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One of my friends has a 2007 Lincoln MKX that's been having some electrical issues lately, the battery drains to dead overnight. It's been looked at by several people (including dealership), the first few of who pretty much had no idea, and suggested basic things like replacing the battery, which was done but to no avail.

I wanted to look at it briefly and actually get to the core of the issue instead of speculating, so I bought a 100A current shunt and voltage/current readout, hooked it in, and started to check it out. The battery was sourcing 1.4A with everything off. I started going through the fuses, and found a few that caused the current reading to drop, but ultimately I didn't have enough time on my hands due to other obligations to go through and definitively find the problem before my friend needed this car working again.

Instead, it was taken to another friend who is pretty smart and works on vehicular electronics. He says he found the issue fairly quickly. I heard 2nd hand that it was either corroded or incorrectly soldered terminals/terminal wire.

I don't know this friend well, but I know he is pretty smart. Still, it struck me as odd. I would have thought corroded terminals or incorrectly soldered battery cables would cause an open circuit if anything, not parasitic current drain. But perhaps I don't know electricity as well as I thought I did (degree is in computer engineering, so I've done a fair bit of EE, but not automotive electrical).

So how could corroded terminals or incorrectly soldered battery cable cause the behavior I saw? Even so, how would that explain the current drop I saw on multiple fuses? I saw 0.5A drop pulling the radio fuse out, 0.5A on the cluster fuse, and smaller drops on a few other fuses.

Update:

Car is dead again, as I suspected it would be. Never left the driveway this morning. Guy who "fixed" it is very smart, but the family member who dropped it off with him gave him no context, no background, no mention of what I found, probably didnt even tell him the battery kept dying overnight. No clue why, but now we might actually get somewhere. I got in direct touch with him and told him everything. Thanks for confirming my suspicions.

  • Correcting the corrosion will most likely help the battery to charge but won't change the cause of the drain. – Solar Mike Feb 10 at 6:38
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    Pure speculation, but poor voltage supply can put some components into failure modes where they drain more current when trying to work with the reduced voltage. This is quite common with DC-DC converters, though many have separate low-voltage cutoff to combat it. – jpa Feb 10 at 10:38
  • How old is the battery ? – Criggie Feb 11 at 10:17
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The effect of corroded or poorly soldered battery terminals is responsible for excessive voltage drop, not parasitic current drain.

They are not the root cause of the parasitic drain.

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    Nevertheless, while you're fixing it up anyway it definitely won't hurt to replace them. Excessive voltage drop leads to other problems, but still electrical. No point in collecting a heap of problems in the same area. – Mast Feb 10 at 9:57
  • Ok, I think this makes intuitive sense. So the corroded or poorly soldered connections essentially have a higher resistance, therefore dropping a larger percentage of the available battery voltage. – krb686 Feb 10 at 10:57
  • This is only half true some of the time, which makes it not terribly useful. Certainly corrosion products can create conductive pathways, and poor solder jobs can create bridges, shorts, stray wire frays, etc. It's not possible to conclude on the evidence given that corrosion is not a possible cause or that OP's friend came to the incorrect conclusion. – J... Feb 11 at 14:33
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Have a look at this picture:

enter image description here

From here.

Corrosion (the green stuff) can conduct electricity.

If one wire in that picture went to the battery positive terminal and the other went to the battery negative terminal, then it would act like a connection between the two. Not quite a short circuit, but not an open circuit either.

If the cause of the problem was corrosion, then I expect it was something like in the picture.

