Battery Reconditioning Tools

Specific Gravity is defined as the ratio of the density of a solution to the density of water, typically measured with a hydrometer. By definition, water has a specific gravity of one. In a lead-acid battery, the electrolyte is a diluted solution of sulfuric acid and water. In a fully charged battery, the electrolyte is approximately 36% sulfuric acid by weight, or 25% by volume, with the remainder water.

State of Charge : Looking inside Batteries

The specific gravity of the electrolyte is related to the battery state of charge, depending on the design electrolyte concentration and temperature. In a fully charged flooded lead-acid battery, the specific gravity of the electrolyte is typically in the range of 1.250 to 1.280 at a
temperature of 27 oC, meaning that the density of the electrolyte is between 1.25 and 1.28 times that of pure water. When the battery is discharged, the hydrogen (H+) and sulfate (SO4 2-) ions from the sulfuric acid solution combine with the active materials in the positive and negative plates to form lead sulfate (PbSO4), decreasing the specific gravity of the electrolyte. As the battery is discharged to greater depths, the sulfuric acid solution becomes diluted until there are no ions left in solution.

At this point the battery is fully discharged, and the electrolyte is essentially water with a specific gravity of one. Concentrated sulfuric acid has a very low freezing point (less than -50 oC) while water has a much higher freezing point of 0 oC. This has important implications in that the freezing point of the electrolyte in a lead-acid battery varies with the concentration or specific gravity of the electrolyte. As the battery becomes discharged, the specific gravity decreases resulting in a higher freezing point for the electrolyte.

Adjustments to Specific Gravity

In very cold or tropical climates, the specific gravity of the sulfuric acid solution in lead-acid batteries is often adjusted from the typical range of
1.250 to 1.280. In tropical climates where freezing temperatures do not occur, the electrolyte specific gravity may be reduced to between 1.210
and 1.230 in some battery designs. This lower concentration electrolyte will lessen the degradation of the separators and grids and prolong the battery's useful service life.

However, the lower specific gravity decreases the storage capacity and high discharge rate performance of the battery. Generally, these factors
are offset by the fact that the battery is generally operating at higher than normal temperatures in tropical climates.

All batteries will fail at some point, when they have been in use for longer periods of time. But premature battery failure is one of the main frustrations people face. Common thinking is that turning devices off improve battery life, but what happens is the opposite. Devices consume more power when they are starting up, so turning them off is not such a good idea if they are going to be switched on frequently.

Sulfation is a normal process that occurs in lead-acid batteries "like car batteries" resulting from prolonged operation at partial states of charge. Even batteries which are frequently fully charged suffer from the effects of sulfation as the battery ages. The sulfation process involves the growth of lead sulfate crystals on the positive plate, decreasing the active area and capacity of the cell. During normal battery discharge,
the active materials of the plates are converted to lead sulfate. The deeper the discharge, the greater the amount of active material that is
converted to lead sulfate. During recharge, the lead sulfate is converted back into lead dioxide and sponge lead on the positive and negative
plates, respectively. If the battery is recharged soon after being discharged, the lead sulfate converts easily back into the active
materials.

However, if a lead-acid battery is left at less than full state of charge for prolonged periods (days or weeks), the lead sulfate crystallizes on the
plate and inhibits the conversion back to the active materials during recharge. The crystals essentially "lock away" active material and
prevent it from reforming into lead and lead dioxide, effectively reducing the capacity of the battery. If the lead sulfate crystals grow too large,
they can cause physical damage to the plates.

Sulfation also leads to higher internal resistance within the battery, making it more difficult to recharge. Sulfation is a common problem experienced with lead-acid batteries in many PV applications. As the PV array is sized to meet the load under average conditions, the battery
must sometimes be used to supply reserve energy during periods of excessive load usage or below average insolation. As a consequence,
batteries in most PV systems normally operate for some length of time over the course of a year at partial states of charge, resulting in some
degree of sulfation.

How to Minimize Premature Dead and Sulfation in Lead Acid Batteries

The longer the period and greater the depth of discharge, the greater the extent of sulfation. To minimize sulfation of lead acid batteries in
photovoltaic systems, the PV array is generally designed to recharge the battery on the average daily conditions during the worst insolation
month of the year. By sizing for the worst month's weather, the PV array has the best chance of minimizing the seasonal battery depth of
discharge.

In hybrid systems using a backup source such as a generator or wind turbine, the backup source can be effectively used to keep the batteries
fully charged even if the PV array can not. In general, proper battery and array sizing, as well as periodic equalization charges can minimize
the onset of sulfation.

If the plate of any cell inside the battery is sulfated, this sulfation increase the internal resistance of the cell causing an excess of current
flows through the cell. This increase in current, cause overheating and then a short circuit inside the cell. In VRLA with fire retardant case the
effect are shown in the picture above if the same problem was experienced in flooded sealed VRLA battery or car battery the results
will be an exploded battery and a lot of acid on the floor or in car engine.

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