Auto Battery Charger for 6 or 12 Volt Sytems
Intro: Auto Battery Charger for 6 or 12 Volt Sytems
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
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
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.
STEP 4: Wire 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.
236 Comments
Jayleonfb99 2 years ago
Phil B 2 years ago
Charles L.C 2 years ago
I have a question. To keep from buying a trickle charger. Can I take a 6 amp battery charger. And make it charge like a trickle charger? It has a 6 volt, and a 12 volt setting. I need it to charge a 12 volt lawn mower battery. Thank You.
Phil B 2 years ago
JimK155 6 years ago
Hi I have just purchased a 12V ATV battery(AGM) .I have a older 10 amp battery 12V / 6V charger with overload protection. My new battery says to charge at 1.2A for 5 to 10 hrs .Do I need to purchase a new charger ?
Phil B 6 years ago
https://www.optimabatteries.com/en-us/support/charging/charging-agm-battery
NoskillsrequiredN 6 years ago
Hi there, buddy, do you have any idea there is a relation between amps needed and the capacitor on the bridge rectifier side i mean how do you choose the capacitor there must be something...
optimusprime 17722 6 years ago
Phil if i may , . I have 2 battery chargers ,the one i use the most i have done a check on it,, and the output is aprox 12 volts . .But the older one is only puting out 10 volts aprox .Do you know what that reason would be?.I thank you for the diagram i will make one a soon as i can .
Phil B 6 years ago
Or, a connection on the secondary side may be presenting a high resistance that is dropping the output voltage. The resistance could be corrosion or oxidation that you cannot see without taking a connection apart, especially if it uses a bolt or a screw.
Thank you for looking. As I said I am only guessing at possibilities.
ReynaldoB9 6 years ago
Can we use an ammeter after the bridge rectidier in serie with the load to monitor the level of the charge?
No amps consumption means the battery is already charged?
Phil B 6 years ago
ReynaldoB9 6 years ago
Thanks for your answer !
rrn1 6 years ago
Phil B 6 years ago
horusapollo 6 years ago
I have 12v to 17v dc generator can I use this charge a 12v battery and then the 12v battery to a invetor and then use 150 volts. Please help
Phil B 6 years ago
I believe you can. The inverter should have enough wattage to power the current draw of whatever you are using. So, a 3 Ampere current draw at 150 volts would require an inverter capable of handling 450 Watts or more. (Volts x Amperes = Watts)
syed subhan 7 years ago
Phil B 7 years ago
syed subhan 7 years ago
ManishJ24 7 years ago