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i have a 2012 altima that will intermittenly drain the battery. i have had it for over a week at a very well know automotive electronic specialist and the battery is good, the altenator is good and when it is checked the power draw is very low. then after 3 days the battery is completely dead. does anyone have any idea what is causing this? thanks
 

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A properly working charging system puts out about 13.2 to 15.0 volts, but this is a general spec, and the factory service manual should be referenced for the correct charging system voltage specifications for a particular vehicle. A battery should have a static charge of 12.2-12.6 volts. If a battery is not good, the charging system may not be able to charge properly. If a vehicle is not charging properly and the battery is good, the first thing to do is to turn the ignition switch to the "ON" position without starting the engine and make sure the charging system warning light is operating. If the bulb is burnt out, the charging system will not charge. If the bulb is OK but still does not illuminate, the circuit must be tested. If the warning lamp does illuminate, then the next thing to check is to make sure the circuit between the battery positive post, or fusible link, to the connection in back of the alternator is good. On Nissans, this will be a thick (approx. 10 gauge) white wire to the "BAT" post on the back of the alternator. It's not uncommon for this wire to get corroded and burn up, creating resistance in the circuit. So, before assuming an alternator is bad, make sure this circuit is good and battery voltage is getting to the alternator. It's also important to make sure the alternator belt is tight and not slipping and the battery connections are clean and tight.

Assuming the charging system voltage is ok and the battery is good, the next thing to do would be to have a parasitic draw test performed; there should not be more than a 50 milliamp draw on the system with the ignition switch in the "OFF" position. If there is a higher draw, you need to do some testing to find out where the draw is coming from. Remove fuses one at a time until the draw goes away or falls into acceptable range.

The later-model cars have adopted what Nissan calls “power generation voltage variable controls.” This system monitors battery current and varies voltage to the regulator, reducing the load on the engine and resulting in better gas mileage. By varying the voltage to the alternator, engine load due to power generation of the alternator is reduced and fuel consumption is decreased. The battery current sensor is installed on the battery cable at the negative terminal. The battery current sensor detects the charging/discharging current of the battery and sends a voltage signal to the ECM according to the current value detected.
 
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