Greetings Im currently having issues with my battery draining over an extended period of time. My car was running just fine for a little over a month, but all of a sudden one morning my battery was almost dead, so i charged it up with a battery charger and brought to get it looked at by a mechanic and he did a battery and alternator test and told me that it was showing green across the board then about 4 days later when i was about to leave to go hang out with some friends my battery was drained again. I tried doing a alternator test with my multimeter and found that my alternator was showing about a 12.7v while the car and accessories were on, Im not too versed in this area but is there anything Im possibly missing something? am I doing something wrong? Very stuck and kinda in a bad spot. I keep thinking its my alternator but it could be a bad battery too. Just need some direction on where to look. Vehicle is a
2009 Mazda RX8 Sport with about 150,000 with a rebuilt engine.
See Scotty's video on Parasitic Draw.
The only draw i see is the 11v for running the security system for my vehicle. When an alternator is starting to get bad do they sometimes miss the charge or they just dont charge all together? Because it seems that the alternator is inconsistently charging the battery, as of this morning my car is starting like normal.
With a meter, the battery voltage should be about 12.75 volts with the car off. With the car idling, the voltage should be about 14.5 with nothing else on. If not, or if the voltage fluctuates, you need a new alternator.
Got it thanks for your time
found that my alternator was showing about a 12.7v while the car and accessories were on
can you please describe how you measure this?
I put my leads on my multimeter on the battery terminal, then i start my vehicle and i turn everything on to see if I was getting the right amount of voltage while the car was running with accessories on
OK try it again, but this time put the neg lead on the alternator case, and the pos lead on the pos stud on the alternator.
What reading ahould i be looking for
tell us the measured voltage when you do this.
Just check, when the vehicle is idleing 14.1v and when the car is off 12.6v
So with the engine running, you said the voltage measured on the alternator was 14.1v, but 12.7v on the battery. They should be the same. That means you have a bad connection between the alternator and battery.
Now, determine if the fault lies in the positive, or negative (ground) leg of the circuit:
With the engine running, measure voltage:
- between the pos stud on the alternator, and pos stud on the battery
- between alternator case, and the neg stud on battery
It should be less 1.0v . If it's higher, then the fault lies between those points. You can keep probing further like this to narrow down the location further.
Ground leg, things to check:
- alternator assembly: make sure the mountaing bolts are snugged down tight, surfaces are clean and bright, and there's no insulating spacers. The alternator makes its ground connection through the housing, directly to the engine block.
- engine ground strap: There should be a braided strap going from the engine block to the body of the vehicle. Those straps can rot out over time. Make sure it's in good condition, and both ends are clean and properly secured. Replace if necessary.
- Battery: There will be a cable connecting the neg post to the body of the vehicle. Again make sure cable is in good condition, and both ends are clean and secure. If the body where it connects is rusty then rub it with sandpaper until you see bright shiny metal.
Positive leg, things to check:
- alternator cable: There should be one cable connecting directly from the alternator pos stud to the pos post on the battery. Make sure it hasn't been damaged, and both ends are clean and secure. There should also be an inline "fusible link" somewhere along this cable. It may be hidden under the insulation. If you do not measure electrical continuity end-to-end on this cable, then the fusible link has burned out. Replace it.
12.5 volts with engine off and about 14.5 volts at idle, again.