Battery voltage slowly dropping

90Hatchback

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Oct 30, 2018
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My '90 LX is not a daily driver. I put a brand new battery in it when I purchased it four years ago. I would try to start it once a week, but sometimes wouldn't start it for a couple weeks. It got to the point that it would start dying by the time I would try to start it. When I started monitoring the voltage, I would notice it would get down to 11.98 volts, but would still start. While running, it would get as high as 14.5'ish and then would fall to 12.43 or so after sitting for a few hours. It would then lose .01 volts every few hours.

On Feb 9 at noon, the old battery was at 12.43v.
at 7:30 P.M. it was at 12.41v.
Next morning at 8:00 A.M. it was at 12.40v.
By 2:00 P.M. it was at 12.39v.
At 11:00 P.M. it was still at 12.39v.
On the 11th at 9:00 A.M. it was at 12.38v
At 10:00 A.M. I disconnected the battery cables and the battery read 12.43v.

This was the typical trend with this battery until it would drop to 11.98v and I would start it (thinking it wouldn't start), but it started up normally (not as strong at my new battery (which has about 50 more cranking amps), but it didn't struggle to fire up. I don't know how far it would drop when it wouldn't start (I wasn't measuring it at the time).

IS THIS ^ SLOW DROP IN VOLTAGE NORMAL FOR A BATTERY. AND, WHEN DISCONNECTING THE BATTERY, IS IT NORMAL FOR THE VOLTAGE TO GO BACK UP TO A "NORMAL" VOLTAGE. OR, DOES THIS SLOW DRAW INDICATE I HAVE A SHORT OR SLOW DRAIN SOMEWHERE THAT ISN'T NORMAL?

I then installed a new battery which measured 12.46v before connecting the cables.

On Feb 14, the new battery was at 12.42v at 10:00 A.M. (I didn't document this well, but I believe I purchased the new battery on the night of the 13th and measured it at 12.46 off the shelf. The next morning before installing the battery, it measured 12.42 before and right after attaching the battery cables).
At 5:00 P.M. it was at 12.41v.
On Feb 15 at 9:00 A.M. it was at 12.40v where it stayed for three days until
Feb 18 at 9:00 A.M it was at 12.39v.
Next morning it was still at 12.39v.

Yesterday, I pulled fuse #11 and #16 after running the car for 20-30 miles.
I parked it at 10:00 A.M. (didn't measure the voltage).
At 3:00 P.M. the battery was at 12.50v.
Last night at 9:00 P.M. it was at 12.50v.
I reinstalled fuse #16 (suspected offending circuit because this is where I pulled constant hot for my alarm recently).
Today at 9:00 A.M. it's flickering between 12.49 and 12.50v but steadied at 12.50v.

WHAT SHOULD I EXPECT FROM THIS NEW BATTERY IF THERE IS NO ABNORMAL DRAW ON THE BATTERY? SHOULD IT LOSE .01v OVER A FEW HOURS, OR A FEW DAYS? OR, SHOULD IT MAINTAIN A "NORMAL" VOLTAGE?

Since the battery stayed at 12.50v all day yesterday, I was thinking that if I installed fuse #16 and it only dropped .01v over the next few days, that that circuit probably isn't the problem. If I see a drop of .02v over the next day or so, that circuit might well be at least part of the problem. But, my question really is, should there always be a slow constant draw on the system? I'm suspecting yes, because if any car sits for long enough, it won't start. But, how quickly should the voltage be dropping???

(While taking all these measurements, I moved the probes of my multi-meter around to make sure my readying wasn't from a bad connection).
 
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90Hatchback

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I really don't think I'm over-thinking it. I'm not asking if I should keep charging my battery whenever I turn it off. I'm asking if the draw I'm seeing on the battery is NORMAL or if I need to start searching for a short or some other issue! Hence all the bold statements in my top post. Try re-reading the top post...but only read the bold parts. Then, go back over the rest for clarification purposes as needed.
 

General karthief

wonder how much it would cost to ship you a pair
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Ok, let me say this another way, a car battery voltage will fluctuate some, my stang sits outside for sometimes a month between drives, if you start your stang every couple weeks or so your good as long as it runs long enough to dissipate the moisture inside the engine and keep the battery charged, I suggested the battery tender to keep you from having to start and run the car to keep the battery charged, I think you do not have a problem with something draining your battery.
 

Mustang5L5

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There's will always be some draw. Not much, but some. The radio memory and ECU draw a tiny bit to retain their memory.

You should really grab a digital multimeter and hook it up in series with the battery and get an actual current draw measurement.

Nothing you posted seems to set off alarms in my head as to issues. My battery has been disconnected for a month so I can go see what voltage it's sitting at, but proper sitting voltage should be about 12.5 volts.

If you don't measure a significant current draw, then I'd recommend a good battery tender. I usually put my car on one if I don't drive the car for more than 3 weeks or so.

