Specific Gravity (SG) and State of Charge.

The charging and discharging of all lead acid batteries rely upon reversible chemical reactions between lead, lead dioxide, sulphuric acid, lead sulphate and water. The actual chemical reactions do not really matter for the purposes of this explanation but while we're here we might as well go the full mile. I have tried my hardest to word this document in the most user-friendly, unscientific language I can manage. The final, ultimate reactions are as follows......

(charging is in the direction to the left - discharging is in the direction to the right)

At the positive plate:-

PbO2 + 3H+ + HSO4- + 2e- <--> PbSO4 + 2H2O

At the negative plate:-

Pb + HSO4- <--> PbSO4 + H+ + 2e-

Note that these equations simply show the directions in which the reactions take place. For instance the top equation does not mean there will be no PbO2 (lead dioxide) in a partially discharged state. It means the direction of discharging (to the right) is converting the PbO2 (lead dioxide) on the left, ultimately, into PbSO4 (lead sulphate) on the right.

Also note that at certain times in between these two states there will be other chemicals present due to incomplete chemical reactions.

The 2 references to the 2e- are electrons that we add or remove during charging and discharging. These are intimately related with the equation and therefore the chemical reactions.

Look at the bottom equation. The right hand side represents the chemicals present at the negative plate when completely discharged. This shows the composition of the plate to be entirely PbSO4 (lead sulphate). The H+ shows there to be positive hydrogen ions available in the electrolyte.

Now look at the top equation. This shows the composition of the positive plate consisting entirely of lead sulphate (PbSO4) and the electrolyte (or rather what is left of it) consisting of pure water.

Now we all know that in order to charge a battery it is necessary to "force" positive current into the positive battery post. For historical reasons (the convention of direction of current flow was decided upon before

anyone actually knew what it was - as it happens they guessed the direction wrong - but the convention remains) what this actually means is that we remove electrons from the positive post.

We do this by connecting a source of electrons (a battery charger) between the positive and negative battery posts in such a manner that the source tries to force electrons into the negative post and remove them from the positive post.

Now refer back to the right hand side of the bottom equation. The PbSO4 (lead sulphate) and the H+ (Hydrogen ion) are already there. By charging we force electrons into the negative plate which adds more of the 2e- in the equation. Discharging produces these electrons which then flow out of the negative post - in order to recharge we need to introduce more electrons to force the chemical reaction the other way i.e. to the left. Forcing the chemical reaction to the left of the equation converts the Lead sulphate, hydrogen ion and electrons into pure lead and HSO4 (which when in water is sulphuric acid).

In order to force these electrons into the negative plate it is necessary to remove the same number from the positive plate. Adding something to one side of an equation, is the same at taking it away from the other side. So referring to the top equation, we need to remove electrons from the right hand side (the discharged state). This is obviously the same as adding them to the left hand side. So removing electrons from the positive plate forces the equation from the right hand side, to the left hand side. Thus converting the lead sulphate and water into lead dioxide, hydrogen ions and sulphuric acid.

If none of that made sense do not worry. The effect can be just as easily explained in plain english.

Here are the equations written in english.

Again, the discharge direction is from left to right, charging is from right to left.

At the positive plate:-

Lead dioxide + sulphuric acid + electrons <--> lead sulphate + water

At the negaitve plate:-

Lead + sulphuric acid <--> lead sulphate + electrons

In these equations I have ignored the free hydrogen ions as they are not required in order to understand what happens.

At the negative plate, moving left to right (discharging), the lead and sulphuric acid is turned into lead sulphate and electrons. The lead sulphate is basically what the plate turns into during discharging, the electrons are the electric current that the reaction produces. These electrons (the discharge current) pass from the negative post, through the loads, and return to the positive post.

At the positive post the returned electrons react with the lead dioxide and the sulphuric acid, these are then turned into lead sulphate and water.

During charging, we force the electron flow in the opposite direction (using a charger) and exactly the opposite reactions take place.

