Charging

Correct charging is one of the most important factors to consider when using valve regulated lead acid batteries. Battery performance and service life will be directly affected by the efficiency of the charger selected. The basic charging methods are:

Constant Voltage Charging

Constant Current Charging

Taper Current Charging

Two Stage Constant Voltage Charging

Constant Voltage Charging

Charging at constant voltage is the most suitable and commonly used method for charging valve regulated lead acid batteries. Figures 10 - 15 show the charging characteristics of NP batteries when charged by constant voltage chargers at 2.275 volts/cell, 2.40 volts/cell and 2.50 volts/cell when the initial charging current is controlled at 0.1C Amps and 0.25C Amps.

Figure 9 shows one example of a constant voltage charging circuit. In this circuit, the initial charging current is limited by the series resistance R1.

Fig 9

The recommended float charge voltage for NP type batteries at 20°C is 2.275vpc ± 0.005v, this should be the measured average for the total battery, however when measured within a battery network or string the allowable tolerances can be expected between 2.25vpc and 2.3vpc.

Fig 10. Charging Characteristics

Fig 11. Charging Characteristics

Fig 12. Charging Characteristics

Fig 13. Charging Characteristics

Fig 14. Charging Characteristics

Fig 15. Charging Characteristics

Charging Efficiency

The charging efficiency (¼) of a battery is expressed by the following formula:

The charging efficiency varies depending upon the state of charge of the battery, temperatures and charging rates Figure 30 illustrates the concept of the state of charge and charging efficiency. As shown in Figure 31, Yuasa NP batteries exhibit very high charging efficiency, even at low charging rates, unlike some nickel cadmium batteries.

Fig 30. Charging Efficiency vs. State of Charge

Fig 31. Charging Efficiency

Expected Service Life of NP Batteries

There are a number of factors that will affect the length of cyclic service of a battery. The most significant are ambient operating temperature, discharge rate, depth of discharge, and the manner in which the battery is recharged. Generally speaking, the most important factor is depth of discharge. Figure 32 illustrates the effects of depth of discharge on cyclic life.

Fig 32. Cycle Service Life in Relation to Depth of Discharge

The relationship between the number of cycles which can be expected and the depth of discharge is readily apparent. If an extended cycle life is required then it is common practice to select a battery with a larger capacity than the one that is required to carry the load. 1 the specified discharge rate over the specified time depth of discharge will be shallower and cyclic life will be longer.

Float Service Life

NP batteries are designed to operate in float/standby service for approximately 5 yrs (NP+NPH) 7-10 yrs (NPL) based upon a normal service condition in which float charge voltage is maintained between 2.275vpc ± 0.005 volts per cell in an ambient temperature of approximately 20'C. Figure 33 shows the float service life characteristics of NP batteries when discharged once every three months to 100% depth of discharge.

Fig 33. Float Service Life (Standard NP)

In a normal float service, where the charging voltage is maintained at 2.275vpc ± 0.005 volts per cell (see Fig. 34), the gases generated inside an NP battery are continually recombined into the negative plates and return to the water content of the electrolyte. Therefore, electrical capacity is effectively not lost due to the "drying Lip" of the electrolyte; the loss of capacity and eventual end of service life is brought about by the gradual corrosion of the electrodes. It should be noted that this corrosive process will be accelerated by high ambient operating temperatures and/or high charging voltage. When designing a float service system, always consider the following: LENGTH OF SERVICE LIFE WILL BE DIRECTLY AFFECTED BY THE NUMBER OF DISCHARGE CYCLES, DEPTH OF DISCHARGE, AMBIENT TEMPERATURE AND CHARGING VOLTAGE.

Fig 34. Relationship Between Float Charge Voltage and Battery Life (20°C)

Charging Voltage

The charging voltage should be chosen according to the type of service in which the battery will be used. Generally, the following voltages are used:

For float (standby) use ...... 2.275vpc ± 0.005 volts per cell

For cyclic use .................. 2.40 to 2.50 volts per cell

In a constant voltage charging system, a large amount of current will flow during the initial stage of charging but will decrease as the charging progresses. When charging at 2.275 volts per cell, the current at the final stage of charging will drop typically to a value of between 0.0005C Amps and 0.004C Amps The charged volume in ampere hours, shown on the vertical axis of Figures 10 - 15 (pages 14-16), indicate the ratio of charged ampere hours to the previously discharged ampere hours. When a battery has been charged up to a level of 100% of the discharged ampere hours, the electrical energy stored and available for discharge will be 90% or more, of the energy applied during charging.

Charging voltage should be regulated in relation to the ambient temperature. When the temperature is higher, the charging voltage should be lower and conversely when the temperature is lower, the charging voltage should be higher. For specific recommendations, please refer to the section on Temperature Compensation on page 25. Similarly, charged volume (measured in ampere hours) realised over a given time will vary in direct relation to the ambient temperature; the higher the ambient temperature, the higher the charged volume in a given period of time and the lower the ambient temperature, the lower the charged volume in the same given period of time. Figure 25 shows the relationship between volume and temperature.

Fig 25. Charging Characteristics at Different Temperatures

Initial Charge Current Limit

A discharged battery will accept a high charging current at the initial stage of charging. High charging current can cause abnormal internal heating which may damage the battery. Therefore, when applying a suitable voltage to recharge a battery that is being used in a recycling application it is necessary to limit the charging current to a value of 0.25C Amps. However, in float/standby use, Yuasa NP batteries are designed so that even if the available charging current is higher than the recommended limit, they will not accept more than 2C Amps and the charging current will fall to a relatively small value in a very brief period of time. Normally, therefore, in the majority of float/standby applications no current limit is required. Figure 26 shows current acceptance in NP batteries charged at a constant voltage of 2.30 vpc without current limit.

