Battery is good, alternator is good. when car is off the battery is drained

A bad alternator no longer charges the battery, the battery will die once it is drained. This will occur more rapidly when using equipment that runs off of the battery( lights, heater blower, windshield wipers etc.) even electronic ignition will run down a battery. Your battery just went over night the alternator may have stopped working long before. Go to an auto parts store that offers free testing to make sure it is your alternator before making repairs.
Good luck.

charge the battery then remove the negative battery cable and place a test light between the cable and the battery remove fuses until the light goes dim also unplug and unbolt the alternator as bad diodes can draw on the system it can charge and still drain the battery down

place a test light between the neg battery cable and the battery see how bright the light is remove fuses til the light dims that is the circuit draining the battery glove box lights and alternator diodes so you may need to unplug the alternator it could charge and still draw down the battery when not running

03 Mustang GT (Parasitic) Battery Drain Fix

Hello,
I am seeing a lot of mustangs are having battery draining issues and I wish to share some simple solutions found while working on a friends 03 GT 4.6ltr.

In a quiet area, key switched off, hood up, Listen carefully close to alternator. I heard a high pitched hissing/whistle sound at low volume, came from back of alternator. This sound was the voltage regulator/rectifier staying on when key is off due to faulty diodes inside the voltage regulator/rectifier circuit, can cause battery to go dead after car not ran in about 2-7 days.

After having the alternator tested free at a parts store, test showed alternator was good because the faulty diodes in the alternator did not show up on the test, the machine did not test for faulty drain back issues that were present.
Alternator on-car-fix:

There was battery voltage running at all times through the two-wire connector on back of alternator. Disconnect the battery first for safety and to reduce chance of short circuit. The yellow wire/with white stripe is hot while car is off so I traced this from the 2-wire connector on the alternator then closer towards driver side front fender. Next I cut the Yellow/White Stripe wire, spliced a good connection wire to it and ran it through a hole in the fire-wall to the fusebox(under dash), connected it to a 15a fuse that does not get used but has power when key is turned on but no power when key is off.

The purpose for re-routing the hot wire to the alternator, is to turn off the alternator charging circuit when the key is turned off so it may not run down the battery. I am seeing good results with this fix(or patch) so far. No More Parasitic Battery Drain!

Another fix can be to change the small pulley on the alternator to a larger pulley so the alternator does not spin too fast, putting too much energy through the diodes can cause a charging system to act a fool. So if you have charging issues and have an alternator changed, putting a larger pulley on can make sense turn into saved dollars(especially on high revving engines).

I hope this info helps to solve some draining battery and/or alternator issues.

for reading,

TyBorg1

Are you sure that you have a problem with something draining the battery, or do you have a problem with the alternator not charging the battery? I would have the alternator tested to make sure of that before spending hours and hours troubleshooting electrical problems.

Basically, to check a problem like this you will need a clamp on amp meter. Then you pull fuses from your fuse panel and see where the drain is. Make sure you have a good layout of your fuse panel and fuse sizes (a digital photo is also a good idea) before starting, so you don't introduce problems.

MAKE SURE BATTERY CABLES IS CLEAN FREE FROM CORROSSION. MAKE SURE BATTERY CABLES GOOD TIGHT.IF ALL IS OKAY. WELL I HAVE SEEN A GOOD BATTERY CAN ALSO BE BAD.I KNOWN PERSON BOUGHT NEW BATTERY DIDNT LAST A WEEK.IT HAD A FACTORY DEFECT ALTERNATOR WAS GOOD.DO A QUICK CHECK TO SEE IF ALTERNATOR CHARGING BATTERY.CRANK VECHICLE UP CHECK VOLTAGE OUT PUT AT BATTERY.IT SHOULD BE 13.5 - 14.5 VOLTS.IF NOT ALTERNATOR NOT WORKING CHECK ALTERNATOR FUSE AND WIRES.MAKE SURE THEY ARE INTACT.IF ALTERNATOR GOOD.HAVE THE BATTERY LOAD TEST TO SEE IF YOU HAVE A BAD CELL IN IT.IF BATTERY IS GOOD.BATTERY DRAIN CAN BE CAUSED BY RADIO STAYING ON.THE INTERIOR LAMP STAYS ON.FAULTY PCM SHORTING OUT.FAULTY DOOR LOCK SWITCH.FAULTY BLOWER MOTOR RELAY.FAULTY ALTERNATOR DIODE.FAULTY STARTER SOLENOID SHORTING CONTACTS WILL DRAIN BATERY.

Check the voltage regulator if there's coming out of it.
you may have a good alternator but if nothing goes at
the alternator, then, it results to a drained battery. Just
replace your regulator, if this is not working.

DID U CHECK THE STARTER TO SEE IF IT IS GOOD!!!MAKE SURE THE WIRE IS CONNECTED REAL GOOD TO THE STARTER TOO!!! B/C IT WILL DRAIN YOUR BATTERY!!! THE SAME WITH THE ALTERNATOR IT TOO WILL DRAIN YOUR BATTERY!!

have the alternator checked it could test good and still be bad but it can also drain the battery 9 times out of 10 its the alternator thats draining the battery