13-05-2011, 03:39 PM
There are many ways of batterycharging but constant-currentcharging, in particular, is apopular method for lead-acid and Ni-Cd batteries. In this circuit, the batteryis charged with a constant current thatis generally one-tenth of the batterycapacity in ampere-hours. So for a4.5Ah battery, constant charging currentwould be 450 mA.This battery charger has the followingfeatures:1. It can charge 6V, 9V and 12V batteries.Batteries rated at other voltagescan be charged by changing the valuesof zener diodes ZD1 and ZD2.2. Constant current can be set asper the battery capacity by using apotmeter and multimeter in series withthe battery.3. Once the battery is fully charged,it will attain certain voltage level (e.g.13.5-14.2V in the case of a 12V battery),give indication and the charger willswitch off automatically. You need notremove the battery from the circuit.4. If the battery is discharged belowa limit, it will give deep-dischargeindication.5. Quiescent current is less than 5mA and mostly due to zeners.6. DC source voltage (VCC) rangesfrom 9V to 24V.7. The charger is short-circuit protected.D1 is a low-forward-drop schottkydiode SB560 having peak reverse voltage(PRV) of 60V at 5A or a 1N5822diode having 40V PRV at 3A. Normally,the minimum DC source voltageshould be ‘D1 drop+Full chargedbattery voltage+VDSS+ R2 drop,’ whichis approximately ‘Full charged batteryvoltage+5V.’ For example, if we takefull-charge voltage as 14V for a 12Vbattery, the source voltage should be14+5=19V.For the sake of simplicity, this constant-current battery charger circuit isdivided into three sections: constantcurrentsource, overcharge protectionand deep-discharge protection sections.The constant-current source isbuilt around MOSFET T5, transistorT1, diodes D1 and D2, resistors R1, R2,R10 and R11, and potmeter VR1. DiodeD2 is a low-temperature-coefficient,highly stable reference diode LM236-5.LM336-5 can also be used with reducedoperating temperature range of 0 to+70°C. Gate-source voltage (VGS) of T5is set by adjusting VR1 slightly above4V. By setting VGS, charging currentcan be fixed depending on the batterycapacity.
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