| 3. CHARGING CHARARCTERISTICS |
| 3.1 | CHARGING METHODS |
There are four charging methods:-
3.1.1 Constant Voltage Charging
Charging at constant voltage is the most suitable and commonly used method for charging DiaMec batteries. The charger voltage must be stabilized in a narrow range and with a device to suppress the initial current to less than 0.3C. The initial current limitation can be a accomplished by a constant-current regulator, a properly designed output-voltage from the power transformer, or by designing the overall impedance of the circuit (such as using a current regulating resistor). During the final stage of charge, the current decreases automatically. Fig. 1 & 2 show constant voltage charger circuits provided with constant voltage function, composed of transformers, transistors, silicon diodes, IC's, etc.
Fig. 1 | Fig. 2 |
It is desirable for the charger to be temperature-compensated. For more details, please refer to section 3.5.
3.1.2 Constant Current Charging
It is an effective method for supplementary charge of many batteries at one time in series during storage, but the charging time must be strictly controlled. It is because if the charging is continued at the same rate for an extended period of time after the battery reached a fully-charged state, battery voltage rises excessively, water decomposes, heat generates, and a severe overcharge may occur resulting a heavy damage to the battery. For a maximized life, it is not recommended to repeatedly use constant current charging for refreshing batteries.
3.1.3 Taper-current Charging
In this system, the charging current drops gradually as the charging proceeds. It shall be accompanied by using a power transformer with a secondary voltage which is considerably higher than the battery voltage and a suitably high-resistance in the circuit for current limitation. A charging cut-off circuit should be incorporated in the charger to prevent overcharge. It can then be utilized for industrial uses for charging multiple numbers of batteries and for trickle charging system.
3.1.4 Two-step Combination Charging
This method employs two steps of charging. It can be constant current-constant current, constant current-constant voltage, etc. The switching from the first step to the second can be carried out by a battery voltage sensor, time control, or charge current sensor.
| 3.2 | CHARGING APPLICATION TIPS |
Battery life is affected by the charger's performance and the battery's operating conditions. Charger selection depends on the battery usage which may be cycle use or standby use (either under trickle charge or float charge operation). Please refer to Table 1.
Table 1. Charging method & battery application
| Application | Standby / Backup use | Standby use | During Storage | |
| Charging Method | I Trickle ChargeOperation | II Float ChargeOperation | III Cyclic Charge Operation | IV Refresh ChargeDuring Storage |
| Constant VoltageCharging | Regulation range of controlled voltage (20°C,68°F):-6V Batteries : 6.75V to 6.9V8V Batteries : 9.00V to 9.20V12V Batteries : 13.5V to 13.8V | Regulation range of controlled voltage (20°C,68°F):-6V Batteries : 7.2V to 7.5V8V Batteries : 9.6V to 10V12V Batteries : 14.4V to 15.0V | ||
| This method can providea short-time charge | The charge's current capacity must be big enough to maintain the specified charging voltage during float | Short-time charge allowed | ||
| Same model batteries, under the same storage, can be charged in series, otherwise they should be recharged in separate groups | ||||
| The charge voltage must be stabilized. Otherwise, battery may be overcharged or discharged.The charger should be temperature compensated when using battery in a wide range of ambient temperature | ||||
| Constant CurrentCharging | Not Recommended | Not Recommended | Not Recommended | Charging current:-Approx. 0.1CCharging timeControl is strictly recommended.Otherwise, charge may occur.No temperature compensation is needed. |
| Tapered CurrentCharging | Not Recommended | Not Recommended | Not Recommended | Not Recommended |
| Two-Stepscombination charging | Two-step constant current charge is highly recommended1) approx. 0.4C at the first step.2)0.002C-0.005C at the second stepA time control or a charging voltage detection device is required to transfer from the first step to the second | |||
| Note: C rate in the table refer to current as a percentage of nominal capacity Example: For model DM12-7.2 (7.2AH) 0.3C = 0.3x 7.2A = 2.16Amp | ||||
3.2.1 Trickle-charge standby application
Under standby use, batteries are normally kept in fully-charged condition, and serves as a power supply to the load when AC power fails. Under trickle charge operation, AC power is normally supplied for operating the equipment, while charging the batteries which are not connected to the load. If the AC power fails, a relay circuit connects the batteries to the load and battery power is supplied. A two-rate charger or a constant voltage charger can be used, while the first method is highly recommended.
