Do it Yourself: How to clean the battery terminals

Whether the auxiliary battery or house battery they are invariably charged from the engine generator or alternator. As yours is 1992 it is probably an alternator.

The charging of the second battery could be wired through a switch but is most commonly wired through a relay that is connected to and energised by the ignition light. This ensures the second battery is isolated when the engine isn't running and when started and the ignition light goes out the relay connects the second battery.

This effectively connects the two batteries together and while some charging of the second battery takes place it usually isn't a great success due to several factors, firstly the second battery is usually physically distant and so the connecting wires are longer, often much longer. The age and capacity of the second battery is also a major factor and finally the amount of care lavished on the second battery is usually very little and so it likely spends a great deal of time, especially during winter, less than fully charged and so a reduced capacity due to sulphation is inevitable.

To compound matters, perhaps the alternator is no longer producing the rated output.
Food for thought...

It would help if you had provided more details of the publication. There are many claims online of people "reconditioning" lead acid batteries - car and truck batteries.

There is only one way to recondition a battery and that is to dismantle it, clean the parts and examine them and rebuild with whatever new parts are needed, fill with fresh acid and recharge.

Most online reconditioners are really just using kerbside mechanic methods to extend the life of a failing battery. Mostly these methods are variations on emptying and washing out the battery and replacing the electrolyte - in one case with pickling vinegar, and then recharging.

Not all batteries can be saved. Those that have a damaged and short-circuited cell, those that are heavily sulphated or those with heavily degraded plates due to excessive mechanical shock or vibration...

the sulphuric acid reacts with the lead in the plates with the result that the acid breaks down and tends to go water and the lead becomes lead sulphate
when a battery is dead flat, the plates are all lead sulphate and the density of the acid is approaching that of water
that can be tested with a hydrometer
When a charge is applied to the battery terminals , the process is reversed and the lead sulphate returns as lead and the water ( low ph acid) returns to sulphuric acid

This is a high charging on the battery, as it is overcharged . this can be failure of the alternator rectifier or the regulator.
so use a voltmeter and confirm the voltage range to rectify the fault.
A high charging of this nature can damage the battery in the long run.

Sulfuric Acid goes in your battery. Could this be what you were after? It is available at any auto parts store

'Grows up' is a funny phrase. The only thing I can think that grows in a car is Lead Sulphate crystals that grow on a battery from mising lead and sulphuric acid