A fully charged battery has a positive plate made up of Lead dioxide, a negative plate made up of pure lead and an electrolyte of sulphuric acid.

As the battery is discharged, both plates change into lead sulphate and the electrolyte changes into water.

That more or less sums up how the lead acid battery works. This applies to wet cells, gel cells, AGM, carbon fibre etc etc etc. It applies to all lead acid batteries. Minor other chemical reactions take place in each different type that affect other parameters such as water loss, required charge voltages etc. But the actual chemical reactions that produce the power, and that take place during charging remain the same.

Now imagine a battery with a pure lead dioxide positive plate, a pure lead negative plate and an electrolyte of sulphuric acid and water (pure sulphuric acid is too strong). This represents a fully charged battery, in brand new condition.

This battery will have a certain Specific Gravity (SG) and the at rest, fully charged voltage will be a certain voltage. Let's say the voltage is 12.65 volts and the SG is 1.285 (the actual SG and voltage varies between different types, makes and models of battery).

Now let's discharge this battery. This results in both plates converting part of their chemical composition into lead sulphate and the electrolyte becoming slightly weaker i.e. the sulphuric acid part will be used up slightly and overall the electrolyte becomes weaker consisting of less acid and more water.

We now recharge the battery, thus forcing the chemical reaction the other way. The lead sulphate is turned back into sulphuric acid in the electrolyte, pure lead in the negative plate and lead dioxide in the positive plate. When the chemical reactions stop, the battery is fully charged again. Keeping the charger connected longer will achieve nothing. The chemical reactions have stopped.

That is the theory anyway. In practice there are several pitfalls.

Firstly, not all of the lead sulphate will be converted back into lead and lead dioxide. A tiny amount of lead sulphate will remain on the plates. This obviously means less lead and lead dioxide is available for the next reaction (discharge), it also means the electrolyte is slightly weaker (as the sulphate part is now partially stuck to the lead).

So taking another SG reading will now result in a very slightly lower reading. It might now be 1.2849. The battery voltage (after a rest period to remove the surface charge) will be identical at 12.65 volts.

This battery now has marginally less capacity than it had prior to the first discharge cycle. You can keep the charger on for as long as you like. It will not change anything. The SG reading will get lower and lower each time the battery is used. The plates will get more and more covered in lead sulphate.

This has several effects.

A. The plates now have a partial coating of lead sulphate on them. This means less lead and lead dioxide is in contact with the electrolyte. This increases the internal resistance of the battery. This means the battery can now produce a lower maximum discharge current.

B. The electrolyte is now weaker. This, again, increases the internal resistance of the battery with the same effect as A. above (water has a higher resistance than sulphuric acid).

C. As there is now less lead, less lead dioxide and less effective electrolyte, the battery can store less charge. The battery is effectively now a smaller battery than it was when it was new.

And each time the battery is discharged and recharged this same thing happens. The plates get sulphated up. The electrolyte gets weaker. The battery loses some capacity.

The SG readings will get lower, but the battery voltage will remain the same for a specific state of charge. This is why the battery voltage simply cannot be used to show the condition of a battery. The battery could be almost completely sulphated up, and totally useless as a battery, but as long as all the plates still have even a tiny contact area with the electrolyte, the at rest voltage will remain identical to that of a brand new battery of the same make and model.

An often neglected aspect of the SG reading getting lower as a battery ages is that the SG reading cannot be used to show the state of charge.

It can be used to show when the battery is completely flat, that never changes (the SG will be 1.000). It can be used to show the condition of the battery if the state of charge is known. But the fully charged SG reading gets lower as the battery ages so it cannot be used to show the state of charge.

In our example above the SG reading started off at 1.285 when fully charged and brand new. After 12 months use the battery could be fully charged for say 24 hours (or until the charge current has fallen to a very low level) which effectively means the battery is as fully charged as it can ever be, yet the SG reading will be substantially lower, say 1.265. If this SG reading is compared to the data sheet for the battery then this might show the battery to be at 80% charge state. It is at 80% charge state. But it is at 80% charge state of the charge state of a brand new battery. It is not at 80% charge state for that particular battery. It is at 100% charge state for that particular battery.

