Battery Draining But Parasitic Draw and Alternator Test OK - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 26 Old 04-18-2019, 08:36 PM Thread Starter
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Battery Draining But Parasitic Draw and Alternator Test OK

Having some kind of weird issue with my '91 GT.

The battery will die after about two weeks.

The alternator is brand new and charging at 14.4

Checked the alt with a diode test, checked out perfect... no reading when positive lead was on power post and negative lead was on casing. Reading of .87 when reversed with positive on the casing and negative on the power post.

Checked for parasitic draw and got a reading of .02 amps (or 20 milliamps) which is well within range.

I did test the draw by turning on the car alarm and with the alarm active it stayed at around .027 amps BUT when the alarm was turned off it jumped up to .078 and stayed there.

What am I missing here?

If anyone has any suggestions I'd love to hear them.

Thanks for any help!

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post #2 of 26 Old 04-19-2019, 08:40 AM
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what brand battery


89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #3 of 26 Old 04-19-2019, 10:17 AM
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How old is this battery? I had a battery that was fine if it was charged. Everything seemed good. But you had one shot at starting because if it cranked more than 5 or 6 seconds it would barely turn over on the next attempt. If it was fully charged and sat for 24 hours, it would barely turn over.

Bad battery. Might not be the problem. But don't ignore the possibility.
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post #4 of 26 Old 04-19-2019, 10:46 AM Thread Starter
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It's a fairly new battery, an "Autocraft Silver"

Battery tests out fine, drops to 10v at startup which is within spec. I suppose I could get a bigger battery, more CCA... I'm beginning to wonder if it's some kind of draw from the alarm.

The alarm has been on the car since it was new in '91 so I'm wondering if it's got some internal issues.

With the draw test when the alarm was active it stayed within range at 30 milliamps, but when it was turned off it jumped up to 80 milliamps and then after 30 seconds or so, went back down to 20 ish milliamps.

That might be because the lights momentarily turn on with the alarm, but I'm not certain.
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post #5 of 26 Old 04-19-2019, 12:01 PM
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YOU have checked current draw on all circuits?

Vanity mirror?

Can drain the battery quickly for example

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #6 of 26 Old 04-19-2019, 01:05 PM
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I had a problem like that once, when my Autometer Monster Tach was wired to a constant 12V+. Changing to a switched 12V+ solved that issue. So make sure you don't have something aftermarket drawing on the battery when the car isn't running.

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post #7 of 26 Old 04-19-2019, 02:13 PM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
YOU have checked current draw on all circuits?

Vanity mirror?

Can drain the battery quickly for example
I'm not sure what you mean by "checked current draw on all circuits."

I did a parasitic draw test from the battery and the reading was within spec at 27 milliamps so that would be testing all circuits since there's no other power source on the car aside from the battery.

If the vanity mirror was causing a draw that was more than 27 milliamps it would have shown wouldn't it?
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post #8 of 26 Old 04-19-2019, 04:02 PM
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that is correct

if your battery is draining from 27 mA then batt is not good

so are you 100% there is only a 27mA if so then batt is dead

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #9 of 26 Old 04-19-2019, 05:26 PM
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You removed the negative cable and put your Voltmeter between cable and negative battery post? And measured and if high start pulling fuses to see when it goes down? Just curious how you are testing.
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post #10 of 26 Old 04-19-2019, 05:29 PM
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Batteries are tough. I’ve had ones test just fine and only actually swapping a new battery fixed the issue. I probably didn’t load test it correctly, but hey....

On the flip side, if you buy a new battery, look at NorthStar battery’s. Made in USA and are as good as it gets.


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post #11 of 26 Old 04-19-2019, 08:04 PM Thread Starter
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that is correct

if your battery is draining from 27 mA then batt is not good

so are you 100% there is only a 27mA if so then batt is dead
I don't even know if you really know what you're talking about here.

I did a parasitic draw test on the car. The draw was 27 mA.

The battery is measured in volts not amps or milliamps so I don't even know what you're saying.
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post #12 of 26 Old 04-19-2019, 08:05 PM Thread Starter
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Originally Posted by BlackBeauty View Post
You removed the negative cable and put your Voltmeter between cable and negative battery post? And measured and if high start pulling fuses to see when it goes down? Just curious how you are testing.
Correct.
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post #13 of 26 Old 04-19-2019, 08:26 PM
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Change to an Interstate battery. By far the best batteries I've ever had. The problem is probably the autocraft silver battery.
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post #14 of 26 Old 04-19-2019, 09:12 PM
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You’ve never had a NorthStar I bet...

