This circuit can extend the life of the Gel Cell battery. Because it is a lower voltage charging (adjust the voltage of 13.4V). Thus no overheating.
While the circuit is running, the LED indicates charging. And the battery voltage rises and a full current as we setting before. Then, the current-flow decreases to zero. At the same time, the LED will go out.
Gel cell battery charger circuit working
In the circuit above, We use the LM317 as 1.5A Adjustable Positive Voltage Regulator. It converts the input DC voltage into the stable voltage to charge the battery. Which we adjust the 5K potentiometer-VR1 for 13.4V output.
We set the output current by the current sensing resistor( R3) in the ground or ( – ) terminal.
The R3 is a current limiting. You can choose it to set the charging current.
- 300mA = 2.2 ohms, 1 watts
- 500mA = 1 ohms, 1 watts
- 1,300mA = 0.47 ohms, 1 watts
When the current flow through R3. This results in a voltage across the base and emitter of transistor-Q1. It is a forward-biased. Thus, Q1 conduct current to LED1 and adj of IC1.
The red LED1 will show charging battery. When the battery voltage reaches, the current will drop to a few milliamps. And it makes the voltage across Q1 and LED1 to lower. When it current drops about 5%, the LED turns off and the current drops to almost zero.
The AC adapter
We use an AC adapter as a power source of this. As circuit above, the plug pack 500mA DC for charging current of 300mA. But my son draws the circuit to mistake. We are sorry. The current of AC adapter should more than of 1500mA at 15V to 18V.
- 300mA output requires the plug pack 500mA up.
- 500mA output requires the plug pack 650mA up.
- 1,300mA output requires the plug pack 1,500mA up.
Parts you will need
IC1 = IC1 = LM317_1.5A Adjustable Positive Voltage Regulator
C1,C2 = 0.1uF 50V_Ceramic capacitors
R1 = 470 ohms _ 0.25W Resistor
R2 = 2.2K _ 0.25W Resistor
VR1 = 5K _ potentiometer
R3 = 1 ohms 1 watts Resistor
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