Myserious battery drain - MustangForums.com


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Old 09-11-2017, 10:21 PM   #1
vargas
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Default Myserious battery drain

Gents and gals,

So its been years since i been here havent messed around with the Mustang much until now....

So alternator, battery, parasitic draw, load test, alternator diode test,.......all test check out fine. I CAN NOT find what causes my battery to drain .2V ever ~24hours 😠

If i drive my car everyday the car is fine but if it is sitting over 24+hours the battery will drain to12.4V then the following day maybe 12.2V.

I have an optima red top and a ultima (oreileys) alternator.

I have read other forums of other makes and models but nothing helps. Im wondering if my mustang people have had this issue before.
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Old 09-12-2017, 12:11 PM   #2
Z28KLR
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Disconnect the battery negative cable for 24-48 hours, see if voltage holds steady. Since you mentioned already testing/observing parasitic draw and found nothing there, I would isolate the battery and see what happens.
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Old 09-12-2017, 02:34 PM   #3
imp
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Quote:
Originally Posted by vargas View Post
Gents and gals,

So alternator, battery, parasitic draw, load test, alternator diode test,.......all test check out fine. I CAN NOT find what causes my battery to drain .2V ever ~24hours 😠

If i drive my car everyday the car is fine but if it is sitting over 24+hours the battery will drain to12.4V then the following day maybe 12.2V. ....

First, a battery can have one cell discharging itself through faulty separator plate(s). This can be detected if one has the time to charge fully, disconnected from any load, check specific gravity of each cell immediately, then the next day. If any one cell registers lower than the rest, throw away the battery, regardless of any "test machine" results. Sp. Gr. is easily checked using an inexpensive battery hydrometer.


Second, battery can discharge slowly through the alternator itself. This can be determined by disconnecting the alternator and observing the battery itself.


Third, parasitic discharge can be via "leaky" capacitors in control modules, this being common in newer vehicles having air bags, etc.


Fourth, BEST way, measure parasitic current flow at the battery itself using an ammeter. If it is within some reasonable range, say, 5 amps, or less, remove fuses from their fuseholders one by one, watching the ammeter each time to find the one carrying the parasitic current. Consult diagrams to determine which equipment that fuse feeds. imp
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