Alum battery conversion...WHICH ALUM TO USE

[lcstier]

After reading many posts on this forum and others I am confused
on which Alum to purchase to convert batteries.
After reading last week I purchased some Laboratory Reagent:
Aluminium ammonium sulfate AlNh4(SO4)2 which I thought
would be the best. But after reading and watching videos (JB)
on the 1951 battery conversion I'm not sure the ALum I purchased is the best
product. I would appreciate any info on the best product to purchase,
not just one that will work.
A big thanks in advance.
Linnie Stierstorfer

[Tom C]

Alum Powder - Used for Pickling, baking and as a preservative - My Spice Sage

this is what I have used. alum powder for baking.

Tom C

[D Rhodes]

If anyone looks at this old post, I have been using Aluminium sulphate (O12-S3.2Al)with good results.
I think the one Tom used is the Potassium aluminium sulphate type.

[James Milner]

As far as I can remember JB used aluminium ammonium sulphate. But said that aluminium sulphate is just as good.
I experimented on a number of sulphates. Sodium sulphate works as well. PH is acidic and I suspect over time can soften the lead paste plates, but may be useful on dealing with stubborn sulphation.

The aluminium sulphate has some special hydrolysis properties. PH can be mildly alkaline to mildly acidic. Although it generally is acidic. It can form very large complexes hence transport large quantity of charges with each particle. The ammonium ions I believe form aluminium complexes of slightly different shape but the sulphate will carry more charge. The chemistry relating to the aluminium complexes is very complicated but there are some publicly available scientific papers available if you are really interested in the detail.
My advice is charge your batteries with the sg and recondition as best you can before you substitute the acid for alum. Expect to lose about a volt off the standing voltage and exercise the batteries. DO NOT USE TAP WATER to mix your alum. It introduces other metals which contaminate your lead plates. Use distilled water.