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:

• Why the switchboard was designed this way.
• How the charger should be used.
• How to take proper care of batteries.

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 V_b 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:

• With the capacity of the battery. This graph goes for a 70 Ah battery, which is already quite a large one. At the same voltage and state of charge, a small, low capacity battery will absorb a smaller charging current than this one.
• With the kind of battery. Car batteries might react differently than solar batteries.
• With the temperature. This graph probably goes for a temperature of 15 °C. At a higher temperature, a battery will accept a higher charging current, so then all lines will shift towards the left.
• With the condition of the battery. A worn-out battery will accept only a very small charging current.

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 V_b as determined by the charger. Relevant data are:

• The normal charging current. This is the charging current when the regulator does not reduce the field current, so when the charger delivers its full electrical power output. Assuming that the electrical power is 165 W (see par. 4.1) and that the voltage is 14 V, this charging current is 11.8 A.
  To be more precise, one could take into account that the voltage increases slightly as the battery becomes more and more charged. This would mean that the charging current will decrease slightly. However, the difference with the constant current of 11.8 A is negligible.
• The voltage setting of the regulator: 14.7 V (see par. 4.9.4).
  Also it is assumed that the regulator works perfectly: Below 14.7 V, it does not reduce the field current at all while when reaching 14.7 V, it reacts so sharply that the voltage can not rise any further.
• The total resistance between the battery poles and the point where the regulator senses the voltage: 0.065 Ohm (see par. 4.9.6).

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.

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 V_r of 14.7 V, the battery voltage V_b 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 V_r sensed by the regulator and battery voltage V_b. Voltage V_r will remain constant at 14.7 V while the voltage drop V_r - V_b equals I * 0.065. This means that the equation representing the relationship between I and V_b is:

V_b = 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 V_b = 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 V_b 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:

• Electrolysis of water, meaning that water is transformed into hydrogen gas and oxigen gas (the explosive gas mixture that is formed during battery charging). The water that is lost this way, has to be replenished by topping up with destilled water once in a while.
• Corrosion of the grid of the positive plate, see annex C:* corrosion.. These undesirable reactions go only slowly because the current is so low. Still they go faster than in a car because the voltage regulator has been readjusted to a slightly higher voltage. Therefor a battery must be disconnected once point D is reached.

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.

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 V_b but use the indicator on the switchboard, thus measuring voltage V_i.

A battery can be considered fully charged when:

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

and:

The voltage V_i 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 V_i 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 V_i 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:

• The charger runs short of water so its power output is just enough for a few amperes charging current.
• The nozzle is partially blocked, with the same effect on the power output.
• There is something wrong with the field current, maybe too many or too few lamps are switched on, the connections are poor or the brushes are wet.
• The current indicator is wrong and underestimates the actual current. The most likely cause for this is corrosion of the surfaces in the indicator switch (operate the switch a few times to see if it goes back to a normal value). In this case, the voltage measurement part probably still functions.

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 V_i 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:

• Generally, temperature will be higher than the 15 °C for which fig. 4.27 goes. At a higher temperature, a battery accepts charging current more easily, so when reaching point C, it will be charged a bit more than fig. 4.27 suggests.
• Suppose the operator will check the battery only once every hour. This means that on average, batteries will be disconnected half an hour after they have passed point C and then they will be charged considerably more (see next paragraph). So point C is the minimum at which a battery can be considered charged enough. On average, batteries will be charged quite a bit more because the operator will only check once in a while.
• Allowing batteries to be disconnected a bit early means that total charging time (see next paragraph) will be shorter, which is important because:
  • It means more convenience for users. They can bring their battery late and still have it recharged the same day.
  • It also means that the charger can serve more users without having to charge during the night.
• Allowing that batteries are disconnected a bit early reduces the risc that they will be seriously overcharged, which would damage them just like undercharging does.
• It is important that operators have a simple and clear criterium for when a battery is charged well enough and point C provides this.

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 V_i 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 V_i can be calculated as:

V_i = V_r -0.01 * I

From fig. 4.28, charging times can be read:

• After 3 hours and 50 minutes, the charging process passes point B and the voltage regulator starts reducing the field current. The state of charge is just above 80 %.
• After 5 hours and 30 minutes, the charging process passes point C and the battery can be considered just charged.
• After 7 hours, the charging process reaches point D and the battery is charged completely. Charging any further means overcharging and will only damage the battery.

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.

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

• The initial state of charge of the battery was not 20 %. Then the charging process starts at another `state of charge' line, so point A moves to this state of charge line.
• Batteries have a lower capacity. Then going from one state of charge line to the next higher one will take shorter so total charging time will be less. Also the state of charge lines in the graph of fig. 4.27 will move towards the left since a lower capacity battery can absorb a lower charging current (at the same state of charge and battery voltage). Consequently point B on the charging characteristic will move towards the 70 % or even 60 % state of charge line.
  If battery capacity would be higher than 70 Ah, of course things change in the opposite direction, so total charging time will be longer, point B will move towards the 90 % or 95 % state of charge line etc.
  Worn-out batteries will react more or less like batteries with a lower capacity.
• Battery temperature is high. The graph from VAN DER MEER, 1990 that was copied in fig. 4.27 probably goes for a temperature of 15 °C. If the temperature is higher, batteries can absorb the same charging current at a lower battery voltage, so all state of charge lines move downwards. Unfortunately no graphs like fig. 4.27 are available for higher temperatures.
• The charger itself has a higher or lower capacity than the 165 W these calculations were based on.

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 V_i (the other voltages can not be measured easily) and charging current against charging time. Advantages of making these graphs are:

• From making these graphs and comparing them with fig. 4.28, operators will learn more about the charging process, and thus learn to handle batteries better.
• Operators can use the graphs to estimate how much more time it will take before a battery is charged well enough. This will save them the effort to go to the charger often to check.
• It will be noticed much earlier if some batteries lose capacity as they wear out (then compared to the graph that was made with a new battery, a worn out battery is fully charged too early). Maybe the cause can be found and remedied. At least no batteries will wear out completely and become useless prematurely without warning.
• The information from the graphs might be useful to simplify and improve the procedures for charging batteries.

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

• The setting of the voltage regulator. This determines the maximum value that the voltage V_r at the regulator can reach, and through this, also voltage V_i at the indicator and battery voltage V_b.
• The internal resistance R_i
• The maximum charging current I

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 R_i 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 V_r - V_b 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 V_b 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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