DC VOLTAGE experts...

[ERA Chas]

This is not a ‘favorite battery’ or ‘Optima vs The World’ thread. It’s actually a request for generalized info about battery characteristics-not which is the ‘best’ battery so no pissing contests please.

The 2 year old Interstate in my street car (which had been perfect up to now) had begun to show 11.9 to 12.1 of late. I have been getting a ‘battery’ icon dash light which would come on at start up and glow 10 or 20 seconds. Sort of like an old time ‘generator light’. No CEL’s or codes showing. Headlights always bright and heaviest draws no problem.

The car is garage kept with 60K miles. It’s mid 40’s in the garage with ambients in the mid 20’s outside. At idle, it’s 14.6-14.7. So I’m thinking the charging system is fine and the battery has a bad cell or some other deficiency that won’t allow it to hold 12.6 or so which I believe is what’s normal for the conditions. I do not have a load tester or any other diagnostic equipment.

The 1 year old Optima in my Cobra will hold 12.5-12.6 without the Battery Tender. I use that during the cold weather. The Interstate is the premium which has a 30 month free replacement, 7 year pro-rated. It’s 80 amp, 640cca, 800 ca and 100RC. So I pulled it and got a free replacement. I know that if the car were outside on a 20’s day or snow storm, it wouldn’t start.

The new battery was 12.1 with a manufacturing date of August. I put my 10 amp charger on it for 20 minutes and it got to 12.7. At idle it was 14.7. Finished install and went for ½ hour drive. After that it was 12.3 and 14.6 at idle and I expected higher. I’m feeling that there may be a drain in the system. Forgot to mention upon disconnection to remove, I used a ‘settings saver’ which uses a 9 volt. Testing a new battery, it was 9.4. Total time disconnected was 3 ½ hours and when I removed the 9 volt it was 8.0.

So-what if any test procedures other than laboriously pulling a fuse at a time and seeing if the drain (if there is one) stops? Assuming that finds a draining circuit, then a wire/switch/relay test of that circuit? That is beyond my skill, equipment and brainpower. OR-is the battery fine at low 12’s in 40 degree and colder weather? If so, why the dash light? Again-didn’t have the problem for 5 years on the original battery and 2 years on the second until now. Any experienced answers appreciated.

[Bobcat]

Charles ... when I was traveling heavily , I used to leave my truck in the garage for a week or two at a time without starting it ( had a company car ) and never had any battery problems . Then I started having some cranking problems and finally a dead battery . Started leaving it on a Battery Tender when I was gone , but had to replace it after a little over 2 years . Long story short .... the new cars today always have a small drain due to all the computer/electronic circuits etc ( theft alarms etc ) to let them keep their presets etc, according to the tech at the Ford dealer . Could that be what is happening and it becomes more of a factor in colder weather as your battery ages ?
As a battery ages , the plates sulfate up some . When it isn`t being kept full charge , that means the plates will sulfate that more quickly and then the battery won`t take a charge like it used to do and it goes downhill even quicker . I have a battery in my Galaxie that is a replica of an old tar top and has been kept on a Battery Tender its whole life and it`s almost 7 years old now ..... hope I didn`t just jinx myself . Why you didn`t have a problem with the 5 year old one , I don`t know .

Bob

[Blas]

at 80 Degrees F
A fully charged battery (100%) will show 12.68V
12.24V at 50% charge
12.06V at 25% charge
Adgust these V #s by 0.04V for every 10 degrees colder temp

At 30 degrees a fully charged battery should show 12.588V

14.6 or 14.7 is fine for the charging system...

Have the battery checked with a load tester...

Blas

[Rick Parker]

The regulator may be on the way out, but the alternator is still capable of developing a full charge.