This sulphating of the plates and reduced electrolyte strength are just two of the factors involved in battery ageing. They can, to a considerable extent, be alleviated (not removed) by careful charging.

A typical three stage charger will charge at a voltage of 14.4 volts for wet cell batteries. This is an accepted standard charge voltage. But when charged at this voltage the batteries must be periodically equalised. This is a form of controlled overcharge at a much higher voltage than usual (often in the region of 16 volts or more). This has the effect of removing much of the sulphate and converting it back into useable electrolyte.

Another method is to use a higher charge voltage all the time. But care has to be taken as this can damage other loads that are connected at the same time. Also charging repetetively at higher charge voltages can cause other problems that reduce battery life such as plate corrosion.

Many modern chargers now charge at a higher voltage than the old standard of 14.4 volts. 14.8 to 15.2 is becoming quite common for wet cells. This has the effect of carrying out a mild equalisation on every charge. This can go a long way to reducing plate sulphation. But again care has to be taken, as if this is done too much the plates and interconnecting conductors can become corroded due to the constant higher strength of the electrolyte.

As the battery becomes discharged, the plates both turn to lead sulphate. If the battery is left in this state of charge for any appreciable length of time, the sulphate crystalises. It is then impossible to remove. Recharging simply will not convert any of the crystalised sulphate back into sulphuric acid. This causes 3 major problems.....

A. The sulphate is no longer useable in the electrolyte, thus the electrolyte is weaker, and therefore less effective. This also increases the internal resistance of the battery.

B. The crystalised sulphate on the plates reduces the area of the plates in contact with the electrolyte. This means less available chemical reactions and also, again, increases the internal resistance.

C. There is now less sulphate available to take part in the chemical reactions so naturally the battery has less capacity. This is why it is so important to keep lead acid batteries charged up all the time.

Now we turn to another aspect of battery ageing. Each time the battery is charged some of the lead and lead dioxide falls off the plates to the bottom of the battery. The higher the charge voltage and/or the higher the charge current, the more falls off. This is known as plate shedding.

You can see that recharging lead acid batteries is a compromise between too low a charge voltage causing the plates to sulphate up and too high a charge voltage causing plate shedding and internal battery corrosion.

Also note that charging at any voltage electrolyses the water into hydrogen and oxygen. And the higher the charge voltage, the more water is used. In most "open" batteries this water is simply lost and has to be replaced. In wet cell maintenance free batteries it cannot be replaced but it still gets lost which is why these batteries won't last very long when used for deep cycing with fast recharge times. In AGM and Gel batteries a very high proportion (not all) of it is automatically reconverted back into water (they are inherently recombinant). For wet cell batteries "catalyser caps" are available which convert the hydrogen and oxygen back into water which drips back into the electrolyte. The effectiveness of these is in some doubt and often hotly debated.

I hope I have explained how a lead acid battery works, shown the relationship between SG readings and state of charge (and how one cannot be used to show the other) and explained the importance of correct charging.

In summary:-

Each time a battery is discharged and recharged some of it's available life is used up.

Charging at too low a voltage causes plate sulphation and reduced battery capacity and life.

Charging at too high a voltage causes plate corrosion, plate shedding (falling off to the bottom of the battery) and reduced battery life.

Battery voltage is no inidication whatsoever of battery condition.

Battery SG readings are no indication whatsoever of battery state of charge.


We have received many emails stating that the equations shown at the top of this page are incorrect. This is stated because so many people seem to know that the chemical formula for sulphuric acid is H2SO4. This is correct. However this formula only shows the relative proportions of each element. It does not take into account the fact that in the real world diluted H2SO4 reacts to produce H+ + HSO4- which still shows the same relative proportions of Hydrogen, Sulphur and Oxygen however it also correctly shows that they have actually separated into a Hydrogen+ Ion and an HSO4- Ion which makes the actual chemical reaction equations easier to understand.


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