Whilst almost anything is possible - and more will be - some information is required in advance by the charger. This starts with the voltage and to some extent the capacity. There are quite a few chargers that can cope with varying capacities and to a lesser extent voltages. They do this by measuring the temperature of the cell(s) under charge. Indeed many battery packs have an inbuilt temperature sensor.
When Ni-Cad & Ni-M-Hy types cells are fully charged and/or cannot chemically convert the energy anymore, their temperature rises quite substantially. This can be used to "assume" that a fully charged state has now arrived
Another, though less reliable way is to monitor the rise in cell voltage. There's a snag here! If one or more cells in a pack become shorted (as can happen with dendrite growth in Ni-Cads), the expected final terminal voltage can never be achieved. This is what makes the temperature measuring method much safer. Another issue in its favour is that even cells that have lost their original capacity will be properly detected for a fully charged condition.
Here you see why it is so important that the cells in a battery should be evenly matched. We need them all to be charged/discharged at the the same rate & time.
POSTSCRIPT - Another method of detecting when a Ni-Cad or Ni-M-Hy battery is fully cahrged has come to my notice. It is called the "minus Delta U" calculation which in this case works thus: When above battery types are charged with a constant current their voltage rises continuously to amaximum which falls slightly if the charge is maintained. This fall can be used to terminate the charge.
UPDATE - whether covered elsewhere here or not there's another point to make concerning pulse battery charging. This was a common method on older cars equipped with a dynamo that delivered DC at a voltage that varies with RPM. A "Regulator" was fitted to ensure that charging currents did not exceed a sensible level for too long. The period of each pulse was controlled by voltage & current operated solenoid which interrupted the current flow often diverting it via a resistor. Thus were two levels of cuurent applied in pulses the duration of which depended on the battery state of charge. This practice gave something else that later alternator circuits did not give which concerns the formation of sulfate on the plates. This happens when the battery is in a wholly or partially discharged state and since it effectively insulates the area of the plates to which it has adhered, it effectively lowers the battery capacity. It now seems that the older pulsed charging systems could somehow dissolve any such formation more effectively than a constant current/voltage system and this has brought us to the electronic pulse charger. In a car it is the job of the ECU. However, external to the car specialised electronically controlled pulsed lead-acid battery charging and conditioning brings those advantages back.
Nor does the story end there. Other battery types, in particular Ni-Cad & Ni-M-Hy types can be charged up much quicker with the pulse method. It seems that the chemistry conversion is more effiicient when charged in pulses. Of those chargers I have seen the pulses are about 1A with a duration of 25 to 50% or in other words something like 1 to 2 secs in 4
Monday 14 September 2009
Thursday 3 September 2009
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?
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?
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