[patrickt]

Of course you are taking the readings right off the battery poles and not from the cable clamps, right? Bad connections are the first suspect. No chance of a slipping serpentine, right? That's the second culprit with an intermittent charge light. Assuming you want to do a parasitic check, I PDFed the pages from my GM service manuals. You can download them here: http://208.255.159.239/drain.pdf but here's a snap of the critical calculation for current draw depending on your battery specs:

[patrickt]

Chas., there's a trick that will occasionally fool you when doing a parasitic drain test. That special tool referenced in the GM Service manual is really not special at all; it's just a plain On/Off switch on your battery cable. But what it allows you to do is conveniently attach your multimeter in parallel before you disconnect the On/Off switch (thus forcing all current through the multimeter). This is a very important step in your parasitic testing. Here is an example that I thought up that will explain why. Suppose you have a dimmer switch on the light in your living room. If you run it all the way down so that the light is just barely glowing it will create a nice, romantic atmosphere. But if the power goes off, and then comes back on, the light will not come back on at that low level. You must then turn the dimmer switch up high, the light will come back on, and then you can dial it back down to the low level. The same can be true for a parasitic drain on your car. Removing your battery cable, and then attaching an amp gauge, can actually stop the drain right before you test for it. You must first attach your multimeter in parallel and then remove the battery cable, thus maintaining a constant circuit that will support the parasitic drain.

Yes, I can hear you now: "Thank you Pat, you're a genius."

[ERA Chas]

Todays update:
Battery tested 12.1 in 45 deg. garage. Upon start up, no dash warning light this time. (?) Went for 15 minute drive and after shut off, battery showed 12.5. Everything functions fine in the car.

A big thanks to all for fact-filled, insightful answers. For the record, terminals are immaculate, grounds are tight-I keep everything like the Cobra-perfect as I can get it.

I will get a load test but this second Megatron is acting much as the first-doesn't seem to be a battery fault to me now. I'm unfortunately suspecting a drain.

Pat, your research and comments are appreciated but the installation is nowhere near as friendly for testing as our Cobras. In typical German fashion, the battery is at the base of the windshield, against the firewall, buried under covers and all the leads are only long enough to disconnect them. Huge PIA and not much hook-up room. Plus the batt weighs 49 pounds and is in a 'mail slot'. Although I follow the literature you provided, I think that the harder part would be identifying the actual draining source-not just that there is one. And forget attaching that GM style shutoff switch for parallel testing-the pos terminal end has two other cables bolted to it which I believe feed the car's 1000's of computers.

I plan to watch it for a while and if significant loss causes the dash light to show, I suspect I'll need dealer circuit testing and diagnosis-although I seldom trust dealer service departments.

And PT, get that hearing defect checked-out...

[FWB]

had similar test results when troubleshooting my lincoln....a diode was fried in the alt. so it tested good but charging was not what it should be. it took an educated guess to fix by just replacing the alternator. maybe your situation.....maybe not.......

fred

[patrickt]

The dealer will have a special tool for that car for a parasitic test. Maybe a code has been thrown that does not set the dash light? That's what you hope for, and their scan tool will spot it. If the battery passes the load test, and the charging system passes the charge test, then a parasitic drain is the only option. Most modern day computer systems have parasitic protection build in to the coding (like if you leave the dome light on). It still wouldn't hurt to look at the car in the pitch black of the garage and see if you can see a light coming from the trunk or glove box. That only happens once in a lifetime, though.

[ERA Chas]

Originally Posted by Blas

at 80 Degrees F
A fully charged battery (100%) will show 12.68V
12.24V at 50% charge
12.06V at 25% charge
Adgust these V #s by 0.04V for every 10 degrees colder temp

At 30 degrees a fully charged battery should show 12.588V

14.6 or 14.7 is fine for the charging system...

Have the battery checked with a load tester...

Blas

Blas,
I must be missing something:
Using the figures above and your formula, I get 12.48V at 30 deg F.
.04 x 5 (50 degrees colder than the 80 degree value)= .2V.

12.68V (the 80 degree full charge value) - .2V= 12.48V

How do you get 12.588V for the 30 degree value?