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post #15 of 26 Old 04-20-2019, 08:44 AM
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Quote:
Originally Posted by Imatk View Post
I don't even know if you really know what you're talking about here.

I did a parasitic draw test on the car. The draw was 27 mA.

The battery is measured in volts not amps or milliamps so I don't even know what you're saying.
oh my........

take a step back, read the battery label

then ask yourself why battery voltage goes down while you place a load on it.

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #16 of 26 Old 04-20-2019, 11:45 AM
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Interstate is good and I've had good luck with Deka. Deka was US made last I knew. In PA I think.

EDIT:
Yes made in the USA and yes made in PA.

http://www.eastpennmanufacturing.com...ns/automotive/
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post #17 of 26 Old 04-21-2019, 07:30 AM
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I am throwing a hail mary on easter here...but when you say you did a parasitic draw test....did you use the meter and pull each fuse one by one? Just want to make sure a step wasn't overlooked...
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post #18 of 26 Old 04-21-2019, 09:40 AM
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Quote:
Originally Posted by jamessr23 View Post
I am throwing a hail mary on easter here...but when you say you did a parasitic draw test....did you use the meter and pull each fuse one by one? Just want to make sure a step wasn't overlooked...
i'll assume the poster removed a battery connection, and placed a ammeter in series with it

therefore did a total load/current draw for the car
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post #19 of 26 Old 04-21-2019, 10:38 AM Thread Starter
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Quote:
Originally Posted by jamessr23 View Post
I am throwing a hail mary on easter here...but when you say you did a parasitic draw test....did you use the meter and pull each fuse one by one? Just want to make sure a step wasn't overlooked...
I did the draw test, but didn't need to pull any fuses since the draw was well below a "normal" reading of 50 milliamps.
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post #20 of 26 Old 04-21-2019, 11:05 AM Thread Starter
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oh my........

take a step back, read the battery label

then ask yourself why battery voltage goes down while you place a load on it.
Ok I understand what you're saying now... I think. You're saying that if the draw is only 27 mA then the battery is bad because it shouldn't be going dead at that low of a draw?
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post #21 of 26 Old 04-21-2019, 11:06 AM Thread Starter
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Quote:
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i'll assume the poster removed a battery connection, and placed a ammeter in series with it

therefore did a total load/current draw for the car
Correct.
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post #22 of 26 Old 04-21-2019, 11:56 AM
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my motorcraft 95 rc, reserve capacity rating

95/2.4 = ~40 amp hours

therefore 0.027amp draw is 40/0.027= 1481 hours at that current draw or 61 days

I think your measured parasitic draw is too low to be a problem, battery or charging system is where i would be looking

if you have a DC clamp meter you could compare alt current production and compare it to your system draw, and see if you are at a deficit

I have a 130amp alt, and it can only put a max of, from memory, 65-70amps at idle when i measured

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #23 of 26 Old 04-22-2019, 11:12 AM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
my motorcraft 95 rc, reserve capacity rating

95/2.4 = ~40 amp hours

therefore 0.027amp draw is 40/0.027= 1481 hours at that current draw or 61 days

I think your measured parasitic draw is too low to be a problem, battery or charging system is where i would be looking

if you have a DC clamp meter you could compare alt current production and compare it to your system draw, and see if you are at a deficit

I have a 130amp alt, and it can only put a max of, from memory, 65-70amps at idle when i measured
I measured the voltage drop on starting the car and it didn't drop below 10 and the alt is charging at 14.4 or so... I'm thinking it's some kind of weird intermittent thing.

A wire that's shorting but only occasionally.

Friggin electrical gremlins drive me nuts.

I've pulled the fuses from the car alarm. Going to drive it a couple weeks and keep monitoring and see if that might be the culprit.
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post #24 of 26 Old 04-22-2019, 11:35 AM
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does your battery light in the dash work?

does it ever come on while driving?

do you have a big stereo, electric fan, drive with the blower on high speed?

these loads, can easily put a battery into deficit as the alt cant keep up at idle.

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #25 of 26 Old 04-24-2019, 08:08 PM
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Quote:
Originally Posted by tjm73 View Post
Interstate is good and I've had good luck with Deka. Deka was US made last I knew. In PA I think.

EDIT:
Yes made in the USA and yes made in PA.

East Penn Manufacturing | Automotive batteries
Correct.
AAA batteries are Deka with a 3 year full replacement warranty.

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post #26 of 26 Old 05-01-2019, 05:04 PM
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I would look for a relay going bad. They can stick closed causing a high drain. It is often intermittent making it hard to find.
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