4.9.5 Charging characteristic previous Introduction

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Battery charging is what the charger is all about so it makes sense to have a closer look at what happens during the charging process. Understanding the charging process will also help understanding some other things better:

The charging process depends on the electrical characteristics of both the battery and the charger. The characteristics of the charger are well defined and fairly stable. The characteristics of the battery however are much more uncertain. In fig. 4.27 an example of such a battery characteristic is given. For each state of charge, there is a line in fig. 4.27 that shows the relation between battery voltage Vb and charging current I. So if the charger supplies a constant current I, the corresponding battery voltage can be read from the graph. Or if the charger would supply a constant voltage (it doesn't), you could find the corresponding charging current. Of course after charging a while, the battery will get charged and you have to look to the next higher state of charge line. Problem is that such battery characteristics could vary widely:

Fig. 4.27: Relation between charging current, voltage and state of charge (copied from VAN DER MEER, 1990).

Fig. 4.27 also illustrates the `danger zone' for battery charging. In the top right corner, the state of charge lines are not drawn because charging with a high current and a high battery voltage would cause immediate damage to the battery (see annex C: Active material..). To be safe, the charging current should be no more than one tenth of battery capacity (so 7 A for this battery) once battery voltage rises above 14.1 V (VAN DER MEER, 1990).

Batteries will also wear out gradually when they are overcharged with a much lower current, but for a long time. Then the damaging mechanism is corrosion of the grid of the positive plates (see annex C: *Corrosion..) and water loss, which means that they have to be topped up with destilled water. This will happen when batteries are not disconnected from the charger once they are completely charged.

In fig. 4.27 the charging characteristic of the charger can be drawn. This is a line that represents the relationship between charging current and battery voltage Vb as determined by the charger. Relevant data are:

The charging characteristic is represented by the lines connecting the points A, B, C and D in fig. 4.27:

  1. Starting point for charging for solar battery (car batteries should not be discharged further than to than 50 %).
  2. Voltage regulator starts reducing field current.
  3. Fully charged, battery can be disconnected.
  4. Absolute end point of charging. The relation between charging current and battery voltage

Suppose that when charging is just started, the state of charge of the battery is 20 % (only solar batteries can be discharged safely until 20 %, car batteries should not be discharged further than to 50 %). Then the current will be the normal charging current of 11.8 A. So the first point to be drawn in fig. 4.27 is the point with current = 11.8 on the 20 % state of charge line. This is starting point A.

As charging continues, initially the current will remain the normal charging current of 11.8 A. Meanwhile the battery becomes more and more charged so the lines representing higher states of charge are crossed. This means that from starting point A in fig. 4.27 a vertical line going upwards appears.

To find where this vertical line stops, one has to calculate when the voltage regulator will start reducing the field current. Between the point where the regulator is connected and the battery poles, there is a voltage drop of 11.8 * 0.065 = 0.77 V. So if the regulator starts reducing the field current when it senses a voltage Vr of 14.7 V, the battery voltage Vb at that moment must be 14.7 - 0.77 = 13.93 V. This is point B in fig. 4.27.

Now charging continues and battery voltage will rise further, but the current decreases. To find out how the current decreases, one has to look at the voltage drop mentioned above, which represents the difference between the voltage Vr sensed by the regulator and battery voltage Vb. Voltage Vr will remain constant at 14.7 V while the voltage drop Vr - Vb equals I * 0.065. This means that the equation representing the relationship between I and Vb is:

Vb = 14.7 -0.065 * I

This is the equation for a line going through point B and crossing the Y-axis of the graph at a voltage Vb = 14.7 V.

In practice, the line stops where it reaches the line for 110 -120 % charged. This is the end point D of the charging process. Even charging a battery for a week would not make the current drop further and voltage Vb reach 14.7 V. By now, the energy that is still pumped into the battery is not used for the chemical reaction that goes with battery charging (see annex C: *chemical reactions), but for undesirable reactions that occur during overcharging:

The charging characteristic in fig. 4.27 just nicks the corner of the danger zone for battery charging. When battery voltage reaches 14.2 V, the current is still 7.7 A instead of the 7 A (one tenth of battery capacity) that is recommended by VAN DER MEER, 1990. Getting just this little into the corner of the danger zone won't harm the battery at all. But it shows how important it is that the charger complies with the data that were used in calculating the charging characteristic. If the regulator would be adjusted slightly higher and the resistance between battery poles and regulator would turn out to be a bit lower, the charging characteristic could end up well into the danger zone.

Another interesting point is point C where the battery is just fully charged and charging can be stopped. On first sight, one would choose for point C the point where the charging characteristic crosses the line for 100 % state of charge. Then all the electricity that has been taken from the battery, has just been charged back into it. However, generally it is recommended to charge a battery with a little bit more electricity than has been taken from it, so point C should be a little towards the left from the 100 % state of charge line. When is a battery charged enough

Operators need a simple criterium for deciding whether a battery is charged enough and can be disconnected, or whether it needs to be charged further. Generally, they will not measure battery voltage Vb but use the indicator on the switchboard, thus measuring voltage Vi.

A battery can be considered fully charged when:

The current has dropped below 4 A (see note below);


The voltage Vi as measured with the indicator has risen above 14.5 V.

