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Battery Sulphation


Chris Hobson
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Another battery topic close to my heart - sulphation (or sulfation if you are a Yank) of lead acid batteries.

First let us look at the chemistry. The anode (positive plate) of a lead is a made up of lead dioxide (PbO2). The cathode (negative plate) is made up spongy lead (Pb). Both of these materials are soft and are mechanically supported by a grid made of a lead- antinomy alloy, a lead - calcium alloy or a lead -antimony selenium alloy. Each with their own inherent pros and cons but that is not the focus of my post. 

Let us deal with the oxidation reaction at cathode first 

Lead (Pb) reacts with sulphuric acid in the electrolyte as follows:

        Pb(lead)  + H2SO4 (sulphuric acid)  => PbSO4 (lead sulphate)+ 2H+ (hydrogen ions)  + 2e(electrons).

These electrons then accumulate on the cathode(-) and provide the "extra" electrons for current flow.

At the anode the reduction reaction at the anode(+) is

PbO2 (lead dioxde)  + H2SO4 (sulphuric acid)  + 2e- (electrons)  + 2H+ (hydrogen ions)  => PbSO4 (lead sulphate)  + 2H2O (water).

Note the electrons for this reduction reaction pass from cathode to anode through your inverter powering your TV so you can watch the rugby (you wouldn't be watching the cricket now would you!). The hydrogen ions (H+) needed for this reaction migrate through the electrolyte from the cathode (-) to the anode(+). That is why the sheets that separate the plates in a battery need to be porous.

Interesting to note that the final product during discharge at both anode and cathode is lead sulphate (PbSO4). A completely discharged battery in a perfectly balanced theoretical battery would be contain only lead sulphate and water - an amazing thought.

These reactions are reversed by the application of charge turning the lead sulphate back into lead and lead oxide at the cathode and anode.

If the battery suffers from chronic under-charging the lead sulphate starts to crystallise and harden reducing active material both lead and electrolyte for the battery. Prevention is occasional long charging periods (14 - 16 hours). it has been found that the same batteries in golf carts last longer than those in motorised wheelchairs since the golf cart generally gets a full charge overnight. I with my off-grid system cannot connect to grid for that length of time. Any pointers from folk in the industry? Is a full daily charge to SOC of 100% sufficient to prevent crystallisation?

I have heard folk charging their batteries with grid power rather than wait for solar in the morning to prevent sulphation. I think they are missing the boat the crystals don't grow that quickly. The small crystals are broken down with charge it is the larger hard crystals that are a problem. 

 

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