You can also take the battery to a local parts store and have it load tested.
 
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jrichker

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Troubleshooting battery drains

Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.

The ideal method is to disconnect the negative terminal, and connect a Digital Multimeter (DVM) between the negative terminal on the battery and the negative cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto-range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.

See http://assets.fluke.com/appnotes/automotive/beatbook.pdf. You will need the Adobe Acrobat viewer which is also a free download – http://www.adobe.com/products/acrobat/readstep2.html

If you can’t find the current drain by pulling fuses, then the alternator is suspect or any aftermarket sound or lighting equipment that may be on the car. Disconnect the alternator output power plug, then disconnect any aftermarket sound or lighting equipment while watching the current on the DVM.
 
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Mustang5L5

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Tested my battery

12.8 volts after sitting over a month without being connected to anything
 
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90Hatchback

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Tested my battery

12.8 volts after sitting over a month without being connected to anything

My brand new battery (Duralast Gold) was just 12.46 off the shelf.

You should really grab a digital multimeter and hook it up in series with the battery and get an actual current draw measurement.

I was really asking to see if ^ this was necessary, or if the draw I was seeing was "normal". It seemed a bit high to me, that's why I asked. I'll get to this as soon as I have the time.

Thanks for all the responses!
 

Mustang5L5

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I was doing a few electrical things in my car tonight and checked my draw real quick

0.01 amps at rest

Common items that can induce a draw while the car sits..

Underhood light
Vanity mirror lights
Internal alternator short
 

90Hatchback

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So...when I disconnected the negative battery cable and put my multimeter between the negative cable and negative post on the battery, my alarm would go off.
Would this indicate the alarm isn't installed properly, or do I just need to unplug the alarm for this test? And, if I unplug the alarm, how will I confirm the alarm isn't drawing power when it shouldn't be?
 
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Mustang5L5

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You'll have to disable the alarm. Some tend to trip when power is cut to the vehicle.
 
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Blown88GT

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So...when I disconnected the negative battery cable and put my multimeter between the negative cable and negative post on the battery, my alarm would go off until it blew the wire out of the probe of my multimeter (literally less than a second).
It would indicate you don't know how to use a multi-meter.
Probably blew the multi-meter fuse. You have to use the 10A scale not the mA scale.
 
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90Hatchback

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It would indicate you don't know how to use a multi-meter.
Probably blew the multi-meter fuse. You have to use the 10A scale not the mA scale.

Might not wanna respond when your response is based solely on assumptions. Especially when your assumptions are wrong! The fuse of the multimeter isn't blown, AND I was using the 10A scale!
 

Blown88GT

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Sorry about that. When you asked what you were doing wrong, that's what came to mind.
I apologize, my bad.
Your test indicates that the battery was drawing more than 10A with engine off.

Back to your 1st & 11th post.
It's possible that 1 or more diodes in the diode pack of the alternator are shorted. They usually fail open, not shorted, but I've seen electrical equipment (ATS) where a diode shorted & killed the controller. Didn't know until the tech replaced the controller & it blew out a chip right in front of us. I still have the pics.

He indicated this as a possible failure in post #7, last paragraph.

I would recommend getting the alternator tested.
 

WillWho?

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That's what I was kinda thinking might be the problem. I'm pretty sure I put a new alternator in when I purchased the car 4 or 5 years ago. So, I was doubting that was the issue. But, more and more things keep pointing in that direction.
Thanks.

It one of the diodes is not functioning, it is likely allowing some AC current to leak into the rest of the system. Even a tiny bit of AC current in a DC system will create random problems. Fortunately, the multimeter is the easiest way to test. With the car running, set the multimeter to AC voltage and read between the battery posts. Set the range high and slowly dial the range lower.

How does a bad diode drain a battery at rest? A diode directs current flow in one direction. In this case, away from the alternator. If one is not functioning, the battery will send power through to the alternator where the current will seek out ways to mess with your mind. (If you work with electronics, the term EMF does not always refer to electro magnetic field. Those little electrical m f .... you do the math...)

Some of the alarms installed back in the early 90's had a circuit that disabled them if the voltage dropped below the point that it would prevent the car from starting. Usually the first clue was the remote lock feature would not work so you used the key to enter. As soon as you turned the key in the ignition, the alarm would start and you used the fob to disable it. Then the car would start, but it would be a weak start. This is when you began troubleshooting for a parasitic battery draw.

If you are up for it, you can install an auxiliary fuse box that is independent of the main fuse panel. Connect any aftermarket accessories to the Aux panel. Now you have solid connections to power without using fuse taps. Your main fuse panel looks neat and you can keep the wiring to the Aux panel looking neat too. The Aux panel can be under the hood, just avoid mounting it too close to heat sources. Also, mount it so that it is easily accessible and not in the way of performing routine maintenance. The directions will tell you how to make some of the power switched and some always hot. This is a 30 to 45 minute install for someone handy with tools but not an electrical guru.
 
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