![]() Interior lights, or a trunk or underhood light that fails to go out can also sap power from the battery when a vehicle sits overnight. Leaving the lights on can drain a battery fairly quickly. This can run the battery down fairly quickly if the vehicle sits for long periods of time, is driven only infrequently or for short trips, or has a weak battery or low charging system output.Ībnormal key-off power drains can also run down a battery. Because of this, the key-off power drain can be fairly high in many late model vehicles (80 milliamps to several hundred milliamps). ![]() Many go into sleep mode and shut down after a certain period of time to reduce the power draw, but others (such as the antitheft system, keyless entry system and PCM keep-alive memory) are always on. The electronic modules in today’s vehicles draw a small amount of power from the battery to keep their memories alive when the vehicle isn’t running. Even so, their average service life is typically five to six years depending on use.Īllowing a vehicle to sit for a long period of time without being driven (say a week or more) can allow the battery to run down. Gel-cell batteries that do not contain liquid acid electrolyte are better in this respect because they are less affected by evaporation. The average service life of a conventional lead-acid car battery is only about four to five years, and typically a year or so less in extremely hot climates. This will reduce the battery’s ability to accept and hold a charge, and drastically shorten the battery’s life. If the battery is allowed to run down or discharge excessively and is not fully recharged within a few days, the lead plates inside the battery can become permanently sulfated. But automotive lead-acid batteries must be maintained at or near full charge for the cells to last. But it is simple, cost-effective and generally provides an adequate power for most automotive applications. Lead-acid battery technology is actually ancient. It might take 20 to 30 minutes or more of normal driving to fully recharge the battery. If the battery is low when a vehicle is first started, it takes some time for the charging system to bring the battery back up to full charge. Police cars are murder on alternators and batteries because they spend so much time idling with high electrical loads on the charging system (lights, radios, heater or A/C, etc.). Consequently, if the engine is left idling for a long period of time with the headlights, A/C, defrosters, radio or other accessories on, it can overtax the charging system and drain the battery. Even so, most alternators can’t achieve maximum output until engine speed reaches about 3,000 RPM or higher. The powertrain control module in most late-model vehicles controls charging output, so that the PCM can boost the charging curve a bit when demands are high at low engine speed. The alternator’s output is lowest at idle, and increases with engine speed. Normally, the battery is only used to crank the engine, to provide power for lights and accessories when the engine is not running and provide supplemental power when the demands of the vehicle’s electrical system exceed the output of the alternator. ![]() The alternator’s job is two-fold: to supply current for the vehicle’s electrical system and to maintain the battery at full charge. Many so-called battery problems are not the battery, but a charging fault. Any of these things may occur if the battery is low or the alternator is not producing its normal charging output. The engine may lack power, misfire or stall. Electronic gauges may give strange or erratic readings. The turn signals may not blink normally when the switch is flipped to either side. For example, low voltage may cause the airbag or ABS warning lights to come on. ![]()
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