When designing a charger, it is recommended that suitable circuitry is employed to prevent damage to the charger caused by short circuiting the charger output or connecting it in reverse polarity to the battery. The use of current limiting and heat sensing circuits fitted within the charger are normally sufficient for the purpose.

Fig 26. Constant Voltage Charge Characteristics with no Current Limit

Charge Output Regulation and Accuracy

To ensure the correct voltage is set accurately, when adjusting the output voltage of a constant voltage charger, all adjustments must be made with the charger "ON LOAD". Adjusting the output voltage with the charger in an "OFF LOAD" condition may result in undercharging. The constant voltage range required by a battery is always defined as the voltage range applied to a battery which is fully charged. Therefore, a charger having the output characteristics illustrated in Figure 27, should be adjusted with the output voltage based on point A The most important factor in adjusting charger output voltage is the accuracy at point A, which should be in the range of 2.275vpc ± 0.005 volts per cell however this accuracy is not normally required over the entire range of the load. A charger adjusted in accordance with Figure 27 will never damage a battery, even if the charger has the characteristics shown by the broken line in Figure 27.

Fig 27. Output Voltage Adjustment

Top Charging

Since any battery loses capacity through self discharge, it is recommended that, prior to putting the battery into service, a process called "top charging" be applied to any battery which has been stored for a long period of time.

Excluding conditions in which storage temperatures have been abnormally high, top charging is recommended within the following parameters:

In order to successfully top charge a battery stored for more than 12 months, the open circuit voltage must be checked to ensure that it is higher than 2.0 volts per cell.

Therefore ALWAYS check the open circuit voltage FIRST. If the open circuit voltage of the battery is 2.0 vpc or lower, please refer to us prior to attempting to "Top Charge".

Recovery Charge After Deep Discharge

When a battery has been subjected to deep discharge (commonly referred to as over discharge), the amount of electrical energy which has been discharged can be 1.5 to 2.0 times greater than the rated capacity of the battery. Consequently, a battery which has been over discharged requires a longer charging period than normal. Please note from Figure 28 that as a result of increased internal resistance, the charging current accepted by an over discharged NP battery during the initial stage of charging will be quite small, but will increase rapidly over approximately the first 30 minutes until the internal resistance has been overcome, then normal, full recovery charging characteristics resume.

Fig 28. Charging Characteristics of NP 6-12 After Deep Discharge

Because of this initial small charge current, in an over discharged battery, as described above, unless due consideration is given to this fact then if the charging regime uses current monitoring for determining either the state of charge and/or for signalling that the switching point has been reached for reducing the voltage to a float/standby value (as is the normal case in a multi-stage charger), the charger could be 'tricked' into entering further stages before completing earlier ones. In other words the charger may give a false "full charge" indication, or may initiate charge at the float voltage figure, instead of at a higher voltage level.

Temperature Compensation

As temperature rises, electrochemical activity in a battery increases and conversely decreases as temperature falls. Therefore, as the temperature rises, the charging voltage should be reduced to prevent overcharge and increased, as the temperature falls, to avoid undercharge. In general, in order to attain optimum service life, the use of a temperature compensated charger is recommended. The recommended compensation factor for NP batteries is -3 mV/°C/Cell (for float/standby) and -4 mV/°C/Cell, (cyclic use). The standard centre point for temperature compensation is 20°C. Figure 29 shows the relationship between temperatures and charging voltages in both cyclic and float/standby applications.

Fig 29. Relationship Between Charging Voltage and Temperature

In practice where there are short term temperature fluctuations between 5°C and 40°C, temperature compensation is not absolutely essential. However, it is desirable to set the voltage at a value shown in Figure 29 which, as closely as possible, corresponds to the average ambient temperature of the battery during its service life.

When designing a charger equipped with temperature compensation, the temperature sensor must sense only the temperature of the battery. Therefore, consideration should be given to thermally isolating the battery and temperature sensor from other heat generating components in the system.

Solar Powered Charging

A battery is an indispensable component of any solar powered system designed for demand energy use. Since solar cells have inherent constant voltage characteristics, NP batteries can be charged directly from the solar array using a simple diode regulated circuit as shown in Figure 24.

Fig 24. Block Diagram of a Solar Powered Charging System

In designing a solar powered system, consideration should be given to the fact that in addition to normal periods of darkness, weather conditions may be such that solar energy is limited, or virtually unavailable for long periods of time. In extreme cases, a system may have to operate for 10 to 20 days with little or no power available for charging. Therefore, when selecting the correct battery for a solar application, the capacity should be determined based upon maximum load conditions for the maximum period of time the system may be expected to be without adequate solar input.

In many instances the battery capacity will be 10 to 50 times greater than the maximum output of the solar panels. Under these circumstances, the maximum output of the solar array should be dedicated to charging the battery with no load sharing or intervening control devices of any kind.

Naturally, in cases where the output of the solar array exceeds the capacity of the battery, and weather conditions are such that the potential for overcharging the battery exists, appropriate regulated charging circuitry between the solar panels and the battery is recommended.

Remote sites and other outdoor applications is where most solar powered systems are to be normally found. When designing a solar powered system for this class of application, a great deal of consideration must be given to environmental conditions. For example, enclosures which may be used to house batteries and other equipment may be subject to extremely high internal temperatures when exposed to direct sunlight. Under such conditions, insulating the enclosure and/or treating the surface of the enclosure with a highly reflective, heat resistive material is highly recommended.

In general, when designing a solar powered system, consultation with the manufacturers of both the solar panel and the battery is strongly advised.