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So my battery light on my 2004 explorer is coming on intermittently (block light with a plus and minus symbol). Recently my battery died while I was at work, and after I got a jump I took it to O'Reilly and they tested my starter, battery, and alternator - all of which checked out fine. So, what caused my battery to die? And why is this light coming on? Any ideas would be appreciated. Thanks!

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If the battery symbol lights up that usually means "Check Alternator". You can confirm this by consulting your vehicle's owner's manual.

It is possible for bad alternators to produce too much current. In such situations, most modern cars protect the battery with an overcharge-protection system that stops the current supply from the alternator.

This kind of failure would explain the presence of the "Check Alternator" light and you battery going flat.

I expect that when the alternator was tested the engine was idling, at which point the current being generated was acceptable (current increases with engine load). This would explain why the alternator checked out OK.

One way to confirm an over-charging alternator would be to measure the voltage across the alternator with the engine under load. In my experience, 17-18 V is deemed too high by GM vehicles. You may want to investigate if Ford provides a (usually hidden) diagnostic on-board cluster menu that reads out battery voltage in real time. Even if there is none available, a multimeter could be used to measure the voltage as well.

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Sounds like something is staying on even when the key is out try turning off every thing don't forget the dome light then disconnect the positive side from the battery then touch it to the battery look for a spark if you see one then remove one fuse at a time to find out what is staying on make sure the alarm is off

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