A simple explanation of surface charge (and surface discharge).

This is a term that crops up all over the place. But I don't think I've ever seen a simple explanation of what it is, how it affects recharge times or how it affects discharge times.

So I thought it was time to put this right. It isn't a complicated phenomenon. In fact it's very simple, but it's effects can be quite far reaching.

The first thing to remember is that the process of storing energy in a battery is a chemical reaction. The idea is to turn as much as possible of the lead sulphate (which form the plates of a flat battery) into lead and lead dioxide (which form the plates of a fully charged battery). But this chemical reaction is actually quite slow.

Initially the chemical reaction only takes place on the surface of the plates, where they are in contact with the electrolyte. It takes some time for this chemical reaction to start to penetrate deep into the plates. This is effectively the "surface charge"

It can mean that a very low charge current is going into the batteries because the surfaces of the plates are fully charged. But wait some time, and the surface of the plates will become slightly discharged. In effect they have discharged into the "deeper innards" of the plates. This is effectively what limits the speed at which lead acid batteries can be charged

To clarify this point, the speed at which a lead acid battery can be charged is limited by the rate at which the chemical reactions take place. Increasng the charge current will, of course, speed up the chemical reactions (and thereby decrease the total charge time) but there is a limit to this speed. Increasing the charge rate above this limit will not increase the speed of the chemical reactions. It will simply produce gasses within the battery.

The maximum rate the chemical reactions can take place reduces as the battery reaches higher charge states. Initially (in a low charge state) the chemical reactions can take place very quickly, but they are still limited (when the battery is flat), as the charge current is limited by the increased internal resistance of the battery.

As the battery approaches higher charge states, the internal resistance falls which permits a higher charge current to flow, but there becomes less lead sulphate to take part in the chemical reactions of charging (that of converting lead sulphate back into lead, lead dioxide and sulphuric acid) so the chemical reactions take place more slowly.

The effect of increased internal resistance when flat explains why a flat battery can take a long time before starting to accept a charge.

The effect of reduced chemical reaction rate when in a high state of charge partly explains why the charge rate

reduces towards the end of the charge cycle and why it can take so long to get the last few percent of charge into the battery.

But this effect is more pronounced on the surface of the plates.

This means that as the battery approaches full charge, when the charge current has fallen to a low level (and the charger switched off), if the charger is then switched back on, a higher charge current will flow. Simply because the batteries were not fully charged, they were only "surface charged".

Simple really!

But the consequences can be quite nasty.

If we try to charge the batteries too fast, with a large charger, then they will not become fully charged. Only the surfaces will be charged. Deep inside the plates they will still be in a low state of charge. This causes the "insides" of the plates to sulphate up (a phenomenon usually associated with the surface of the plates).

"Surface charge" also has further consequences.

Remember Peukert's effect? If we instantly start to discharge a "surface charged" battery then we are effectively discharging just the surfaces of the plates. This means we are effectively discharging a smaller battery (think about it). As we are discharging a smaller battery with the same discharge current then the Peukert corrected discharge current will be much higher (again think about it). Thus Peukert's effect will be higher immediately following a charge. And the faster the charge was put into the battery, the higher Peukert's effect will be immediately following the charge cycle.

This means that discharging a battery immediately after charging (whilst there is still some surface charge present) will give us less available power than waiting for the surface charge to permeate deeper into the plates. If we wait some time for the surface charge to permeate deeper into the plates, we will get more available power from the battery.

Now I'm not suggesting anyone do this as with a normal sized charger the effect will be minimal, maybe 10% difference at the most. But if an enormous charger has been used, then the batteries' state of charge will consist almost entirely of surface charge.

Surface charge can also cause oversized chargers to terminate the acceptance cycle too early and go into float charge well before the batteries are actually fully charged.

A full, timed, acceptance cycle is the way to cure this problem. Irrespective of what the acceptance charge current has dropped to, the acceptance cycle should continue for a certain period and most chargers do not continue this cycle long enough. Most (there were a few exceptions) older chargers (say from the 1980s and 1990s) seem to have acceptance cycles of around 1 to 3 hours. This really isn't long enough. Many modern chargers now run the acceptance cycle for much longer periods. 8 hours is becoming common. This is much more satisfactory and does ensure that the last remaining 10 percent or so of charge is put into the batteries.

This is fine for an AC powered charger, but obviously running an engine for another 8 hours simply to get the last 5% or 10% charge into the batteries is somewhat impractical. This is, quite simply, unfortunate. Lead acid batteries take a long time to fully recharge. There is nothing we can do about this when using 3 stage chargers.

Surface Discharge and battery recovery

Now if you think about why this surface charge effect happened in the first place, (i.e. the chemical reactions not having time to balance throughout the depths of the plates) you will realise there will be a similar effect during discharging. The surfaces of the plates become discharged, but deep within the plates they remain in a high state of charge.

This explains why a battery can appear to be flat after powering a very heavy load. Yet when we try it again some time later, it seems to have power available. This is called "battery recovery". Quite simply the inner depths of the plates have recharged the surface of the plates (not exactly true, to say that the chemicals have become more evenly distributed would be more accurate).

A common example of this is when trying to start a tired engine in the cold. After several attempts the battery will not crank the engine fast enough to start it. Wait 15 minutes, and you get a few more goes.

These effects are known respectively as "surface charge" and "surface discharge".

The first is commonly heard, the second not as often as it is usually referred to as "battery recovery" but it is the same effect in reverse. Only the name differs.


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