Lead sulfation actually starts when you remove the charging voltage of a fully charged lead-acid battery. The lead sulfate crystals are converted back to lead during the normal charging cycle. The real question is, if all of the lead sulfate crystals are not turned back into lead, how long does it take before they become so hard that they can not be converted? The answer is that varies–it could be weeks or months and depends on a number of factors such as the quality of the lead, temperature, plate chemistry, porosity, Depth-of-Discharge (DoD), electrolyte stratification, and many more factors.
During the normal discharge process, lead and sulfur combine into soft lead sulfate crystals are formed in the pores and on the surfaces of the positive and negative plates inside a lead-acid battery.
When a battery is left in a discharged condition, continually undercharged, or the electrolyte level is below the top of the plates or stratified, some of the soft lead sulfate re-crystallizes into hard lead sulfate. It cannot be re-converted during subsequent recharging. This creation of hard crystals is commonly called permanent or hard “sulfation”.
When it is present, the battery shows a higher voltage than its true voltage; thus, fooling the voltage regulator into thinking that the battery is fully charged. This causes the charger to prematurely lower its output voltage or current, leaving the battery undercharged.
The longer sulfation occurs, the larger and harder the lead sulfate crystals become. The positive plates will be light brown and the negative plates will be dull, off white. These crystals lessen a battery’s capacity and ability to be recharged. This is because deep cycle and some starting batteries are typically used for short periods, vacations, weekend trips, etc., and then are stored the rest of the year to slowly self-discharge. Starting batteries are normally used several times a month, so sulfation rarely becomes a problem unless they are undercharged or the plates are not covered with electrolyte.
As a consequence of parasitic load and natural self-discharge, permanent sulfation occurs as the lead-acid battery discharges while in long term storage. (Parasitic load is the constant electrical load present on a battery while it is installed in a vehicle even when the power is turned off. The load is from the continuous operation of appliances, such as a clock, security system, maintenance of radio station presets, etc.)
While disconnecting the negative battery cable will eliminate the parasitic load, it has no effect on the natural self-discharge of a car battery. Self-discharge is accelerated by temperature. For batteries that are over 77° F (25° C), the self-discharge rate doubles with a 18° F (10° C) rise in temperature.
Thus, sulfation can be a huge problem for lead-acid batteries not being used, sitting on a dealer’s shelf, or in a parked vehicle, especially in HOT temperatures.