Lead sulfate crystals accumulate on negative electrodes of lead-acid batteries as we discharge them. This lead sulfation largely reverses out when we recharge them again. However, a small residue remains behind and becomes permanent as it hardens. We discuss ways to slow this down so the battery lasts longer than it otherwise perhaps would.
How the Discharge Rate Affects Lead Sulfation
Lead sulfate crystals spread throughout the cross section of the electrode plate. This has a sponge-like consistency allowing the diluted sulfuric acid electrolyte to permeate. In this way, the reaction occurs throughout the plate, which is most efficient.
However, something different happens if we discharge the battery at a very fast rate. In this case, the suffused electrolyte consumes at a faster rate than the electrode can allow through. The reaction thus concentrates on the outer wall of the electrode where the electrolyte can replenish in time. This may cause dense lead sulfation that permanently diminishes the performance of the inner part of the electrode.
How This Situation Progressively Worsens
The lead sulfation crystals grow larger as their surface area increases. It becomes increasingly difficult to break them down by recharging. Especially since the diluted acid solution remains strong, because less of it can reach the inner part of the electrode plate.
Battery scientists call this phenomenon hard lead sulfation. The crystals progressively insulate the electrode from the electrolyte, thereby reducing battery power, capacity, and efficiency. This situation is largely irreversible owing to secondary side effects that develop.
It follows from this analysis we should only discharge lead acid batteries at the right rate. We can achieve this by matching batteries to devices, and refreshing them frequently so they remain between 80% and 100% charged. However, note this will only slow down the lead sulfation rate. No battery lasts forever because of the chemistry inside.
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Preview Image: Lead Sulfate
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