It is handy to have a small battery charger for your automobile, especially if someone parks it in your garage for the night with a door just slightly ajar. In that case, the dome light will remain on all night and the battery will be very low in the morning.

I made this charger when I ordered some electronic parts and received a 120 to 12 volt AC transformer with center tap. Sending it back would have cost as much as the transformer. Similar transformers can be found at places like Radio Shack. The transformer puts out about 3 Amps., so it is ideal as a trickle charger. A charge will require about 12 hours. But, it has gotten me out of several jams. Some friends have also borrowed it when their batteries were dead and it worked for them.

The center tap allowed the output to be either 6 or 12 volts. At the time I made this charger I helped to maintain an older farm tractor with a 6 volt electrical system.

Note: The output of the transformer is actually about 13.4 volts. When the voltage passes through the diodes in the rectifier it drops 0.6 volt for each of two diodes to roughly 12 volts. If you look for a 12 volt transformer you may at first be frustrated because you can find only 13.4 volt transformers.

Step 1: The circuit and what you need

The circuit is very simple.

I used a piece of plywood for a base. I already had a double pole toggle switch. I used a piece of scrap aluminum cabinet angle to mount the switch. Any piece of aluminum or steel could be bent to do the job. I used a 4 Amp. bridge rectifier from Radio Shack. I also got aligator clips already connected to a cord to connect to the auto battery. This was also from Radio Shack. The AC cord and plug came from a neighbor's discarded televison set that was put out on the curb on garbage day. You will need some screws, soldering iron, and a hot glue gun.

Step 2: Mount the transformer and attach the line cord

Use screws to mount the transformer on the plywood base. Solder the ends of the power cord to the primary terminals of the transformer. I used a hot glue gun to cover the solder joints in order to protect against electrical shock. Hot glue is great for this. Be patient and let the first layer of glue harden so you can build the glue up for adequate protection.

From the photo you can see the details of the strain relief I made to hold the cord in place.

Step 3: Mount the switch and wire it.

Use the circuit diagram to wire the transformer secondary terminals to the switch. The aluminum angle is held to the base with screws. Label the switch positions for 6 and 12 volts.

Step 4: Wire the rectifier.

Of course you can use individual diodes to make a bridge rectifier. I found it easier to get a rectifier ready to use. I bent the input (AC) leads in one direction and the output (DC) leads in the other direction. This made a convenient base or support for the rectifier.

It is a good idea to use a heat sink when soldering diodes to protect them from too much heat. Put a rubber band on the handles of a needle nose plier and clamp the plier jaws on the lead you want to solder.

Watch the output polarity so the + terminal on the rectifier connects to the wire for the red aligator clip. I simply glued the bridge rectifier to the plywood with hot glue. Notice the strain relief for the output cord.

To use: Select 6 or 12 volts with the switch. Connect the red aligator clip to the red battery terminal and the black to the black. Set the base of the charger someplace safe. Plug in the AC cord. Disconnect the AC cord after 12 or more hours. Then disconnect the aligator clips from the battery. This prevents sparking that could possibly ignite hydrogen gas from the charging.

