Automatic battery float charger circuit: This article aims to develops a battery charger circuit using the principal of float charging technique. Float charger also refer as smart charger, maintenances charger or storage charger because it charges the battery at the same rate at which it self-discharges.
The main reason of using float charger is, it protects battery from overcharging and deep discharging. Thus, you can connect float charger circuit to a battery for indefinitely interval of time i.e. there is no need to disconnect the charger circuit from the battery. The project ‘Automatic battery float charger circuit’ is designed to charge 12V sealed lead acid battery.
Circuit Description of Automatic Battery Float Charger:
The circuit is shown in figure 1. For description we had divided the entire circuit into three main sections.
Rectifier Section: – This section is designed around a step-down transformer and a bridge rectifier. We had used here 230V to 15V, 1A transformer which lower the AC mains to 15V AC. These step-down voltage (15V) is changed to pulsating DC using bridge rectifier. Bridge rectifier is build around four rectifier diodes. Capacitor C1 filter the pulsating part of DC and smoothen the wave.
Current Limiter Circuit: – Current limiting circuit is built around LM317 (IC1) variable voltage regulator IC. Here we had designed current limiting circuit for 0.3 Amps because data sheet of battery recommended so. In order to design current limiting circuit, we had to calculate the value of resistor R1.
Generally, in market the value of resistor 4.1667 is not available thus we had used nearest value of resistor i.e. 4.7 .
If we had used 4.7 resistor the current limit to
Voltage Regulator Circuit: – Voltage regulator circuit is also build around LM317 variable voltage regulator IC. Let the max charging voltage will be 13.75V and we had used a diode for blocking reverse voltage from battery. thus we had produce the output of 13.75V + 0.7V
The standard formula for calculation Vout is
Here we had design the circuit for 14.45 thus we had to find the value of resistor R1 and R2
In order to get the value of 2320 we had made a series combination of 1.5 and 820 .
Automatic cutoff Section: – The circuit of cutoff section is build around op-amp LM358. Op-amp LM358 is configured in comparator mode. Op-amp under open-loop operation is indeed a basic comparator circuit for comparing signals at its input terminals. The comparator can be used as a non-inverting or an inverting circuit. here we had used inviting circuit. For the operation, Vin is applied to the non-inverting input terminal of the op-amp and the reference voltage (VR) is connected at the inverting input. In this case, the comparator output will be in the ‘high’ (or 1) state (= + VZ) when (Vin – VR) > 0; and it will be in the ‘low’ or ‘0’ state (=-VD) otherwise.
When the battery is fully charged the relay become de-energized and vice-versa.
Relay Switching Circuit: – The switch is designed using a NPN transistor (BC547). When battery is charged the transistor start to connect and hence energized relay. Similarly, when battery is fully charged the transistor is switch off and hence relay is deenergized.
Transistor T2 is used because when battery not connected to battery terminal 0V is appeared at inverting input. Hence, the output of op-amp becomes high. To avoid this condition, the PNP transistor grounds the base of BC547. Hence output is entirely controlled.
Glowing LED1 indicates power supply of the circuit.
Glowing LED2 indicates the battery is charging.
Glowing LED3 indicates the battery is fully charged.
PARTS LIST OF AUTOMATIC FLOAT CHARGER CIRCUIT
|Resistors (all ¼-watt, ± 5% Carbon)|
|R1 = 4.7 Ω, 2W
R2 = 220 Ω
R3, R12 = 1.5 KΩ
R4 = 820 Ω
R5, R9, R10 = 1.2 KΩ
R6 – R8 = 1 KΩ
R11 = 750 Ω
R13 = 10 KΩ
|C1 = 1000 µF, 30V|
|IC1, IC2 = LM317
IC3 = LM7805
IC4 = LM358
T1 = BC547
T2 = 2N2907
D1 – D6 = 1N4007
LED1 = 5mm, Blue color LED
LED2 = 5mm, Red Color LED
LED3 = 5mm, Green Color LED
|X1 = 230V to 15V, 1A Step Down Transformer
RL1 = 5V relay
12V, 3A Battery to be charged