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Author Topic: Lead-acid batteries - voltage vs temperature  (Read 5030 times)
johnd
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« on: November 22, 2010, 02:23:13 PM »

Does anyone here have access to an authoritative book on lead-acid battery characteristics?

I'm try to establish how the no-load off-charge voltage varies (say at 100% SoC) with temperature for say a standard flooded battery. In my ignorance, I had imagined that this was straightforward, well-established information. But browsing around the web, there seems to be huge disagreement. Some sources seem to suggest that the 100% SoC voltage increases with increasing temperature. Other sources suggest quite the reverse, ie that there's a negative temperature coefficient. And then the third view, is that temperature has relatively little (essentially negligible) effect on this parameter. Anyone know what the truth might be - it must be out there somewhere?

Note that I'm not asking about temperature effects on charging voltage nor discharge capacity nor recommended storage temperature - all of which are more clearly documented. But temperature vs open-circuit voltage seems to have confused most web-page writers.
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Justme
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« Reply #1 on: November 22, 2010, 06:02:10 PM »

My understanding with cold was that the V was not affected (at 100% soc) but the ability for the chemical reaction to take place was reduced so under loads / after discharges the V will drop (more than normal for the amount of load compared to the bank true size) to represent the available SOC at that temp for the cold banks relative size. Discharge a bat to say 80% soc when cold then warm the bat & the SOC will be the same at 80% but the total capacity will be higher as its 80% of the true capacity.
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johnrae
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« Reply #2 on: November 22, 2010, 06:20:53 PM »

The electro-chemical potential (otherwise known as the cell voltage) between lead and lead-oxide (the plate materials) is a physical constant determined by the molecular structure of the two metals and is to all effects and purposes independant of temperature. As Justme states however the chemical reaction of current flow is dependant on temperature.
jack
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rogeriko
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« Reply #3 on: November 22, 2010, 08:22:31 PM »

Should be enough information here to make your head spin

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
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johnd
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« Reply #4 on: November 22, 2010, 09:27:57 PM »

Should be enough information here to make your head spin

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Yes but sadly that's just another resource that doesn't - as far as I can see - answer or address the specific question I asked.
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billi
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« Reply #5 on: November 23, 2010, 05:12:07 AM »

Quote
Yes but sadly that's just another resource that doesn't - as far as I can see - answer or address the specific question I asked.

my battery has lesser capacity in winter  , cause colder , but enough charge from three renewable ideas to live with that  hysteria

thats how i addressed that specific question  Grin

billi
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johnrae
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« Reply #6 on: November 23, 2010, 09:59:10 AM »

In the event of information not being available do what every enquiring mind has done since the dawn of time --- answer your own question.

Fully charge a battery until you are convinced it's at 100% soc.  leave for about 6 hours to stabilise at room temperature, measure voltage.

Place in fridge, leave for several hours, measure voltage

Bring back into room temperature, leave for several hours measure voltage

Set freezer to -10C, place battery inside, leave for several hours  etc etc

Draw yourself a graph of terminal voltage versus temperature and become an instant source of knowledge

jack
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billi
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« Reply #7 on: November 23, 2010, 11:40:00 AM »

Jack

this graph seems right in my eyes

But sure  would be intresting to see the results from your test  Smiley



Billi


* Discover_temperature_effects_charging.jpg (116.86 KB, 709x489 - viewed 3505 times.)
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EccentricAnomaly
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« Reply #8 on: November 23, 2010, 01:43:12 PM »

Billi, the question is about voltage, not capacity.
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billi
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« Reply #9 on: November 23, 2010, 01:49:31 PM »

...... i know  Cheesy


* Discover_temperature_effects_charging.jpg (43.73 KB, 684x133 - viewed 2563 times.)
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Fintray
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« Reply #10 on: November 23, 2010, 01:50:34 PM »

I think Billis' graph answers the question asked as it shows voltage increasing along with increasing temperature.
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EccentricAnomaly
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« Reply #11 on: November 23, 2010, 05:57:51 PM »

...... i know  Cheesy

Oops, yes, so you do, sorry.
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johnd
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« Reply #12 on: November 23, 2010, 06:13:49 PM »

this graph seems right in my eyes

Thanks - that's useful and looks like the data has been carefully collected. But is there a source reference please and/or any explanation of where the data came from? And any indication of whether it was a flooded battery, AGM etc?

JGD
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Amy
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« Reply #13 on: November 23, 2010, 06:29:54 PM »

There is only a 0.4 v difference across the 60 deg range. Hardly seems worth thinking about.



Temperature Effects on Batteries

Battery capacity (how many amp-hours it can hold) is reduced as temperature goes down, and increased as temperature goes up. This is why your car battery dies on a cold winter morning, even though it worked fine the previous afternoon. If your batteries spend part of the year shivering in the cold, the reduced capacity has to be taken into account when sizing the system batteries. The standard rating for batteries is at room temperature - 25 degrees C (about 77 F). At approximately -22 degrees F (-27 C), battery AH capacity drops to 50%. At freezing, capacity is reduced by 20%. Capacity is increased at higher temperatures - at 122 degrees F, battery capacity would be about 12% higher.

Battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell (16.4 volts) at -40 C to 2.3 volts per cell (13.8 volts) at 50 C. This is why you should have temperature compensation on your charger or charge control if your batteries are outside and/or subject to wide temperature variations. Some charge controls have temperature compensation built in (such as Morningstar) - this works fine if the controller is subject to the same temperatures as the batteries. However, if your batteries are outside, and the controller is inside, it does not work that well. Adding another complication is that large battery banks make up a large thermal mass.

Thermal mass means that because they have so much mass, they will change internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10 degrees over 24 hours internally, even though the air temperature varies from 20 to 70 degrees. For this reason, external (add-on) temperature sensors should be attached to one of the POSITIVE plate terminals, and bundled up a little with some type of insulation on the terminal. The sensor will then read very close to the actual internal battery temperature.

Even though battery capacity at high temperatures is higher,  battery life is shortened. Battery capacity is reduced by 50% at -22 degrees F - but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures - for every 15 degrees F over 77, battery life is cut in half. This holds true for ANY type of Lead-Acid battery, whether sealed, gelled, AGM, industrial or whatever. This is actually not as bad as it seems, as the battery will tend to average out the good and bad times. Click on the small graph to see a full size chart of temperature vs capacity.

One last note on temperatures - in some places that have extremely cold or hot conditions, batteries may be sold locally that are NOT standard electrolyte (acid) strengths. The electrolyte may be stronger (for cold) or weaker (for very hot) climates. In such cases, the specific gravity and the voltages may vary from what we show.

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