[digitalsam]

I have an automotive chemistry question. A car battery is a lead Sulfur battery, over time when not used, the lead gets covered in crystals (Sulfation?)

The solution is to drain the battery  of the water and sulfuric acid solution, and add a thick solution of Epsom Salt (Magnesium Sulfate) and put the battery on a slow trickle charge.

My question is, once this is done, none of the tutorials online say that you need to add more sulfuric acid.

HOW WOULD IT BE POSSIBLE? Does a reaction occur that makes more sulfuric acid?

Any help or thoughts? I am not a chemist, be kind.

[Arkcon]

Actually, we have a thread on this topic, that does explain it more clearly:  http://www.chemicalforums.com/index.php?topic=67944.0  Briefly, a completely discharged lead-acid storage battery has the sulfate making up the plates.  http://en.wikipedia.org/wiki/Lead_acid_battery#Discharge  with almost no acid in the solution.  The magnesium sulfate is a good, cheap, reasonably pure (USP grade, which does not mean a pure reagent, but is close enough for this application) conductive electrolyte.  Now the discharged battery will conduct again, and a trickle charge will recharge the battery, converting the plates and creating sulfuric acid in the electrolyte. http://en.wikipedia.org/wiki/Lead_acid_battery#Charging  And yes, you should now dump out the mixed magnesium sulfate-sulfuric acid electrolyte and replace it with the proper sulfuric acid electrolyte.  You don't want the battery to be more conductive then normal, that will enhance auto-electrolysis, a side effect of the battery -- the battery will lose water, and generate hydrogen and oxygen.  And you don't want hydrogen close to a battery, an alternator, an engine full of fuel, your groceries, ... you, etc.

[Corribus]

I could be wrong, but I believe the working theory here is that if a lead sulfate battery becomes TOO discharged (or is discharged too many times, or it's been long since a recharge) excessive sulfate buildup renders the battery irreversibly unchargable.  Lead sulfate is a poor conductor, and if there is too much of it, the crystals just won't dissolve into solution, allowing the battery to be recharged.  (I.e., you've got to have SOME lead metal surface around to conduct the recharging current.)  By placing a metal sulfate in the electrolyte solution and running a recharging current through the battery, you effectively electroplate the electrode surface with metal (in this case magnesium, which is widely available).  This provides enough conductive surface to the electrode to facilitate the reverse electrochemical reaction and get the lead sulfate crystals to dissolve in solution, whereupon the battery can be recharged as normal.

Then you have to replace the magnesium sulfate with a normal acid electrolyte to facilitate the primary battery electrochemical reaction. 

Anyway, that's the explanation I've read.  I'm not sure how well it holds up to scrutiny.  Thoughts?

[Arkcon]

Briefly, many things can go wrong with the lead acid storage batter.  Certainly, excessive charging and discharging can buildup sulfate on the plates.  They're also prone to flaking off of material, due to shock and vibration, freezing of the discharged battery warping the plates, and shorting out of individual cells by buildup.  This magnesium sulfate trick seems like a good way to bring the battery back, but the conductive salts should be flushed out, and the electrolyte replaced, before we reuse the battery -- that's the bottom line.