12.4V Open Circuit Voltage

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George7941

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And still the battery functions very well starting the truck up in -25C weather.

The battery is a Costco(Johnson Controls) Kirkland Signature Size 78 side terminal flooded lead acid battery bought in 2008(for another vehicle, I installed it in my truck a year and a half ago).

I have verified the voltage reading with my trusty bench multimeter(and ScanGauge and Midtronics PBT200 Conductance Tester). When the engine is shut off the OC voltage is 12.8v and it drops to 12.4v in two hours and stays there. The Conductance tester rates the battery at 640CCA and passes it and the battery has never exhibited the slightest sign of weakness in our cold winters.

I just do not have an explanation for the low OC voltage. The parasitic draw on my truck is 16 ma and there is nothing staying on drawing any significant amperage. Leaving the battery on charge overnight on a battery charger does not improve the OC voltage, two hours after the charger is disconnected the OC voltage goes back to 12.4v. It will stay at 12.4v even if do not use the truck for a week.
 
It's a 12v battery & most will read approx 12.2-12.4 fully charged... It isn't suppose to read 12.8v, worry about something else...
 
Voltage is correct for a fully charge battery. The notional charge drop is the "surface charge" droping off.
 
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This is from the Wikipedia article on Automotive batteries


Terminal voltage

The open circuit voltage, is measured when the engine is off and no loads are connected. It can be approximately related to the charge of the battery by:



Open circuit voltage

Approximate
charge

Relative
acid density



12 V

6 V



12.60 V

6.32 V

100%

1.265 g/cm3



12.35 V

6.22 V

75%

1.225 g/cm3



12.10 V

6.12 V

50%

1.190 g/cm3



11.95 V

6.03 V

25%

1.155 g/cm3



11.70 V

6.00 V

0%

1.120 g/cm3



For a fully charged battery it lists 12.6v. For a 75% charged battery battery it lists 12.35v. Yet no matter how long I leave the battery on a charger it never attains an OC voltage of 12.6v
 
Originally Posted By: friendly_jacek
George,
I've got the same issue with a Johnson Controls battery (WMT brand) purchased in 2010.
I posted about it here: http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2727055#Post2727055

Oh, BTW, the people who responded above know squat about acid lead batteries.


Maybe a little harsh? I hope you are not referring to my response on your thread. if you would like further info, here's an easy read, http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf
 
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"Lead acid" is also not quite as simple as it seems. The battery manufacturers twiddle with trace elements in the alloy of the lead plates for many reasons- to make batteries more heat tolerant, more vibration tolerant, more overcharge tolerant, reduce plate pitting as the battery is cycled, reduce sulphation during prolonged sitting, etc. etc. etc. The addition of trace elements also can pull the resting OC voltage off the theoretical norm a bit. If the battery PERFORMS well and has exhibited a particular resting OC voltage since new, I'd worry about something else (as another poster suggested).
 
Those who mention "worrying" are missing the point. One can be curious about something without necessarily worrying about it. In my case, as I mentioned, the battery has never exhibited any sign of weaknesss, so I am not worried about it.

Lead-lead, lead-antimony, lead-calcium and lead-antimony-calcium all supposedly have 12.6v voltage, so I am curious as to why mine is 12.4v.

The article meep linked to adds nothing to this discussion. It also shows the resting voltage to be 12.6v, not 12.4v.
 
Originally Posted By: meep


Maybe a little harsh? I hope you are not referring to my response on your thread. if you would like further info, here's an easy read, http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf



No, I was not referring to you. Thanks for the "easy read" link. The only problem is that the "easy read" doesn't actually deal with resting voltage vs SOC. Go ahead and check it again.
 
Originally Posted By: George7941
Those who mention "worrying" are missing the point. One can be curious about something without necessarily worrying about it. In my case, as I mentioned, the battery has never exhibited any sign of weaknesss, so I am not worried about it.

Lead-lead, lead-antimony, lead-calcium and lead-antimony-calcium all supposedly have 12.6v voltage, so I am curious as to why mine is 12.4v.

The article meep linked to adds nothing to this discussion. It also shows the resting voltage to be 12.6v, not 12.4v.


what you're looking for is temperature compensated state of charge,
12.65 volts is a battery at 100% state of charge at 80°F temperature. I see your location is Ontario....
the colder the battery the lower the open circuit voltage but not down to 12.4 volts though. at 30°F 100% state of charge is 12.588 volts. what might be happening is your meter is not that accurate and unless you calibrate to a known voltage source you don't really know.

check out http://www.batteryfaq.org/
 
Originally Posted By: friendly_jacek
Originally Posted By: meep


Maybe a little harsh? I hope you are not referring to my response on your thread. if you would like further info, here's an easy read, http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf



No, I was not referring to you. Thanks for the "easy read" link. The only problem is that the "easy read" doesn't actually deal with resting voltage vs SOC. Go ahead and check it again.


dude. it's the grey line on page 67. ???

it also points out that voltage alone is not an accurate way to check SOC, which I would think directly deals with the very point--- it's a flawed method for final accuracy, yet it is the specific method we are using. it also points out that SOC-dictated voltage will vary with temperature. though it does not graph that, it does go on to say "by as much as .5V"

a guy tries to help.....
smirk.gif
 
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Originally Posted By: George7941


The article meep linked to adds nothing to this discussion. It also shows the resting voltage to be 12.6v, not 12.4v.


this thread <> manners.

1. article discusses that voltage alone is not a good indicator.
2. article mentions that temperature affects accuracy of SOC by voltage measurement.
3. a casual analysis would also suggest that the entire spectrum of no-load SOC measurement happens inside of ~1.1 V. any parasitic loads will play in impacting interpretation. add in other factors, such as temp... chemistry (why do automatic chargers have different switches for flooded, sealed, deep cycle, etc?). this further suggests that voltage alone is not an ideal indicator of SOC.

I'd counter, that the article darn nears answers the whole question.

#whatarewesmoking
 
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An OCV of 2.4V is good for a 5 year old battery. What was the OCV of the battery when it was new, and fully charged?
 
From batteryfaq.org:

"........The OCV readings for a battery at 100% SoC will vary by plate chemistry, so if possible, check the battery manufacturer's specifications for their State-of-Charge definitions for the battery being measured. Depending on the plate chemistry, the Open Circuit Voltage can range from 12.22 to 13.00 for a fully charged wet Low Maintenance (Sb/Ca) or Standard (Sb/Sb) battery at 80° F (26.7° C). Deep Cycle batteries tend to have higher voltages than car batteries..........."
 
Originally Posted By: friendly_jacek
George, any follow up on yours? I replaced mine under warranty.


My battery is still performing well, in its fifth year of life. I will get at least eight years out of it. I monitor its health with a Conductance Tester and it is losing its CCA very slowly, hence my eight year projection.
 
Originally Posted By: SubLGT
An OCV of 2.4V is good for a 5 year old battery. What was the OCV of the battery when it was new, and fully charged?


I don't know because I did not check. At that time I did not have a ScanGauge.

My ScanGauge underreads voltage by 0.4V, so it actually shows the battery resting voltage to be 12.0v. I have an accurate bench multimeter, so I am confident in the accuracy of my figures.
 
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