TECHNICAL INFORMATION
BATTERY CHARGE
Proper loading is one of the most important factors to consider when using rechargeable lead-acid maintenance free. The performance and service life of the batteries will be directly affected by the efficiency of the charger used. The 3 most common methods of charging are:
- Constant voltage charge
- Constant current charge
- Charging at constant voltage in two stages
Constant Voltage Charge
The constant voltage charging is the most convenient and most commonly used for charging lead-acid batteries sealed. Figures 1 to 6 show the characteristics of battery chargers when they are charged with constant voltage to 2.30 volts / cell, 2.40 Volts / cell and 2.50 Volts / cell when the initial charging current is controlled at 0.1 and 0.25 CA CA.
The following figure shows an example of a circuit for constant voltage charging. In the circuit the initial charging current is limited by the series resistor R1.
Constant current charge
This method of loading is not normally used for battery sealed lead-acid, but is an effective method for charging multiple batteries at the same time, and as an equalizer to correct loading variations in battery capacity between groups. Precautions should be taken when constant current load. If the load is continuous at the same frequency for a period of time after the battery has reached a state of full load, overload may occur resulting in severe damage to the battery.
Charging at constant voltage in two stages
This type of load is recommended for charging sealed lead-acid in a short period of time, keeping the battery fully charged in standby or float charge.
In the initial stage of charging, the battery is charged by constant current. The charge voltage increases as the load continues until it reaches the 2.4 Volts / cell, at which point the charging mode automatically switches to constant voltage load. During the period of constant current load (AB) load current, which has fallen to the point of change is detected, and the charging voltage is switched to a floating level of 2,275 volts / cell.
This charging method is one of the most efficient. Recharge time is minimized during the initial loading stage while the battery is protected from overload by changing the system to float charge at the point of change. The figure illustrates an example of a constant voltage charger and two-stage constant current. Basically this is a circuit of a power supply stabilized, with a current limiting function, using a hybrid integrated circuit of constant voltage. The difference between this circuit and the constant voltage circuit shown in Figure 6 is the addition of VD circuit which serves to increase or decrease the output voltage as a function of variation in the input current. In other words, the output voltage is set to change, along with the resistor R6, the potential proportion of the detected voltage IC circuit that detects the voltage reduction in both terminals of the resistance R1.
By using this method of loading should be taken into account the following:
- Initial charge current: Initial charge current: 0.25CA to 1.0CA
- Charge voltage:
1st stage 2.40V/cell (2.35 to 2.43V/cell)
2nd stage 2.275V/cell (2.25 to 2.30V/cell) - Power change: 1st Stage to 2nd Stage: 0.05CA (0.04 to 0.08CA)
Note: This charging method can not be used in applications where the load and battery are connected in parallel.
Charge Voltage
The charge voltage should be regulated according to the type of service in which the battery will be used. Generally, the following voltages are used::
- In order to use floating or stand by 2,275 to 2.30 volts per cell
- For cyclic use 2.35 to 2.45 volts per cell
In a constant voltage load, a large amount of current flow during the initial stage of loading, and decrease as the charging process continues. When charging at 2.30 volts per cell, the load current at the end of the load stage to fall to 0,002 the number of CA.
When a battery is charged to a level of 100% of ampere-hours discharged, the electrical energy stored and available for download is 90% or more of the energy used during charging.
The charge voltage should be regulated in relation to ambient temperature. When the temperature is high, the charging voltage should be lower. When the temperature is low, the charge voltage should be higher. Similarly, the volume of available battery charge (measured in amp-hours) will vary over time in direct relation to temperature. Cargo volume at a given time period will be larger at higher temperatures and smaller at low temperatures
Initial limit load current
A discharged battery will accept an initial load current higher in the initial stage of loading. High load currents can cause abnormal internal heating which may damage the battery. It is therefore recommended that the load current is usually limited to 0.25CA. However, in standby use, batteries are designed that even when the load current is higher than the recommended limit, accept no more than 2CA, and the load current will be reduced to a relatively small value in a short period time. Therefore, in use stand by, no current limit required.
When designing a charger, it is recommended that a current limiting function is put in the boot loader to prevent failures due to overheating of the transformer or other damage caused by misuse, eg short circuit or reverse polarity.
Load limit (additional charge)
Any battery to lose its self-discharge capacity, it is recommended that a charge is applied to stop any battery that is now stored for a long period of time before connecting the battery. Except for conditions in which the storage temperature has been unusually high load limit is recommended within the following parameters:
| Battery Age | Recommendations |
|---|---|
| Up to 6 months after manufactured | 4 to 6 hours at constant current of 0.1 AC or 15 to 20 hours at constant voltage of 2.40 volts per cell |
| Until 12 months after manufactured | 8 to 10 hours at constant current of 0.1 AC or 20 to 40 hours at a constant voltage of 2.40 volts per cell |
To make a full charging a battery stored for 12 months, the open circuit voltage should be higher than 2.0 volts per cell. In that case, always check the open circuit voltage before applying the load.
Loading recovery from deep discharge
A battery that has been over discharged requires a period of higher than normal load. Figure 14 shows that the charging current accepted by an over discharged battery during the initial stage of charging will be quite small as a result of internal resistance, but increase rapidly over the first 30 minutes until it exceeds the internal resistance.
In view of the above, consider the fact that if the charging method used is by constant voltage at which the charger used for current sensing status indication of either charge or voltage reduction (two-stage charger ) during the initial stage of charging a discharged battery on the charger could give a false indication of full charge or may initiate a charge to float voltage.
Temperature compensation
The electrochemical activity of a battery increases while the temperature. Similarly, while the temperature falls, electrochemical activity decrease. Therefore, conversely, as the temperature increases, the charge voltage should be reduced to prevent overloading and increase it when the temperature drops to prevent low loads. In general, to ensure optimal battery life, it is advisable to use chargers with temperature compensators. The recommended compensation factor for batteries is -3 mV per cell in standby mode and -4 mV per cell for cyclic use. The standard center point for the compensation of the temperature is 20ēC.
When designing a charger equipped with temperature compensation, the temperature sensor is sensing only the temperature of the battery. Therefore, you should consider insulating the battery and temperature sensor other system components that generate heat.
Charge Efficiency
The charge efficiency of a battery is expressed by the following formula:
μ = Ah discharged after charge / Ah delivered to the battery during charging
Charging efficiency varies depending on the state of battery charge, temperature and loading rate. The figure and the left shows the concept of state of charge and loading efficiency. As shown in the figure on the right, shows a high efficiency battery charging even when it is loaded to lower freight rates.
