Parasitic Drain vs Alternator - JeepForum.com
 
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post #1 of 13 Old 09-20-2019, 12:57 PM Thread Starter
Pipedrummer1
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Parasitic Drain vs Alternator

Hi, I was a member a number of years ago, and had to reregister. 04 TJ I6.... Hoping you can help me figure out my electrical issue...

Over the past year or so my battery will be intermittently dead. The Jeep will start fine and then be dead a few days later. I'll charge the battery, and maybe it will be fine two weeks later (I don't get to drive it much anymore). But then again will die out of nowhere after sitting. I last drove it a week ago, and went to start it yesterday and it was dead. The battery showed 4v on my multimeter.

I had some time, and wanted to finally figure this out. I got a new battery, since it was almost 3 years old and had been charged a number of times. Also 4v led me to believe it just flat out failed.

I installed the new battery, and did the parasitic draw test with a MM in series with the negative side. I go 0.0013 milliamps (yes milliamps). This seems extremely low, but leads me to believe I have nothing drawing excess power. I assume that's what the computer draws??

I then turned to the alternator. I was getting 14.15v at idle. With a load (lights,radio and ac blasting) 13.75 at idle, 14.05 opening the throttle from the engine compartment (maybe 2k RPM).

I then tried a diode test, and got 15-20mv AC current at the battery at idle. This is where I'm confused. Some things I'm seeing online say that no AC current should be present, some say anything under 25-30 is ok.

I'm leaning towards the alternator being the issue but I'm not 100% on it. Does anyone know anything else can test/do? Is it the alternator? I just want to nail this issue so I'm more confident in being able to drive it and not worry about a dead battery. Thanks!

Dan

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post #2 of 13 Old 09-20-2019, 02:11 PM
prerunner1982
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If you aren't driving it often and it's draining I wouldn't suspect the alternator, sounds like it's charging fine.
Have you let the vehicle sit with the new battery in it to see if you get the same results?

Jon
1993 XJ Sport 2 door
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post #3 of 13 Old 09-20-2019, 04:16 PM Thread Starter
Pipedrummer1
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It was fine from yesterday to today. Started right up this morning. Was at 12.48v. I'll check it again tomorrow. Just not sure why the battery would have been draining if there's only such a small draw from the computer. Is it possible there WAS more of a drain, and it fixed itself hah??

Also forgot to mention that when it was dead two days ago and at 4v, the tach and Mph needles were down below 0. Had never seen that before when it died in the past. But then again it was never 4v, more like in the 10-11 range...
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post #4 of 13 Old 09-21-2019, 12:04 PM
V8GCZJ
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Alternators have diodes in them to convert AC power to DC. They can fail and drain your battery but it will always happen and show more draw than you posted. If it still happens with the new battery intermittently i would suspect something is staying energized(ignition switch or some other on board device).
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post #5 of 13 Old 09-21-2019, 01:06 PM
DougRz
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Lead acid rule of thumb is 5% self discharge per month. Higher on an older battery and starting from a lower energy storage point as well. Could simply be that.

0.0013 mA can't be right? That's 1.3 microamps. I'd be surprised if your meter read uA. Probably you mean 1.3 milliamps (mA), which is 0.0013 A.

I've tested my car's parasitic draw at 10 mA iirc, when I was doing a power window project. These are small draws from the large capacity of a lead acid. A TJ battery is ~50 amp-hours. It would take 100 hours to draw 1 Ah at a 10mA rate. There are 720 hours in a month. So a monthly self discharge rate of 5% and a draw of around 15%, in my case. Even less parasitic with your 1.3 mA case. A quick Google is showing 25 mA a rule of thumb limit for parasitic. So yes, your 1.3 microamp theoretical would be extremely low, and even 1.3 mA low.

If your alternator was not charging correctly, you'd only get a handful of starts and 1 or 2 hours of engine run time, is my guess.

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post #6 of 13 Old 09-22-2019, 03:50 PM
Jerry Bransford
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Quote:
Originally Posted by Pipedrummer1 View Post
I then tried a diode test, and got 15-20mv AC current at the battery at idle. This is where I'm confused. Some things I'm seeing online say that no AC current should be present, some say anything under 25-30 is ok.
Alternators produce a/c which the diodes convert to DC but a small amount of a/c component is normal since the conversion process is not perfect and it isn't well filtered like it would be out of a DC power supply. Only a generator produces pure DC.

