mA which is quite bright if modern LEDs are used. D7 protects against reverse battery connection. The comparator sections have open collector outputs that simply function as switches to operate the LEDs. ![]() ![]() U1 is an LM339 quad comparator that compares the various voltages from the two dividers. R7 & 8 form a voltage divider to that reduces the battery voltage by a factor of 3. Tied to this is a string of divider resistors (R2-6) that set the various fixed voltage levels. Of course, you may select your own colors if desired.ġ2 Volts Battery Level Indicator Circuit Schematicĭ1 is the voltage reference zener. This battery level indicator offers (5) LEDs that light up progressively as the voltage increases: ![]()
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