Battery Charging

Oh dear - the need to tackle this in a bit more detail arises! Please read the previous section first. There are several ways to charge. We'll get rid of two straight away. "Trickle" & "Fast"

These are each the antithesis of the other. A "trckle" refers to the current which can be continually applied without any damage. Such damage occurs through overheating or chemical "gassing" in the extreme.

A "Fast" charge always does damage and is the trade off for a more immediate restoration to use.

The two most common methods of charging are referred to as the "Constant current" and "Constant voltage" methods. If we study the constant voltage method first, we will see why the other become necessary.

I think we might stay with the idea of a "battery" here - that is a collection of cells that together have a terminal PD (that's the voltage when under a medium discharge load) of say 12Volts. When a lead-acid car battery is in a fully charged state the cells will be slightly over their nominal 2V each. In fact 2.3V is the usual value. There need to be six cells and so the fully charged voltage will be 13.8V {Let's say14V}. The unloaded Voltage is known as the EMF. {ElectroMotive Force}.

It's worth saying that a 12V battery made from Ni-Cad, {or indeed Ni-M-Hy}, cells would need 10 cells. {to equal 12V}. When fully charged these would actually achieve an EMF of 14 to 16Volts.

In either case the voltage from the battery charger needs to be at least as large in order to overcome that standing voltage and thereby impart a charge.

Can you see that, (you must make yourself see this), the charging voltage must be at least equal to 14V. The applied voltage must be able to overcome the standing "surface" charge. We also need to consider some current "limiting" especially if the battery is in a very low state of charge.
Once the battery has "caught up" and is equal to the applied Voltage the charging will cease. It is therefore very safe to leave unattended as overcharging cannot occur. However the rate of charge will diminish over time as the battery voltage rises and becomes ever more equal. The total process slows down (to a stop) and takes longer than it really needs to. {Exponential decay}.

This, then, is the reason for the alternative where the applied voltage is much higher and the charge rate is much more constant over time. This method carries with it the dangers of over-charging mentioned earlier in text.

In practice most good chargers will employ a mixture of both methods for what is wanted here is Maximum speed without loss of capacity or any damage. So we will charge at a medium rate until the cell voltages all rise and then "fold" the current back. Can we do any better?

I'm afraid to tell you we can! The cell voltage will rise to a peak when it is fully charged and this works fine if all the cells are in the same condition. In practice they are often not so equal as the chemicals within them age at different rates.

There's another problem. We might not know how large (how much capacity) a given battery actually has. If this is so, we can't use the charging time as a guide. The answer is to measure the temperature. There will be a rise in temperature when the chemical conversion is done. If we sense that, then we might be on the way to some pretty smart battery charging.

What more do you really need to know?