In fig. 4.27 it shows that when charging current has dropped below 4 A, the charging proces has just passed the 100 % state of charge line.

Note: In fig. 4.28 it shows that voltage Vi at the indicator will rise above 14.5 V long before the current will drop below 4 A. So then the point at which the battery is fully charged, will determined by the criterium that the current should drop below 4 A. Still voltage Vi must be checked to make sure that the current has dropped to less than 4 A because the battery is charged, since there could be other reasons why the current is so low:

So to prevent that batteries that are not yet charged, are disconnected because the current is below 4 A for one of those reasons, also voltage Vi should be checked.

The graph of fig. 4.27 goes for a rather large battery of 70 Ah. For smaller batteries, the state of charge lines would end up closer towards the Y-axis and such batteries might not be fully charged when the current has decreased to 4 A. Therefor small car batteries can be considered charged when the current has dropped to 2 to 3 A (instead of 4 A). So for those batteries, point C should lie at a current of 2 to 3 A. For small batteries, the other points on the charging characteristic remain virtually the same.

VAN DER MEER, 1990 recommends to charge in 10 % to 20 % more than what has been withdrawn from the battery, so when a battery was discharged down to 50 %, it should be recharged up to 105 % -110 %. In fig. 4.27 it can be seen that point C is only a little bit past the 100 % state of charge line, and far away from the 110 % line. So compared to VAN DER MEER's recommendation, batteries would not be charged well enough when they reach point C. Still the choice of having point C at a charging current of 4 A seems wise considering: Voltage and current as a function of charging time

The charging characteristic drawn in fig. 4.27 gives the relation between charging current and battery voltage. Another way of analysing the charging process is by of looking how charging current, battery voltage and some other variables vary over time.

This comes down to calculating how much time it takes to go from one state of charge line to the next. As long as the current is constant at 11.8 A, this is easy: Charging in 10 % of battery capacity means charging 7 Ah (total battery capacity is 70 Ah). So it will take 7 Ah / 11.8 A = 0.593 hours (= 36 minutes) to go from one state of charge line to the next. After passing point B, the current is no longer constant so that for each traject between two state of charge lines, a mean current must be calculated. For this, the current at the beginning and at the end of the traject has been read from fig. 4.27 and the average is taken. To go from 90 % to 95 % state of charge, and from 95 % to 100 % state of charge, of course only 5 % of battery capacity has to be charged in, so only 3.5 Ah. The results of this calculation are presented in fig. 4.28. In this figure, also the voltage Vi at the indicator is printed because this is the voltage that is measured on the switchboard. The resistance of the battery switch + current shunt is estimated at 0.01 Ohm (see par. 4.9.6) so voltage Vi can be calculated as:

Vi = Vr -0.01 * I

From fig. 4.28, charging times can be read:

So ideally, the battery should be disconnected at some time between 51/2 and 7 hours after charging has started.

Fig. 4.28: The charging process as a function of time. Effects of battery type on charging characteristic

These calculations are just an example to illustrate what will happen during charging. Things will be quite different in case:

So the charging characteristic given in fig. 4.27 and fig. 4.28 is of little practical use because things will be different for other batteries or at another temperature. Therefor for each battery type, operators are advised to make their own graph of the charging proces. This could be a simplified version of fig. 4.28, so a graph of indicator voltage Vi (the other voltages can not be measured easily) and charging current against charging time. Advantages of making these graphs are: Effects of the charger on charging characteristic

The charging process would also end up quite different if the characteristics of the charger are different. Relevant characteristics are:

It would go too far to discuss the influence of these 3 charger characteristics on the charging proces. One effect however needs mentioning:

Suppose that the resistance Ri between the battery poles and the point where the regulator is connected, would be 0.13 Ohm instead of the normal value of 0.065 Ohm. It could easily become so high because of bad connections, or because the operator has fitted a longer battery cable so that the battery can stand at a more convenient place during charging.

Then the voltage drop Vr - Vb will be 11.8 A * 0.13 Ohm = 1.53 V! This means that the regulator will already start reducing the field current (point B) when battery voltage Vb is only 13.17 V so state of charge is only 40 % (see fig. 4.27). So the regulator will start reducing the charging current long before the battery is charged and consequently the total charging time will become unnecessary high. Also point C changes, now it will be at 14.7 - 4 * 0.13 = 14.18 V (instead of the normal 14.44 V). This is just slightly to the left of the 100 % state of charge line, but not enough to guarantee that the battery is charged enough when the operator decides to disconnect it.

When this resistance would become even higher than 0.13 Ohm, the effects are worse. Charging will take exeptionally long, when charging current drops to 4 A, batteries will be disconnected but they are not charged properly yet. operators might disconnect batteries even before the current has dropped to 4 A because they feel that the battery has been charged long enough and something must be wrong with the indicator. Batteries that are not charged well enough, are likely to suffer from active material falling out of the positive plate and sulphatizing (see annex C-3 and C-4). Also, users might discharge their battery too deep because they expected a fully charged battery that would last a certain number of days. And finally, there might just not be enough time to charge all batteries because the charging time per battery has become so long.

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