DC VOLTAGE experts...

[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.

[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

[patrickt]

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

[ERA Chas]

Originally Posted by patrickt

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

Here's an update: On Monday I put a 3 hour charge at 10A into the batt and starting was 12.1-finished at 12.7. Next morning it tested 12.7 still-all temps mid 40F. Drove 1/2 hour put away. Retested today at same temps and got 12.3--bummer. Then drove short while.
I have NOT gotten the battery dash icon on startup however at any time.

So I'm testing everyday and watching. Honestly, I can't figure how to do the drain testing as the damn car is so complex. The alternator is in a hellish place so I can't take it out or test for bad diode as source of drain. And I do not have any code-scanning tools for it and if I did, I doubt I can figure 'em out. This is not my forte.

[Ronbo]

Battery voltage changes very little between a charged and discharged state. DMM and electronic gauges have very high input impeadances (around 10M ohms) at this resistance your multimeter is putting a load of 1.2uA (1.2 millionths of an amp). After a battery is charged it takes a little time for the electrolyte to "settle down" to a "at rest" voltage of around 12.5V, a discharged battery with, say, 10% capacity (A/hrs) will still read around 12V or more.

This is why 40yrs ago when the cost of a few feet of wire was not a factor on actually seeing what the charging system was doing, cars had ampmeters. The dash lights (or headlights) will tell you what the volts are.

Think back, you'll remember...

[Ronbo]

Originally Posted by patrickt

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

Quoted from that site "Full-charge voltage on a 12-volt battery is 0.9 volts higher at 32°F than at 70°F. While discharging, a highertemperature will increase battery voltage. There is little temperature effect while a battery is standing."

Their wording is rather bad, or we've discovered a mis-information conspiracy.

I've got 30yrs as an electronic technician, an even more as a sometimes unwilling mechanic. Never saw a battery (any kind) increase voltage with decreased temps.

Lithium batteries were the only ones I've heard of that can maintain voltage at sub-zero temps. Because of their finiky charging and resultant fires they were deemed to unsafe for public use (in the '40s or '50s). Even the lithium ion batteries can light off. (google sony laptop fire)

[patrickt]

All that makes perfect sense. And I prefer an amp gauge as well, but there have been a hundred threads on this forum saying that volt gauges are better. Rick L. is a big champion of volt gauges, but I like my amp gauge.

[Ronbo]

I think it's out of fear of having that much current inside the passenger compartment more than anything.

Which is why they invented fuses BTW...

[patrickt]

Originally Posted by Ronbo

I think it's out of fear of having that much current inside the passenger compartment more than anything.

I agree with that. But the real current is only flowing for a couple of minutes after you have started the car, then the needle drifts back to just an amp or two above zero -- whether you've got the old Ford POS 55amp alternator like mine or one of the new fangled killer 100+ amp alternators.

[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.