[Blas]

Opps...a typo
~0.01v per 10 degrees...approximately...
All numbers taken from a battery/charging service article…

[patrickt]

Originally Posted by ERA Chas

How do you get 12.588V for the 30 degree value?

Chas & Blas, I don't think that's right. A lower temperature will increase battery voltage. Full-charge voltage on a 12-volt battery is 0.9 volts higher at 32°F than at 70°F.

Source: https://azsolarutility.com/Battery_Information.html

[Blas]

"...A lower temperature will increase battery voltage..."

Not on the planet I live on...

[patrickt]

Originally Posted by Blas

"...A lower temperature will increase battery voltage..."

Not on the planet I live on...

The voltage increases as the temp goes down, but the capacity decreases. Here's another chart that shows the fully charged voltage ratings for 12v batteries at various temperatures. It increases as the temperature declines. http://www.powerstream.com/SLA.htm

[Ronbo]

Unless your running EFI or have a stereo you should have only a couple circuits not cut off by the ignition. The "keep alive" memory for a computer (EFI) or the stereo will typically be less than 1ma (1/1000A). (leaking ign. switch is possible)

As stated, a leak at the alternator or regulator, starter solenoid, lights. While monitoring the battery voltage, pull fuses and disconnect the alt big lead. You should notice a couple tenths of a volt jump when you interrupt the leaking circuit.

Most DMM's have a 200ma current scale and usually a 10A scale, start with the 10A and work your way down. (placed in series with battery cable)

The big thing about batteries is the Amp/hours rating (not the CCA rating which is marketing BS for the most part). Typical car battery is in the 85 ~ 100 A/hr range: (1 amp for 100 hrs, 10A for 10 hrs, 100A for 1 hr.... You get the idea).

You'll probably find a leak of .5~2A from what your describing.

Worst thing they ever did was move from ampmeters to voltmeters on cars. (but hey, 12ft of 10ga copper wire is darn near $5!!)

[Ronbo]

Originally Posted by patrickt

The voltage increases as the temp goes down, but the capacity decreases. Here's another chart that shows the fully charged voltage ratings for 12v batteries at various temperatures. It increases as the temperature declines. http://www.powerstream.com/SLA.htm

Patrick those are charging voltages, not battery idle voltages (which will drop with temp.).

Both voltage and current will drop with temp (capacity in A/hrs). Or power if you will (IxE=P) If E (voltage) went up this would negate the decrease in current (I).

I'm sure we're all aware what effect that first freezing night has on that 5yr old battery we were going to change out.

[ERA Chas]

Originally Posted by Blas

Opps...a typo
~0.01v per 10 degrees...approximately...
All numbers taken from a battery/charging service article…

Blas, thanks for the correction-makes more sense now.

[patrickt]

Originally Posted by Ronbo

Patrick those are charging voltages, not battery idle voltages (which will drop with temp.).

That would seem to make sense, and ol' Blas is usally right, but this quote: "Full-charge voltage on a 12-volt battery is 0.9 volts higher at 32°F than at 70°F" seems to be prevalent on several battery related sites. Is it completely wrong, or is there some subtle point we're missing? BTW, I pulled it originally from here: https://azsolarutility.com/Battery_Information.html

[ERA Chas]

Originally Posted by patrickt

The voltage increases as the temp goes down, but the capacity decreases. Here's another chart that shows the fully charged voltage ratings for 12v batteries at various temperatures. It increases as the temperature declines. http://www.powerstream.com/SLA.htm

Copied from your link:
While charging, a lower temperature will increase battery voltage. Full-charge voltage on a 12-volt battery is 0.9 volts higher at 32°F than at 70°F. While discharging, a higher temperature will increase battery voltage.
Emphasis is mine. As Ronbo says also, those are while charging and discharging.

But thanks to all for clarifying.

[patrickt]

Well that's poorly worded alright. Chas, are you going to try and perform the parasitic drain test?