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post #7 of 13 Old 09-23-2019, 07:43 AM Thread Starter
Pipedrummer1
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Thanks for the responses..... The battery has tested at 12.41v each of the last 3 days. I haven't started it at all. As far as the measured parasitic drain, I was fairly sure of the number, but my meter is auto ranging, so I suppose I could have messed up the scale. Regardless, if it was a 1.3ma draw, and the fact that's it's been 3 days without a loss of voltage is suggesting to me that there isn't a parasitic draw.

It also sounds like the amount of AC from the alternator isn't more than expected, and it was giving appropriate voltage to keep the battery charged while running. I'm not sure what was giving me dead batteries then? Is it possible for a circuit to stay energized only sometimes? A sticking relay maybe?

Right now everything seems fine. But this is what had been happening. I would let it sit for a week, and then it would be dead. Or I'd drive it one day, and go to start it the next and it would be dead. I just don't get it....
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post #8 of 13 Old 09-23-2019, 03:45 PM
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Make sure your key cannot be removed from the lock cylinder unless the ignition is switched to OFF. This is a somewhat common issue that will allow you to accidentally leave the ignition in a position other than off, thereby creating an intermittent situation where the battery draw stays higher than it should with the key removed. If this is the case here, you can either replace the worn key/cylinder or make a mental note to exercise extra care to ensure the ignition is properly switched to the off position before removing the key.

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post #9 of 13 Old 10-13-2019, 04:57 PM Thread Starter
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So it's been a little over 2 weeks and my voltage went from 12.40 to 12.25. this is without ever starting the Jeep or putting any load on the battery (under hood light disconnected, never opened doors, etc). Is that type of drop normal given the time period? It started up fine and I ran it for a while at idle and up to 2k rpm for a bit. Will check the resting voltage again in the morning. Thanks
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post #10 of 13 Old 10-13-2019, 05:37 PM
jtec
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I think dougrz explained the Ma draw confusion away.. leaves us to look else where,

My first suspect Any added electrical - audio, lighting, modified electrics, trailer harness, are a place to start looking.

Leave NOTHING plugged into DLC.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #11 of 13 Old 10-13-2019, 07:57 PM Thread Starter
Pipedrummer1
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Everything I've added to the Jeep has been unplugged from the battery/power distribution center prior to looking into all of this. So if there is a drain, I guess it would have to be a stock component.

I need to get it inspected before month's end, so I'm not going to disconnect the battery until I get that done. Need to drive it so it will pass. But after that I'm going to check power draw again I guess.

I'll check the voltage tomorrow and try to trend it over the next week or so, as I drive it so the computer is ready for inspection
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post #12 of 13 Old 10-14-2019, 09:37 AM
prerunner1982
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Quote:
Originally Posted by Pipedrummer1 View Post
So it's been a little over 2 weeks and my voltage went from 12.40 to 12.25. this is without ever starting the Jeep or putting any load on the battery (under hood light disconnected, never opened doors, etc). Is that type of drop normal given the time period? It started up fine and I ran it for a while at idle and up to 2k rpm for a bit. Will check the resting voltage again in the morning. Thanks
Radio and computers maintain memory so they draw all the time. I wouldn't be all that worried with your results to be honest. If it was draining down overnight or over a couple days then perhaps but after 2 weeks that doesn't seem bad. If the vehicle sits you could always get a small solar panel to keep the battery topped off.

Jon
1993 XJ Sport 2 door
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post #13 of 13 Old 10-14-2019, 01:45 PM
Airstreamer67
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Quote:
Originally Posted by Pipedrummer1 View Post
So it's been a little over 2 weeks and my voltage went from 12.40 to 12.25. this is without ever starting the Jeep or putting any load on the battery (under hood light disconnected, never opened doors, etc). Is that type of drop normal given the time period? It started up fine and I ran it for a while at idle and up to 2k rpm for a bit. Will check the resting voltage again in the morning. Thanks
The voltage drop seems normal to me.

I recently started following the voltage drop on a new battery in my car to see how often I would need to charge it when the car is not driven regularly. I found that just sitting there without the car being used, the battery lost about 0.0004 volts per hour.

So, using your example above, during the ~336 hours in the 14-day / two-week period, your battery lost 0.15 volts. That equates to 0.0004 volts per hour.

Looks normal to me.
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