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Specific Gravity Discrepancies


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9 hours ago, WotEver said:

The acid strength is a balance between being high enough to give the required voltage and maximum capacity from the lead/antimony (or lead/calcium or whatever) on the plates while still being low enough to provide good ionic activity. @Dr Bob can probably provide a better description of the latter. I too had a rubbish chemistry teacher who could barely speak English. 

 

7 hours ago, Mike the Boilerman said:

Ok a brief goggle reveals the SG of H2SO4 is 1.840. And an acid/water mix of density 1.277 contains about 30% H2SO4. 

 

 

Quite right Mike. Conc sulphuric acid has the sg of 1.840 and battery acid is diluted down with water. The more water, the weaker the acid and the lower the SG.

As you discharge a battery, the equations above come into play and part of the acid is converted to lead sulphate so the liquid left is weaker acid (not as many SO4 ions left) so lower SG. As the battery is charged, the lead sulphate is turned back to acid so the SG goes up.

Not a clue about the variation in SG of acid that is sold for batteries. I agree the sellers will make a load more profit by adding water. Perhaps there are other chemical reactions going on to increase SG when a battery is first 'wetted'. Are the plates coated with acid during manufacture so less concentrated acid is needed to be added. Clutching at straws here.

 

What I cant understand is why Wotever keeps putting me down on anything about SG's and then invites me to comment on this thread.?

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There's a good description of equalisation in The Battery FAQ:

 

http://jgdarden.com/batteryfaq/carfaq9.htm#equalization

 

The optional EQUALIZATION stage is a controlled 5% to 10% absorption overcharge to equalize and balance the voltage and specific gravity in each cell. Equalizing can reverse the build-up of the chemical effects like electrolyte stratification where acid concentration is greater at the bottom of the battery. It also helps remove sulfate crystals that might have built up on the surface or in the pores of the plates.

 

Bearing in mind the important safety precautions when handling lead acid batts, when EQ is being done you'd want to see the all the batt cells bubbling nicely by the end of the EQ charge.

 

I do think the Trojan type batts need effective monitoring and charging to get the best from them; it does seem that sometimes people half hope they can get away with less, but often it doesn't work out well enough for them in the long term. Maybe reasonable quality leisures would give a bit better result overall in this case, or in the long term lithiums will become economical enough.

 

Perhaps part of the problem is that a canal boat is more of a hobby that competes with many other interests, and not so critical to survival like a blue water sailboat is.

 

Edited by smileypete
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4 hours ago, Dr Bob said:

What I cant understand is why Wotever keeps putting me down on anything about SG's and then invites me to comment on this thread.?

Specifically ‘ionic activity’...

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1 minute ago, WotEver said:

All I learned in chemistry class was that if you squirted dilute H2SO4 from a pipette onto a girl’s legs, gurt big ‘oles appeared in her tights...

 

 

All I learned was that chemical equations don't follow a consistent, stable and reliable set of rules, but rather do something different from what I thought they should, EVERY fekkn time.

 

One would think I'd get some right by chance, but noooooo...... 

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3 minutes ago, Mike the Boilerman said:

All I learned was that chemical equations don't follow a consistent, stable and reliable set of rules, but rather do something different from what I thought they should, EVERY fekkn time.

 

One would think I'd get some right by chance, but noooooo...... 

That was actually my biggest problem with chemistry. With physics or maths you could deduce a result with intuition. With chemistry you had to know what would happen. You had to remember reams of valancies, atomic weights and more. It never ‘clicked’ with me at all.  

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4 minutes ago, WotEver said:

That was actually my biggest problem with chemistry. With physics or maths you could deduce a result with intuition. With chemistry you had to know what would happen. You had to remember reams of valancies, atomic weights and more. It never ‘clicked’ with me at all.  

 

Yes. The master would say 'oh no, X would pair with Q not Z in this equation, and I'd say why? How do we know that? He would say well it just does, now do it again.

 

Grrrr....

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5 hours ago, Mike the Boilerman said:

 

Actually, I'm looking forward to that nice Dr Bob explaining why X pairs with Q not Z.....

 

This would be BRILLIANT if he can!

 

Dr Bob.... HELLO????

Hello!!!!!

 

Didnt you know. Inorganic chemistry is both boring and meaningless. You should instead try organic synthesis reation pathways. Far more fun. That's how you make interesting things.

(ps ... I couldnt find the answer on google)

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6 minutes ago, Dr Bob said:

Hello!!!!!

 

Didnt you know. Inorganic chemistry is both boring and meaningless. You should instead try organic synthesis reation pathways. Far more fun. That's how you make interesting things.

(ps ... I couldnt find the answer on google)

 

I have a chemist friend whose specialist subject is 'excitable materials'. He blows stuff up for a living, YAY!

 

Is that what you mean?

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6 minutes ago, Mike the Boilerman said:

 

I have a chemist friend whose specialist subject is 'excitable materials'. He blows stuff up for a living, YAY!

 

Is that what you mean?

No. It was more along the lines of making things like lysergic acid diethylamide.

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4 minutes ago, Dr Bob said:

I've been gone a long time

Good trip? 

4 minutes ago, Dr Bob said:

I've been gone a long time.

They use it as a plasticiser you know.

I've got a scientific paper on plasticiser somewhere if you are interested

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Just found the following text on another website <http://www.batterysupplies.be/en/products/starter-batteries>

 

The starting battery is designed to deliver large bursts of power for a short time, as is needed to start an engine. Once the engine is started, the battery is recharged by the engine-driven charging system. Starting batteries are intended to have a low depth of discharge on each use. They are constructed of many thin plates with thin separators between the plates, and may have a higher specific gravity electrolyte to reduce internal resistance.

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On 23/07/2018 at 23:25, Mike the Boilerman said:

So why is a fully charged battery 1.277 SG? Is there something special about this particular value? Is it standard neat acid as opposed to concentrated or diluted acid?

 

Chemistry defeats me. I blame the teachers. ;)

 

Depending upon the intended use (and design life) different lead acid batteries have different "fully charged" specific gravities.

For example the large lead acid batteries common in telephone exchanges in the 1950's to 1970's had and original fully charged sg of 1.215. They were designed to last at least 25 years and to  be fully discharged every 2 years to assess capacity.

 

Higher sg's equate to more energy dense (smaller, lighter) batteries, but with higher plate corrosion and shorter life.

 

The concentrated suphuric acid we used to use to make up acid to fill various types of lead acid batteries had an sg of 1.75 (80% concentrate IIRC) and I believe even stronger sulphuric acid was available.

 

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