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Q: Check gauge light is on and Battery voltage is dropping. The battery is failing to stay fully charged.

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Symptom: Note- Symptom seem to be intermittent Check gage light > Voltage reading red line at about 10v
Had O’Reilly’s test charging system - Battery/Generator test OK - EVR test low EVR (according to maintenance manual defaults to 25%)
Believed BTS was sending PCM into default Replaced BTS since cold seemed to be factor
First start OK ran at 14.5v second start returned to 10v on battery gauge. No sign of parasitic drain. Any ideas?

My car has 352000 miles.
My car has an automatic transmission.

Hi there. First the battery will need to be charged to 100% charge of 12.6 volts. Then load test the battery and see if the battery can hold a charge after a load for 30 seconds. If the battery can hold the charge, then the battery is good. If the battery drops down lower than 12.4 volts after the load test, then the battery needs replaced. If the battery is good, check the battery cables and make sure that they are clean and tight. Even though the alternator was checked for a voltage, check the alternator for an amperage and see if its putting out what it is rated for. If the alternator is working properly, then remove the negative battery cable (-) and put a digital meter on the negative battery cable and the negative battery post (-) and see if there is a voltage reference present with the key off and removed. If there is a voltage of 6 volts or more, then something is on in the vehicle. Operate everything in the car one thing at a time and see if the voltage will drop to near zero. what every makes the voltage drop, then that item is causing the issues. If you need further assistance with diagnosing your electrical system, then seek out a professional, such as one from Your Mechanic, to help you.

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