I have also used this charger as a power supply for things like a hot wire cutter. A smoothing capacitor is not necessary because batteries charge better with slightly choppy current.
Hello sir Pls help me out I want to build a 12v/70ah charger Pls how may I do that with transformer and diode
<p>hello sir</p><p> could this work for educational purpose as a source for car wirings circuits?</p><p>or even better buing one battery charger could make a wiring light of car with relay or flasher ( low light consumption) to work?</p>
Yes. The components I used are rated at only 3 amperes. The demands of devices you are demonstrating should not have a combined simultaneous current draw that exceeds the maximum output of your components. And, for the application you have in mind, I would add a 1,000 microfarad electrolytic capacitor at the output terminals of the bridge rectifier to smooth the current. That is good practice if you are using this as a power supply, not just as a battery charger.
Sir<br>Can I charge 12v battery with a 16v 4.5amp power supply ? It is a laptop chargers basicly.. I am using it to charge my battries.. will it damage my battries ?
I would want the voltage of the charger to be closer to the voltage of the battery. There may be a way to drop the voltage, but it might be more difficult and expensive than simply getting the right charger. Much depends on what batteries you are charging. A lead/acid battery would probably be more forgiving than a lithium-ion battery. Someone has done some Instructables on changing the output of switch mode power supplies. But, you would need to open the case of the laptop charger, find the correct resistor and change it to the correct value for the desired output.
Hi sir my name is Austine kauzeni from Tanzania i am a biginner to the electronics and i want to make an inverter than can also charge itself then i can also walkout with it anywhere and continue using without a problem of power drop . But kindly can you help me a circuits diagram?
<p>Hellow Sir,</p><p>I am making a 12 volt egg inkobeater and wants to run with 60 Amp wet battery, I want to make charger for battries need out put 65Amp please guide me While i have tried but that gives me just 7Amp</p>
In the end, the cost will be less if you can simply buy an automotive quick charger. Those have a high output like you need. I do not know where you would get all of the high current capacity parts needed to build your own. A 7 amp. Charger will eventually charge a battery that needs to put out 65 amps., but slowly.
<p>Hello I have a 12 volt solar charger that I want to drop to 6 volts. I see you are able to do this pretty easy. But I am just not seeing how you can drop the voltage from 12 to 6 volts could you better explain this please?</p>
The transformer I used has a center tap. The voltage between it and one of the outside taps is 6 volts. The voltage between two outside taps is 12 volts. Can you tap into your solar cell charger in a way that bypasses half of the cells?<br>
<p>hi sir</p><p>can this charger work with 12V 100Ah valve regulated battery? i bought an old battery from a junk shop nearby. i will use it as a backup power source when brownouts occur here in our place. i can't find removable caps on these batteries for adding water, its like a sealed type battery and only the two terminals are present on both ends.</p>
<p>I would think it could. You would want to avoid over-charging. Maybe you could run the charger through a timer. Have the timer come on for 30 or so minutes each day and see if that is enough to maintain a charge.</p>
<p>thank you very much sir, I'll try to make one. Uhmm.. any recommendation where i could add the timer in the circuit diagram you shown? I love electricals/electronics but I'm just a beginner. There are circuits I browsed on the net and this one you have is the simplest charger circuit I found, while the other ones have this IC parts that I was trying to analyze the works, you know.. kinda like catching a cheeta in the amazon!</p>
<p>ICs can do wonderful things, and are great, if they are really necessary. I do not understand much about most of them. I prefer to make things simple. If you wish to use a timer, add it to switch the AC mains. </p><p>There is a great story about Thomas Edison and Charles Stienmetz. Edison had a very large glass envelope for a special light bulb. He asked Steinmetz to calculate its internal volume. Steinmetz did a variety of calculations and took his results to Edison, but explained the uncertainty in his calculations. Edison asked him why he had not simply filled the glass envelope with water and measured how much water it contained. It is a lesson in simplicity.</p>
<p>that was also what I'm planning to do and cutting the AC mains is the safest way. thank you and I appreciate that much sir.</p>
You are very welcome.
Hi sir<br>First of all tanks for this useful circuit. <br>What's the rectifier's trade (industrial) name?
I am assuming you do not live in the United States. We have a grou of stores called Radio Shack. They sell electronic parts, including some parts. It is almost impossible to know what firm makes parts for Radio Shack. I would believe many are made in either China or other parts of the Pacific Rim.
I think thee may be a part number, but no maker's name.
Exactly. <br>There is no specific group here and we've to buy these pieces in different markets. However I meant the name written on the piece.<br>Thanks for the answer.
hi sir...i made a circuit that works on 6v DC. but i would like to know if this charger can be used to charge my 6 v battery without removing the battery from the holder and without damaging the circuit?
