Restore a Battery with Epsom Salt?

It can buy a few more months of service, according to Battery University.

"Adding chemicals to the electrolyte of flooded lead acid batteries can dissolve the buildup of lead sulfate on the plates and improve the overall battery performance. This treatment has been in use since the 1950s (and perhaps longer) and provides a temporary performance boost for aging batteries. It’s a stopgap measure because in most cases the plates are already worn out through shedding. Chemical additives cannot replace the active material, nor can cracked plates, corroded connectors or damaged separators be restored with an outside remedy.

Extending the service life of an aging battery can be useful as additives are cheap, readily available and worth the experiment for a handyman. These salts may reduce the internal resistance to give a sulfated battery a few extra months of life. Suitable additives are magnesium sulfate (Epsom salt), caustic soda and EDTA (EDTA is a crystalline acid used in industry).
When using Epsom salt, follow these easy steps to treat most starter batteries. Heat about 250ml (8 fl oz or a cup) of distilled water to about 66ºC (150ºF), mix in as much Epson salt as the water can absorb (a few tablespoons) and stir until dissolved. Avoid using too much salt because heavy concentration increases corrosion of the lead plates and the internal connectors.
When pouring the warm solution into the battery, the electrolyte level will raise. Do not remove electrolyte, and only add as much additive as the battery can take. Be careful not to overfill. Do not place un-dissolved Epsom salt directly into the battery because the substance does not dissolve well. In place of Epsom salt, try adding a pinch of caustic soda. Charge the battery after service. The results are not instantaneous and it may take a month for the treatment to work. The outcome is not guaranteed.
Batteries have improved, and additive treatments may be most effective with older battery models, expanding their life by a few months until a replacement is on hand. Modern batteries already include additives that reduce sulfation and corrosion. Industrial users seldom rely on remedial additives to prolong battery life as the system becomes maintenance prone."

Unfortunately, and despite how some will scream otherwise, batteries of all types are consumable items. They wear out, plain and simple.

Better matiancadnand general treatment in use will extend their life, but that action has a relative added cost to it that very few battery snobs will ever admit to.

Most good quality lead acid starting or deep cycle application batteries have atypical service life of ~ 5 years if not more when maintained to bare minimal standards. The same batteries, even when maintained in the same working conditions with optimal maintenance, typically only show fractional gains in service life.

Some place online, there are a number of reports from studies of such batteries in such applications; the overall consensus tended to show that the life expectancy variance between basic maintenance and high level maintenance practices was less than 5 - 10% at best (which typically the gained life expectancy did not cost justify the added work and or gear required to get it).

Basically, adding the cost of several work hours and $100+ worth of gear to a $200 or less new battery or set of them to get 10% or even less extra life out, it didn't make economic sense.

Personally, I played with the try to rejuvenate batteries for years. Rarely did it do anything but give me false hope, only to then leave me stranded someplace needing at least a jump start to get home or to a store to buy a new battery (knowing full well I just screwed myself out of more time and inconvenience than that bit of battery life I gained back was worth).

Now, when a battery in a critical application starts giving me trouble, it gets replaced with a newer one (usually larger and higher quality than the last one at that) and the questionable one gets repurposed to a less critical application like one of the old farm tractors or other machines where a dead battery is a minimal issue to deal with and some old junk battery that wasn't good enough to even work in those applications any more goes in for the core exchange on the new battery.

At most, during the transfer from the critical application to the less so one, the battery gets a good 2 -3 day hard charge (15 - 16 volt slow cook overcharge) to top it off and desulfate what will go willingly and that's it.