Battery drain while running?

[Matthew]

Got a 1996 Jeep Grand Cherokee Laredo 4.0L.

Typical 96 Jeep electrical oddities… but this one, while I’m pretty sure its a bad/dying alternator, I’m honestly not totally sure.

Brand new battery, cold engine, cranks over and starts just fine.

Multimeter reads 14v for a few seconds, then it drops and idles around 13.8v.

Load up the alternator with everything except the radio (removed, as it was dead, all wires capped properly).

Voltage drops to about 13.6v and while fluctuating +/- .08v eventually “drains” down to around 13v. This year doesn’t have an electric cooling fan (on my ’01 Jeep that sucks down .4v to .6v at idle), and as it’s a brand new battery I haven’t let it run to the point where it goes below 13v.

The timeframe we’re talking about is only 15 minutes.

I ‘think’ it’s the alternator, but what I’m wondering is if anyone’s ever seen this kind of behavior, and whether you have ever actively tested for it? Not a lot out there about this type of issue specifically, or maybe my Google-Fu has just failed me.

Topic

[Bill]

I think you’re on the right track with a failing alternator. I would have it professionally tested to be sure.

[dan]

i agree with Wysetech, 13.5-14.5V is rule of thumb for alternator output, if your voltage drops that low under load it sounds like too me you are having a alternator issue. but some further diagnosis wouldn’t hurt anything…

if all ells shows the alternator is dying then of coarse replace 🙂

[Austin]

Agree with the sound of a bad alternator, but never neglect resistance. Doesn’t take long, disconnect your alt. power, and battery power. Throw an ohm meter on it and see what you got.

[george gonzalez]

The only way to tell for sure is to put an amp meter between the battery and the car. Not in the thick cable going to the starter, the thinner one that powers everything else. The newer clamp-on ones make this a lot easier as you don’t have to undo anything.

If the alternator and voltage regulator are working properly, the ammeter should show anything from zero to an amp or three of charge, with the engine idling and you turning the headlights on and off. If you get more than an amp or two change when you turn the lights on, then the alternator or regulator isn’t doing its job.

[dan]

but also check grounds and electrical connections too, forgot too mention that.

[Matthew]

First of all, thanks for the quick replies, everyone.

During the course of troubleshooting this car I did a parasitic drain test, but after the ECU goes to sleep (30 sec or so), drain is only 0.029A or less.

Volt drop on neg terminal to alt casing was about 0.08v.
Volt drop on pos terminal to alt output was about 0.2v.

I’m assuming that if I ohm out the wiring, perfect world scenario would be 0 ohm between any of the major connectors. At what point in ohms would you replace a cable?

Cut that in half and its what a 96 Jeep would probably tolerate. From past research it is apparent that jeeps in general are notoriously picky about parts and electrical, and mine has been no exception.

Thanks again
-Matthew

[EricTheCarGuy]

Sounds like just cleaning the battery terminals would help. It would bring the voltage drop down. More information on testing for problems like this can be found here.

http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems

That said, as long as you’re above 12v at idle with all the accessories off, it’s considered to be charging. It’s when it falls below 12v that it’s an issue. Based on your description I don’t think you have a problem.

[Matthew]

Thanks Eric,
I haven’t found the nerve to let it run down that far, but even while holding the rpms higher than idle, I still observed drain in the battery.

Seems to me that sitting in stop and go traffic on a cold/hot rainy day for half an hour would conceivably drop it that far… So I’m reasonably apprehensive of that diagnosis.

I wouldn’t expect to observe a steady drain of any kind. intermittent, perhaps, such as when you have an electric cooling fan, but not continuous. Shouldn’t it level out at some point and hold more or less in the same voltage?

note: when I first installed the new battery, after idling the car for 20 min or so (as battery had been disconnected for ~2 years) the standing voltage was about 12.8v, after letting it sit for a day and then doing a load test on the alt, it was at 12.6v, next day load test for another 20 min, standing voltage is at ~12.3v.

[EricTheCarGuy]

You’re not seeing a drain. This is the normal operation of the charging system. By nature, it will fluctuate. If it falls below 12.0v, that’s a problem. As long as it’s above 12.0v, it’s charging. It is a 12v system after all. The evidence you’ve presented does not indicate a charging system problem in my opinion.

In your note you’re talking about something completely different. That sounds like a battery issue. As in the article I posted for you, the electrical system starts and ends with the battery. If you don’t have a good battery your charging system won’t be able to keep up. Make sure your battery and it’s connections are good and then go from there.

[Accord]

eric would a bad or damaged ground have the same effect as a dead battery on this vehicle ?

[EricTheCarGuy]

Forget that for now. Load test the battery and see if it’s good. If it’s not, NOTHING else will work properly.

[Matthew]

With a brand new battery (not to say that a brand new battery can’t be bad, I’ve seen much to prove that possibility out on the interwebs) while having all accessories turned on with the engine running, I’m seeing a progressive decrease in the battery voltage that is trending towards the key-off voltage.

Not wanting to wreck a potentially “good” new battery, I haven’t allowed the car to run long enough to see if the battery voltage indeed hits the 12V mark.

So, if my key-off voltage is 12.5v, and the load test hits 13v, we’re right at that 0.5v threshold between a ‘properly charging’ system and one that is suspect, correct?

I’ll be going over the wiring with my multimeter and a gimlet eye this weekend this week/coming weekend. Thanks for your help.

PS: I wasn’t trying to be argumentative. I’ve been through hell with this Jeep, and I really miss the days when it “just worked” I could trust it. Random stalling at idle, WOT, or anywhere in between, “two screw” miracle PCM fix, just to name a couple of issues were why it’s been “on ice” for the last two years, while I had neither the time, money, or impetus to try to get it back on the road. This thing is about as cursed as the HT4100 was in my ’84 Seville!

[Author]

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