Congratulations on making a working charging circuit. You should be able to charge your battery without removing it. If the battery you are charging is part of a circuit with solid state electronics, you might want to protect against accidentally reversing the polarity. An easy way to do that would be to connect the charger with one type of male and female connector for the positive side and another type for the negative side. If the connectors are of different types, the wires will be impossible to connect in the wrong order.
<p>Hi sir, can I use 10A bridge rectifier(BR1010) with 12-0-12V transformer for charging 12V battery used for bikes.</p>
Yes, with one caution. The output current of the transformer should not be greater than 10 amperes or the bridge rectifier could be damaged.
<p>It would be a good idea to add a capacitor in parallel with the battery poles to smooth out the voltage ripple a bit. There are capacitors in almost every electronic device so salvaging one from a broken device shouldn't be so hard. The peaks of the ripple would induce electrolytic bubbling in the battery, unless regulated by a capacitor. Anyway, if no other components are available, this charger could save the day as it is.</p>
An electronics textbook I read a few years ago said batteries charge better if there is a little ripple, and that is why I did not suggest a capacitor across the output. I have temporarily added a capacitor to this charger when I wanted to use it as a power supply.
<p>Just did it, 3A trafo i bought for Rp50000 (about $4), and 4A Brectifier. Used it to charge my motorcycle accu which previously has dropped to 11.4v (holy-S!). For now it's still working just well, 30 mins has passed and the accu voltage raised to 11.7v. It's just a small accu (40Ah?) but i guess i have to wait for more hours.<br>My question is, what voltage do i have to stop charging? I think i'll stop the charging when it reaches 13v. I kinda afraid if it would blew up if i crank it to 14v.<br><br>*excuse me bout my english, not a native*</p>
Your English is much better than a lot of people who claim it as their first language. I have worked at learning a second language and know how difficult it is to use it well. <br><br>If your battery is sealed, it is difficult to know when it is fully charged. You could do some research on the Internet to see if you can find any data on your battery's voltage at a full charge. If your battery has removable caps so you can add water to the cells, any sign of boiling or of depletion of the fluid are indicators of over-charging. To be safe, I would charge until the battery has enough power to crank the engine. I expect the motorcycle has a charging circuit that will maintain the battery as you ride it. <br><br>If the battery does not take a charge, it may need to be replaced.<br><br>Congratulations on a job well done, <br><br>
Sir , how to make a permanent auto charger for 12v
By &quot;permanent&quot; do you mean one wired directly to the battery so it is never removed? You might want to check the cost and availability of a battery conditioner. These have smart circuitry to avoid over charging the battery.
Sir, can a 9v charge my car battery
No. A 12 volt auto battery needs a charger a little larger than 12 volts by a volt or two.
sir, can i use this circuit for 6volt 700mah battery? <br>this is a rc car battery i want to use that in my project so plz help me <br>
I suppose you could. It seems you could use something less, like a power adapter from a device you no longer use. That would be free. If the voltage on the power adapter is too high, a string of diodes will bring the voltage down to the desired level. Be sure the diodes are able to carry the current load. Basic diodes can handle one amp.
sir how to made simple home made 12v Dc to 230v Ac inverter? sir plz tell me
This is a bit outside my area. In theory, a transformer will not work with DC current, but needs AC current. A circuit that rapidly reverses polarity in the 12 volt portion of the circuit is needed. That is then fed into a transformer that converts 12 volts to 230 volts. Here is a so-called simple inverter circuit for 12 to 120 volts. Just use a transformer capable of putting out 230 volts. You will need to consider the maximum current load at which you want the circuit to perform and make certain all components are able to handle that. <br><br>http://circuitdiagram.net/inverter-12v-dc-to-120v-ac.html
How to 100Ah battery Charge
That is a small battery. I would use an old power supply for some household device. The power supply should have a low current output. If it is not already DC rather than AC, add a bridge rectifier. If the voltage is too high, use a variable voltage regulator and configure it for the charging voltage you need. You will likely need to experiment to determine the proper amount of time required to charge the battery.
How to 100Ah battery Charge
Sr plz 100Ah battery charge
sir how much ampere per hour by this charger
<p>Batteries produce a number of amps per hour. This charger produces 3 amps. at 12 volts continuously. A larger capacity transformer would produce more amps. </p>
<p>sir, if i increase the transformer to have more amps, should i also need to increase the rectifier amp rating?</p>
<p>Yes. The rectifier will need to be able to carry the maximum output expected from the transformer.</p>
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<p>sir how can we charge only a 12v battery without a double switch for 6v nd 12v battery... i just need to charge a 12v battery ...so can u plzz give me a circuit diagram without double swith...</p>
Just ignore the center tap on the transformer. Use the bridge rectifier and connect it to the end taps on the transformer.

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Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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