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1994 Seville STS
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Hi, when I first start my 1994 STS the battery volts on the dash displays 14.2. As I drive it for 20 min it lowers to about 13.4 with the radio on. Assuming the alternator is ok is this number the volts being generated from the alternator to charge the battery or is it the volts of the battery? What I’m trying to determine by reading this number is if my battery needs to be replaced. What is confusing to me is if I turn on my lights and wipers the number drops to the high 12s. I guess I’ll disconnect my battery and using my meter check the volts and if it holding a charge Was just wondering the answer to my question above. Thanks, Jim
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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After 2005 or so the charging protocols for batteries and alternators were changed quite a bit so some of what follows does NOT apply to many newer models.

A fully charged "12 volt" lead-acid battery should show 12.6 volts, disconnected from anything for a half hour.

Voltages in the 14.2 - 15.0 range are normal after START, as the voltage regulator jams amps back into the battery.

Any time the engine is running the dash display shows system (not battery) voltage - and that should stay at around 14.0 - 14.6 volts. You can't see it, but amperage output is also varied to carry vehicle electrical loads and to maintain battery charge. Turning on accessories should not appreciably lower observed voltage but will increase amperage output to carry the additional loads.

With the engine off then the dash shows battery voltage, and that should be 12.4 to 12.6 volts.

A running voltage indication of less than 13.8 volts is not good. Something is failing - either the belt is slipping, the alternator (internal voltage regulator) is failing, the electrical wiring and connectors need work, or the battery is failing.
 
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