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  • Makes sense, so corrosive buildup can potentially connect two points that weren't previously connected. In this case though, I think it was stated that corrosion on existing connections (battery terminals) was the cause of the current drain, hence the confusion. I could not imagine corrosion there further dropping resistance than the metal to metal connection, nor is the corrosion connecting a previously open path. – krb686 Feb 10 at 11:03
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    @krb686: The battery was replaced at one point. Any half way competent mechanic would clean the battery terminals on the cables while replacing the battery. Also, the terminals on battery cables aren't soldered. They are clamped. The question doesn't say that corrosion on the battery terminals was the problem, only that corrosion somewhere was the problem. – JRE Feb 10 at 11:07
  • Yes, it was replaced twice actually. So we are fairly certain we had a good battery on our hands. Ok, so you're saying that corrosion somewhere, but not necessarily the battery cables/terminals, could have been the problem, i.e., somewhere that the corrosion could have built up and connected 2 points of a wire/circuit that weren't supposed to be connected in that way. – krb686 Feb 11 at 1:41
  • This sounds very likely. 0.5A for a switched-off radio (which essentially only needs to keep the clock and station memory alive) is far too high, and also the instrument cluster shouldn't pull 0.5A idle either. This sounds like there's some corrosion on a connector that causes stray currents. – WooShell Feb 11 at 11:23
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Probably not.

A parasitic drain requires electricity to be flowing, a corroded or badly soldered terminal would not cause a drain unless it somehow bridged the electricity to the car body or some other path. Usually a corroded terminal will reduce power flow.

So unless the terminal is somehow in contact with something conductive that isn't the cause of the drain. I suspect something has been changed in the story, it's very possible that a corroded or badly soldered connection somewhere in wiring from the battery could cause a battery drain as described.

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You say you didn't have time to do a complete diagnosis. I am wondering if you allowed enough time to let all the modules sleep when checking draw. The corrosion repair could have fixed a charging problem, and there may not have been a draw problem. To confirm all you need to do is another draw test after repairs.

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    Waited about an hour. – krb686 Feb 11 at 1:37
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Your sense is correct.

Battery terminal corrosion is by necessity, a series phenomenon. That is, it can build up resistance, but it is in series with other loads. So unless those loads are flowing current, the terminal itself cannot.

The car, quiescent, should draw a few milliamps for memory "keep-alive" going to computer, radio and clock. Adding resistance to that won't cause a significant current increase. As the 100A shunt confirmed, resistive voltage drop is proportional to current, and at negligible current, voltage drop is also negligible.

The only exception is if the green spoo has splattered across the battery physically; it might form a film across the plastic battery which could leak current between the terminals (or from the + terminal to the metal strap-down), which presumably is grounded to chassis). But that can be resolved by simply wiping with a cloth. Surely after the many people looking at it, this would have been done.

It's been looked at by several people (including dealership)

Yeah, there's why the terminals look janky. Everyone has tried something different.

However, I am surprised by the concept that with 2 people seeing it in rapid succession, the second one would see any corrosion at all. The first person should have cleaned all that up, and it shouldn't come back quickly.

I would not have gotten a 100A shunt. I'd have gotten a 2A shunt or even a 5 ohm 30W power resistor, because fine resolution is more important than accommodating max current with all accessories on, don't have the shunt in place if the car isn't off.

Keep in mind even 100ma of draw is alarming. A car should be able to sit for a couple months without draining a nominal 100AH battery. I left my car out in the winter for 5 months with a 4 year old battery, and it started right up on a 0 degree day. This is normal. Consider all else to be abnormal.

So 500ma being pulled by a radio, that is mind-blowingly unacceptable.

Given the age of the car and the owner's penchant for allowing "friends" to work on it, I would take a hard look at anything aftermarket. For instance that radio - a good reason for a radio to draw 500ma is the volume was turned down but it isn't off.

  • Radios are supposed to make 2 power connections, one to always-hot (for the station memory/clock) and the other to hot-in-Run/Accy (for everything else). A lot of idiots go ahead and hook both wires to always-hot, not least so they can listen to the radio while sitting somewhere with the engine off. That has a consequence.
  • Large aftermarket amplifiers draw so much current that if you hook them through hot-in-Run/Accy, the keyswitch or factory relay can't handle that much current, and it fries wires or components. Bitten now shy, they wire it up to always-hot directly; again, consequences. (they should run it off always-hot but through a contactor whose coil picks up in "run").
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