3.2.2 Float-charge standby application
In this system, the load and the battery are connected in parallel with the rectified power source. This system requires only a constant voltage charger, regardless of the power consumption by the load. As the regulated voltage of a float charger is very close to the open circuit voltage of the battery, major fluctuation in charge voltage may cause battery discharges while on float. Therefore, in general, battery life in float charge is shorter than in trickle charge.
3.2.3 Cyclic application
Cyclic use requires protection against excessive charge and discharge, because the battery may be operated under unfavorable conditions by inexperienced users.
3.2.4 Refresh charge during storage
Constant voltage charge or constant current charge with limited time can be used.
3.2.5 Solar-powered Charging
Battery can be an indispensable component of any solar powered system.
Fig. 3 Block diagram of a Solar-powered Charging System | Naturally, in case where the output of the solar array may exceed the capacity of the battery, or where weather conditions are such that there is potential for overcharging the battery, it is recommended to apply an appropriate regulated circuit between the solar panels and the battery. DiaMec Battery can be charged by the solar array using regulated circuit as shown in Figure 3. As the system is exposed to direct sunlight, usually a highly reflective, heat-resistant surface material is needed. |
It is desirable for the charger to be temperature-compensated. For more details, please refer to section 3.5.
| 3.3 | FLOAT CHARGE CHARACTERISTICS |
| Fig. 4 Constant voltage charge characteristics at initial charge current of 0.1CA and constant voltage of 2.26V per cell at 20°C, 68°F | The time required to complete the charging varies by the discharge depth, initial charge current and temperature. As shown in Figure 4, charging a fully-discharged battery by 0.1CA initial current and constant voltage of 2.26V per cell at 20°C will need about 24 hours. However, under float charge, DiaMec battery has no limitation on initial charge current, so increasing the initial charge current will shorten the charging time. |
| 3.4 | RECOVERY CHARGE AFTER DEEP DISCHARGE |
Battery will be subjected to deep discharge or overcharge when it is discharged below our specified cut-off voltage. Battery life would be shortened and it requires a longer charging period than normal. From Fig.5, please note that as a result of high internal resistance, the charging current acceptable during initial stage of charging will be quite small, but will increase after more than 30 minutes when the internal resistance has been overcome. Then normal charging characteristics resume. Battery capacity may be recovered, but may be less than full capacity, if internal chemical combination is damaged.
Fig. 5 Recovery charge after over-discharge | Deep Discharge Conditions:-
|
| 3.5 | INITIAL CHARGE CURRENT LIMIT |
A discharged battery will accept a high charging current at the initial stage of charging. But continuously high charging-current can cause abnormal internal heating which may damage the battery. Therefore, it is necessary to limit the initial charging current to 0.3C or below, under constant voltage charge in cyclic application.
Under constant-voltage charge, DiaMec battery is designed that it will not accept more than 2C Amp, even the available charging current is higher than the recommended limit. Moreover, the charging current will then continue to fall to a relatively small value. Therefore, normally, no current limit is required for standby applications with recommended constant-voltage charge. But excessive charging current may break the internal connection mechanically.
| 3.6 | TEMPERATURE COMPENSATION |
| Fig. 6 Relationship between charging voltage and temperature  | Electrochemical activity in a battery increases when temperature rises and conversely decreases when temperature falls. Therefore, when temperature rises, the charging voltage should be reduced to prevent overcharge. And when temperature falls, it should be increased to avoid overcharge. Generally, use of a temperature-compensated charger is recommended in order to attain optimum service life. The temperature recommended compensation factors for 6VDiaMec battery are -10mV°C (for cyclic use), when temperature is not 20°C/68°F. Fig. 6 shows the relationship between temperature and charging voltage in both standby and cyclic applications. |
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