2010 Lexus RX 350 Battery dies after 3-4 days

2010 LEXUS RX 350
41,000 MILES • 0.6L • 6 CYL • 2WD • AUTOMATIC
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DCSPENCER
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I have changed the battery with still having problems. I used an amp meter to test for drain had .25 my
I used my meter and tested voltage it showed 12.5 volts then I started the engine and it went to 13.78 volts turned on lights and air conditioner and voltage held. Which at fist looked pretty good I left it running and I noticed the voltage started dropping after about 10 minutes after about 15 minutes the voltage was down to 12.7 volts would this be the alternator or voltage regulator. And the belt look good.
Jun 29, 2015 at 8:00 PM
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CARADIODOC
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Good observation. Battery voltage must be between 13.75 and 14.75 volts with the engine running. 13.78 is not impressive but technically it's okay.

First check the battery voltage with the engine off. It will be 12.6 volts if it's good and fully-charged. If it's closer to 12.2 volts, it's good but discharged. If you find it's around 11 volts or less, it has a shorted cell and must be replaced.

The generator will usually not cause a slow drop in charging voltage. That is due to a failing voltage regulator or a wiring problem in the circuit where it senses system voltage. It can be caused too by a failed diode inside the generator. That will limit the current output to exactly one third of its maximum rating, and that is often not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.

The voltage tests are just the first part of testing the charging system. You need a professional load tester to measure full-load output current and "ripple" voltage. If you find the maximum current is one third of what it should be, replace the generator.
Jun 29, 2015 at 8:53 PM