Batteries - self-discharge vs sulphation

[Col_T]

I suppose it serves me right for thinking about this, but I am a little confused.

What, if any, is the connection between batteries self-discharging and sulphation? By example, is sulphation a direct result of a battery self-discharging e.g. the chemical reaction that 'stores' electricity is sufficiently weak that it breaks down over time? Another, is the apparent drop in voltage the result of sulphation reducing the surface area of the plates?

[Richard10002]

I an no expert, but  would say they are 2 distinctly different things.

i.e. A battery at full original capacity could self discharge at a high rate, just as badly as the same battery if it were badly sulphated.

and a "better" battery at full original capacity could self discharge at a very slow rate, as could the same battery if badly sulphated.

 

Having said that, a battery which is allowed to self discharge, and remain at a state of charge significantly below 100%, will sulphate, just like a battery in normal use and not charged often enough, and to 100% on a regular basis. In the self discharge case, this is not a function of the self discharging, it is a function of the state of charge. 

[MtB]

As I understand it self-discharge is a trivial effect you cannot prevent but over long periods of time, it becomes significant. 

An old, sulphated battery tends to self-discharge more quickly than a battery in good condition. And I guess a battery stored for a long time (i.e. months or years) will self-discharge to the point where sulphation is occurring.

I think about 1% of battery capacity per month (or less) is the order of magnitude of the self-discharge effect so not relevant on batteries in regular use.

 

[cuthound]

No connection whatsoever.

Self discharge increases as the battery ages, probably due to the active material gradually being shed from the plates and providing a parallel path between the plates,  thus discharging them. 

At ambient tempersturws of 20°C it is typically about 3-4% per month for a new battery,  rising to 6% on one approaching it's end of life.

Self discharge increases when batteries are kept at higher ambient temperatures. 

Sulphation is the result of not enough charging, the sulphur that should be in the sulphuric acid remains as lead sulphate on the negative plates, and gradually hardens as the lead sulphate crystal size increases and becomes less convertible the longer the charging is delayed, evenutally becoming completely unconvertable. 

The drop in voltage across the battery is purely a result of the battery being in a partially discharged state. 

Edited March 19, 2017 by cuthound
To add the last sentance.

[Tony Brooks]

In general I agree with Mike but I do not think he is correct in saying a sulphated battery discharges more than an unsulphated one although it may seem to be true.

As batteries age (sulphated or not) the plates shed material that is conductive. This builds up in the bottom of the battery and will eventually lead to internal short circuits, very low at first but gradually getting greater as more is shed. This means an older bate try will tend to self discharge more than a newer one that has not shed material. Older batteries are very likely to be more sulphated than newer ones and their actual capacity will show this but the greater self discharge is more likely to be caused by internal shorts.

Another factor to consider is that sulphate has a greater volume than the original lead oxide so the more a battery sulphates the "thicker" the sulphated plates. This pushes on the separators so over time could puncture one and again cause an internal short circuit. This is more to do with mechanical damage than sulphation but its arguable. This greater volume is why a sulphated battery will often have its ends bulging out.

Now the OP's question.

Self discharge discharges the battery (as one would expect) and as any sort of discharge converts one of the lead oxides into lead sulphate self discharge will result in lead sulphate on one set of plates. Now, if you got to the battery soon enough and recharged it most or all of the lead sulphate would be reconverted to a lead oxide so the net result is no significant sulphation. However self discharge only becomes significant when the batteries are out of use or left discharged for a length of time before charging - like over the winter months. Did the self discharge cause the sulphation or did the lack of prompt charging allow the sulphate to harden sufficiently so it could not be re-converted. I would suggest the latter but once again its arguable.

[Col_T]

Thanks chaps, that's cleared that up for me.