When the battery reaches 13.7 or 13.8v then suddenly charging stops and red color led glows as an indication for full charging. I have set this circuit’s calibration at 13.8v last voltage.After the battery reaches a certain level the transistor bc547 id tuned ON and it drives a relay which in turn changes the COM pin’s link from NC to NO which is connected to the red color LED that will start glowing after relay changeover, indicating the battery charging completion.During charging the circuits takes input via a charger /bridge rectifier charger circuit shown above and through the relay’s COM and NC, the power is directly connected to the battery.In the above circuit, whole charging is governed by BC337 transistor and relay RL1 acts as the changeover.Or I prefer to use the LM7815 voltage regulator circuit ( 1amps limit) or LM338 High power variable PSU circuit( 2- 5Amps) ( both are explained on the site.) Working of the circuit
I prefer to use a 15v DC 1A or 2A adaptor for perfect voltage and current regulation. The circuit diagram for charging control and auto-cut as well as a rectifier for power input with a material list is above. Battery suitable (lead-acid,SMF,SLA,GEL CELL,AUTOMOTIVE) 12v, 1.2ah-20 ah.BUT I RECOMMEND 15V DC 1A OR 2 A DC ADAPTOR SO THAT IT HAS REGULATED VOLTAGE ) or use LM338 Variable PSU Circuit, 2A-5A for bigger batteries. Transformer 0-15V,2Amp-5Amp ( for battery charging rectifier.R1 3.3 Ohm or 3R3, 1 Watt or 2 watts -1pcs.Q1 Transistor Bc547 or bc337(337 will be preferred more)-1pcs.If it does, then I prefer to use the LM7815 voltage regulator circuit ( 1amps limit) or LM338 High power variable PSU circuit( 2- 5Amps) ( both are explained on the site.) I still recommend using either 15v DC ready-made adaptor or use the LM7815 Regulator circuit or LM338 high power variable PSU circuit. After measuring the output voltage, add the diode drop (about 0.65V) and bias LM317 accordingly.Īuthor : Y.M.If in some cases, after the rectification and filtration the output voltage can have an increment of about 2v more due to capacitor. The shut-off voltage point is determined by charging the four cells fully (at 70 mA for 14 hours). Nowadays, 700mAH cells are available in the market, which can be charged at 70 mA for 10 hours. Here, we’ve set the charging voltage at 7.35V for four 1.5V cells. You may determine the charging voltage depending on the NiCd cell specifications by the manufacturer. When it falls below 650 mV, transistor T3 cuts off to drive transistor T2 and, in turn, cuts off transistor T3.Īs a result, relay RL1 de-energises to cut off the charger and red LED1 turns off. As the voltage per cell increases beyond 1.3V, the voltage drop across resistor R4 starts decreasing. Pushing switch S1 latches relay RL1 and the battery cells start charging. Diode D6 connected between the output of IC1 and battery limits the output voltage to about 6.7V, which is used for charging the battery. Preset VR1 is used for adjusting the bias voltage.
In the charging section, regulator IC1 is biased to give about 7.35V. Now even if the pushbutton is released, mains is still available to the primary of the transformer through its normally open (N/O) contacts. This, in turn, turns transistor T1 ‘on’ to energise relay RL1. When a current of over 65 mA flows through the 12V line, it causes a voltage drop of about 650 mV across resistor R4 to drive transistor T3 and cut off transistor T2. Resistor R4 (10-ohm, 0.5W) is connected between the emitter and base of transistor T3. Transistor T1 is driven by pnp transistor T2, which, in turn, is driven by pnp transistor T3. Relay RL1 is connected to the collector of transistor T1.
When you press switch S1 momentarily, the charger starts operating and the power-on LED1 glows to indicate that the charger is ‘on.’ The relay driver section uses pnp transistors T1, T2 and T3 (each BC558) to energise electromagnetic relay RL1. Regulator IC LM317 (IC1) provides the required 12V DC charging voltage.Īuto Turn-Off Battery Charger Circuit Diagram In the AC-to-DC converter section, transformer X1 steps down mains 230V AC to 9V AC at 750 mA, which is rectified by a full-wave rectifier comprising diodes D1 through D4 and filtered by capacitor C1. The circuit is simple and can be divided into AC-to-DC converter, relay driver and charging sections. It can be used to charge partially discharged cells as well. This charger for series-connected 4-cell AA batteries automatically disconnects from mains to stop charging when the batteries are fully charged.