So a few days ago my battery suddenly went dead. It's only 6 months old. Checked under the hood,looks like nothing is wrong. Jumpstart the car, started driving and the EPS light came on a few minutes later, followed by the dash lights turning off. It seemed the alternator wasnt working. Here it is almost a week later, and so far I've bought a used alternator, a new battery, had my original alternator completely rebuilt, and still the battery is losing voltage without recharging. Im lost. All the fuses look good. Everything is hooked up correctly...idk. Any thoughts?!

 

Do you have any accessories? You could have this if you had a large electrical drain. Maybe you have a short somewhere.

You might want to hook up a volt meter and see what's happening. That might give you an idea as to whether you have an erratic or continuous problem.

A long time ago I had a Dodge truck that killed two batteries and an alternator. It was under warranty and the Dealership finally replaced the starter to fix the problem. I'm not sure what was wrong with the starter.

 

Have you had the battery tested? Maybe something internal like a shorted or open plate. Any parts store or shop can test it out.

Utah

 

Any recently installed electronic components? Aftermarket stereo?

 

Nope, nothing new installed. I've put a new battery in already, so it's not that. It kind of just happened all of a sudden.

 

Here's a couple of universal alternator tests (work on every vehicle):

1. Headlight test/jump start: jump the car enough to get it started. Turn on the headlights. Once running, disconnect the jumper cables. If the headlights dim when the jumpers are disconnected, then the alternator is likely not working. (Quick and dirty)

2. (A bit more electrical theory): every alternator needs three things to work: 1) a connection to the battery positive. This can be easily verified with a multimeter. 2) a connection to ground (battery negative. This is provided by a cable or my the casing of the alternator which is grounded through the engine. Also check with a meter. 3) a field. The rotor of the alternator must be energized (it's an electromagnet, always, on cars, don't worry about PMG's) to produce a magnetic field, and create voltage in the stator (what the battery positive is connected to). Find the wire that provides field voltage. This is easy because it will be smaller than the others. It should have battery voltage with the ignition turned on. If there is no voltage, the problem is in the alternator relay or another component- NOT necessarily the alternator itself. To verify alternator function, it's entirely possible (on an internally regulated alternator) to jump this wire to battery positive and confirm a 14v output to the battery. I would not recommend leaving the harness attached as well while doing this in case ECU's are involved.

3. General thoughts: in ten years of being a mechanic, I have seen entirely too many alternators, batteries, and starters replaced unnecessarily. It's usually not them that is the problem. Always use a multimeter to check resistance (or voltage drop, whichever way you prefer), always physically inspect and remove cables for corrosion, always verify voltage is present when it should be.

4. There is no such thing as an alternator "putting out" 12-12.5 volts. That's the battery. (there's also a terminology problem with saying the alternator is "putting out" b/c it implies it is pushing current, but that's a bit nit-picky). Voltage at the battery should immediately rise when the alternator is turning (engine running). If it does not, that alternator is not working (either because it is faulty or the wiring is faulty).

5. If you suspect a parasitic voltage loss (one that exists when the ignition is off) you can measure it with a multimeter by interrupting either the negative or positive battery terminals with a multimeter set to DC amperage. It is exceedingly rare in the absence of improperly installed aftermarket components.

6. You would be amazed how often professional mechanics do not understand this. There's a tendency to assume it's the expensive, complicated component, rather than the ten cent wire termination or the five dollar relay. It makes me sad for the profession. Same thing goes for ECU fault/sensor fault versus wiring fault. Everyone seems to want the end-component to be the cause of the problem, I suspect, because it's easier to understand.

7. Sorry for the rant. Sincerely. Just a pet peeve of mine.

 

No apologies necessary. I know who I'm going to PM next time my battery goes to s#!t. Haha.

 

Lots of good advice from georgealready, just not sure about #4. Seems like he is saying the alternator does not "push current", but of course it does, since that is how it charges the battery and powers the car's electrical system. Maybe I read it wrong?

The S2000 alternator has an internal voltage regulator so the field winding is not available to test. Also, the voltage regulator is controlled by the ELD unit (electrical load detector) that is inside the main under-hood fuse box. It monitors the total current for the car's electrical system, with the exception of a few items like stability control, brake lights, and hazards. The ELD is something I have never messed with so I don't know much about it and how it controls the voltage regulator and the alternator. Sorry.

I agree with slalom44's idea of connecting a voltmeter to the system because that will give you an idea of what is going on. You can connect it to the accessory power socket, that will be close enough to battery voltage for this kind of testing and it will be easy to read when driving. Like george says, as soon as you start the car the battery voltage should rise, up from about 12.5 to around 14v, and stay there for most of the time the car is running. It will occasionally drop lower once the battery is recharged and the ELD cuts back the alternator voltage, but probably 90% of the time it will be around 14v.

 

Just for the hell of it check for ampere draw while everything is off as stated in georgealready number five.

just had that problem with my gmc. Alarm system was the problem it was around .8 amps all the time and after a few days battery would not turn motor. I still have around .2 amps when everything is off but not enough to drain it all the way to a no start. Now I can let the gmc sit for weeks and it will start right up no problem.

Whats normal drain?? no more than .3 amps or so?? I guess with radio memory and other devices there is going to be some ampere reading.

Note do not start car or try to do this with car running. key and all lights need to be off put car key in your pocket. All current "amps" will pass thru meter from open circuit you create by removing one of the terminals then using meter as bridge to reconect flow path. most dmm's have 2 amp fuse. If you have anything on like headlights you will blow the fuse. read up if you are unsure on how to do this!!