There was a question, a few posts back, about why a battery's charge level makes a difference in how quickly a battery
sulfates. I'm going to try and answer that question in layman's terms.
How well charged a battery
is and how quickly it forms battery
killing sulfate is related to the chemical and physical properties of lead, which is a "multivalent" metal that has multiple forms with subtly different properties.
Lead(II) (the dominant form) and lead(IV) (more exotic) are both soluble in strong, concentrated acids, but their solubility drops off as the acid becomes more dilute. Lead (IV), the version of lead that's responsible for battery sulfation, is much less common than lead (II) but it's also more sensitive to the acid concentration.
The chemical reaction that stores and releases the a battery's electrical
charge creates or consumes acid from the battery water. As you charge a lead/acid battery, you convert lead(II) sulfate and water into lead and sulfuric acid. When you discharge a lead/acid battery, you convert water, sulfuric acid and both
lead(II) and lead(IV) into lead(II) sulfate (the good kind) and lead(IV) sulfate (the bad kind).
This is how electricity is released and stored in your battery. When your battery makes lead(II) sulfate, it binds one atom of lead(II) and one sulfate "ion" from the battery acid to release two electrons. In the process, it makes lead(II) sulfate.
The nice thing about lead(II) sulfate is it is pretty easy to reverse this reaction by simply pumping two electrons back into your battery to break the union up and restore charge to your battery.
As I said earlier, there are different forms of lead. Most of the lead in your battery tends to form lead(II), but some of it becomes lead(IV), and lead(IV) can also bind to sulfate ions and release electrons. When your battery makes lead(IV) sulfate, it binds one atom of lead(IV) and two
sulfate ions to release four
Reversing the lead(IV) sulfate reaction is much more difficult, because the electrons have to "attack" two connections, or bonds, between the lead and sulfates simultaneously. It's about seven times harder to convert lead(IV) sulfate back into lead and acid as it is to convert lead(II) sulfate back. (For engineering geeks, the ratio e²:e⁴.)
Now here's the tricky part. As you use up the charge stored in your battery, the acid in your battery water becomes more dilute, and that increases the likelihood that a lead(IV) atom, which prefers more concentrated acids, will be used to make bad-boy lead(IV) sulfate. The more charge you use, the more likely it is that lead(IV) sulfate will form.
When your battery is really, really discharged, things get even worse. Starting at 20-40% of full capacity (depending on battery construction and additives the manufacturer has added to the battery water to prevent it), the battery acid is so dilute that lead(IV) really, really wants to get out of solution, so desperate that it'll actually force two lead(II) sulfates to revert back to their non-sulfate form and donate their sulfate structures to the lead(IV) to, forming bad-boy lead(IV) sulfate, greatly accelerating the process of sulfating your battery.
This is, very obviously, a technical topic, and I've necessarily glossed over a lot of details, but it is an accurate description of what's going on inside your battery. I'd love to know how well people followed what I was saying.