Help: Alternator vs: Battery

Sorry for the long winded post, but I'm not the greatest with electrical and could use some help diagnosing this.

The car has been getting harder to start over the past week or so, cranking a bit slower, like the battery was low….but I just attributed it to the cold weather. So yesterday, I fired it up and let it warm up for about 5 minutes. As I started down the street, it just died….kind of sputtered out like it was running out of gas. Turned it over and it fired right back up, but as soon as I touched the gas, it just died. Tried one more time and it started again, but would only idle for a few seconds and die. I did notice that every time I cranked it over the battery was getting quite low and the run time would get shorter the lower the battery got. So I pushed it back home..(how embarassing)..and tried to jump start….but that didn't work either and now the car wouldn't start at all. I checked the battery, and it was quite low so I put it on a charger overnight. This morning I hooked the cables back up and it started right up and ran great, no problems at all, like nothing happened.

The battery is about 4 years old. Could it be the battery is not taking a charge from the alternator? If that's the case, then shouldn't it only be hard to start, but still run after it's started?

Or is the alternator not doing it's job by keeping the motor running and charging the battery? I'm leaning more towards this as the problems as it seems that I have been running on the battery...and as the battery died, so did the car.

Thoughts anyone?

I just spent (and wasted) a LOT of time diagnosing electrical problems on my roommate's 95 Mercury Mystique (Mistake), and by not following the proper steps, myself and the other person working on it wasted a lot of time trying to solve a simple problem.

The first thing you need to do is get or borrow a voltmeter. You need to check the voltage at the battery and at the alternator both with the car off and with the car running (4 voltage checks). Let us know what they are, and we can tell you where to go next. When you start making guesses without checking voltages, you might as well throw darts at the car and start replacing parts.

Thanks Dan. I'll see what I can do this evening. The measurements at the battery should be simple enough. But with the V8, I don't think I can get anywhere near the alternator........that thing looks like it's tucked in there pretty tight....

Happy, Happy, Joy, Joy!

fuel pump/pickup or fuel filters could cause the same sort of stalling too. Battery sounds like its pretty beat up though.

about a month ago. It's a charging problem as it started and ran great this morning after charging the battery all night.

Have you ever changed the alternator on yours? How difficult is it?

I'd be leaning towards the problem being your alt crapped out on ya. Had the same problem with same symptoms in my mustang. Drive a few feet and car died. Turned 1 or 2 times then that was it. Car wouldn't charge the battery but battery could be charged via other means, jump, charger, etc.
If you get the chance I'd run to a local parts shop like pep boys or something and have em check your battery. Mine kept dying in my 535 and I took mine in. Came to find out my battery had a cell gone bad. I had intermittent problems with starting mine. Usually ran fine but sometimes didn't want to start. Others it would be fine. Anyway yours seems old enough to have it check/replaced. I paid 60 for a new one and was nice to have the peace of mind.

I guess what I'm saying is it sounds like your alt but your battery is also old enough that it might also be adding to the problem.

Just my 2c. I only go by experience and have no further knowledge of cars other than my own and my interest.

sometimes the brushes in the alt will get worn to the point that they will not allow it to charge all of the time (by loosing contact with the armature) so when you check it's output voltage it looks fine but craps out later, cold also plays hell with older batterys, did battle with a 97 528i a month or two back after the 1st cold snap, fully charged the battery would put out like a mother on a load test, and the owner worked at a dodge dealer put the battery through their several thousand dollar battery tester a couple times and it passed, but we left the door open for about 15 minutes with the interior lights on and it was dead as a hammer, so through all of his and our testing it still wound up being a bad battery (which was me and the other mechanic at our shop's hunch from the start)

do the volt meter test and that should tell you the whole story. Should be a little above 12v when not running and upwards of 13.5 when its running.

voltage at he battery running vs not running should be enough shouldnt it????I cant see why the voltage at the altenator will be any different than at the battery.If the altenator is the problem the voltage at the battry when running will be app 12v whereas it should be app 14v

correct Paul.

The alternator and the battery. The bearing in the alternator was quite noisy and the charge going back to the battery was inconsistent. Also the battery was almost dry.....

Damn pricey alternators.....$225 + 50 core at BMA.......$65 for the battery at AutoZone.......much better......runs smoother, idles smoother, lights brighter, starts easier.....it was a good investment. But man, the alternators on the V8's are a pain in the ass......

you can read a significant voltage drop in the connections from the alternator to the battery (not good).

To see the voltage rise when engine start running, effectively tells alternator is sending power, seeing same low voltage only tells check further for bad alternator or open/bad connections.

Depending the point you are taking the reading, you may have battery voltage (pin 14 on data link/diagnose connector) or alternator output voltage (Jump start post, if alternator output post can not be reached). You should get a "Minimum" voltage drop between both points (lower at the battery) if alternator is sending power (turn on lights and A/C to make it more dramatic). This is a good confirmation that current is flowing from the alternator.

Javier