Battery Tests Fine, Low Dark Current, Yet Battery Low After Sitting Several Days

[Number_35]

The title sums it up.

The car is my friend's 2007 Ford Fusion 3.0 AWD.

I charged the battery and load-tested it with an old-school resistive tester - the analog scale shows the battery still having good reserve.

I checked dark current several times, and it's consistently under 30 mA, which I consider acceptable.

If the car sits for several days, the engine won't turn over - the solenoid rattles, but that's it.

There's a sticker on the battery shown a manufacturing date of 2018 03. The battery is 6-1/2 years old, and may have been in the car almost as long.

Would it reasonable to replace the battery even though it tests as good?

Another possibility is a weak starter motor that needs the battery to be fully charged to work. (I went through this with one of my own vehicles a few years ago.)

 

[Analog-1]

Yep over 6 years it's time to replace not worth taking the chance of be stranded when it decides to completely fail and it will sooner then later.

 

[SHOZ]

Make sure the connections are all tight and clean.

 

[ecotourist]

My BMW had an intermittent current draw (heater/AC fan) running the battery down. Battery tested fine, but still often dead when you tried to start the car.

A known feature of the vehicle. "I'm surprised you didn't have to change the xxx part sooner."

 

[Jetronic]

the resistive tester doesn't measure capacity. I suspect that's what happened, reduced capacity. 0.03A*24h*4days is about 4 Ah capacity remaining. I would put a 21W bulb on the battery for an hour or 2 and test with the resistive tester again

 

[eljefino]

^ yeah you're way down on capacity, which is a failure mode. Car will crank at 20 below zero but leave the radio on five minutes and it's done.

 

[Number_35]

^ yeah you're way down on capacity, which is a failure mode. Car will crank at 20 below zero but leave the radio on five minutes and it's done.

That's very similar to how a battery failed in my work truck - started fine in the cold, and then died when I left it at the worksite with hazard lights (4-way flashers) on for an hour.

 

[Number_35]

the resistive tester doesn't measure capacity. I suspect that's what happened, reduced capacity. 0.03A*24h*4days is about 4 Ah capacity remaining. I would put a 21W bulb on the battery for an hour or 2 and test with the resistive tester again

How does one determine the typical capacity (in A-h) of an automotive battery? I realize it's different than the CCA (600 A in this case).

(Edit: One site says that 48 A-h is typical. Let's say the dark current is 25 mA (i.e. 0.025 A). Over 24 hours, that's approximately 0.6 A-h. Over a week, that's approximately 4.2 A-h. I could see that being insignificant on a healthy battery, and critical on an older battery.)

 

[Number_35]

Make sure the connections are all tight and clean.

I cleaned the terminals early on.

 

[Cujet]

How does one determine the typical capacity (in A-hrs) of an automotive battery? I realize it's different than the CCA (600 A in this case).

In general, a small/mid sized car will have a 40AH battery, or maybe a bit more. An F150 might have a battery closer to 70-80AH.

Just looking at the chart below, the AH rating is about 50% more than the weight of the battery..... For a very rough guess.

 

[SHOZ]

I cleaned the terminals early on.

The negative body ground is a problem on some cars where they clamp them over paint and then expect the bolt to carry the current. Or they rust out over time and build resistance.

A good test is ohm from negative terminal to the alternator frame. Should be 0 or the resistance in your meter leads.

 

[Jetronic]

The negative body ground is a problem on some cars where they clamp them over paint and then expect the bolt to carry the current. Or they rust out over time and build resistance.

A good test is ohm from negative terminal to the alternator frame. Should be 0 or the resistance in your meter leads.

Yes, but that shouldn't affect a car that gets a charged battery then stands for a few days parked. I agree it impacts charging though.

 

[Jetronic]

How does one determine the typical capacity (in A-h) of an automotive battery? I realize it's different than the CCA (600 A in this case).

(Edit: One site says that 48 A-h is typical. Let's say the dark current is 25 mA (i.e. 0.025 A). Over 24 hours, that's approximately 0.6 A-h. Over a week, that's approximately 4.2 A-h. I could see that being insignificant on a healthy battery, and critical on an older battery.)

I would consider 48 Ah a small battery, current cars seem to have 60-70Ah batteries on average, and I've seen up to 100Ah, or just over. Regardless, try draining a few Ah, and then do a load test. This should be done fairly quickly, we don't want to harm the battery more than needed. But you know that.

 

[SHOZ]

Yes, but that shouldn't affect a car that gets a charged battery then stands for a few days parked. I agree it impacts charging though.

If a charged battery is going dead after a couple days it's either junk or you have a parasitic draw beyond the usual >.05A

 

[ripcord]

After 6.5 years the positive plates have likely eroded significantly which reduces the reserve capacity of the battery. It may still take a charge and have enough voltage but not have enough capacity remaining to do much more than start the engine a time or two.

Another symptom of aging is increased self discharge. Combine high self discharge with reduced capacity and you get exactly the symptoms you described.

 

[Donald]

The title sums it up.

The car is my friend's 2007 Ford Fusion 3.0 AWD.

I charged the battery and load-tested it with an old-school resistive tester - the analog scale shows the battery still having good reserve.

I checked dark current several times, and it's consistently under 30 mA, which I consider acceptable.

If the car sits for several days, the engine won't turn over - the solenoid rattles, but that's it.

There's a sticker on the battery shown a manufacturing date of 2018 03. The battery is 6-1/2 years old, and may have been in the car almost as long.

Would it reasonable to replace the battery even though it tests as good?

Another possibility is a weak starter motor that needs the battery to be fully charged to work. (I went through this with one of my own vehicles a few years ago.)

A 100 amp load tester or a carbon pile set to half battery CCA?

 

[Number_35]

A 100 amp load tester or a carbon pile set to half battery CCA?

It's a Schumacher with resistive elements like an oven element.

I'm not sure what the current draw is. I'll investigate.

 

[ripcord]

I'm guessing it's a 100 amp one like this? I like them, though, you have to interpret the results to account for the higher capacity of large batteries.

 

[Donald]

It's a Schumacher with resistive elements like an oven element.

I'm not sure what the current draw is. I'll investigate.

If it's got no knobs you adjust it's most likely 100 amp. But the best is to test at 1/2 CCA for 15 seconds. Then look at the voltage just before you stop the test.

 

[dogememe]

At 6+ years old just replace the battery.