Introduction: Car Battery Charger Repair

About: Part software developer, part maker.

Hi Everyone,

I got this battery charger as a gift from my brother but he didn’t know if it worked or not. This is a relatively old charger but if it works, then it should be a nice addition to my tools collection. Starting by visual inspection, I was able to identify that at some point in its life, the charger sustained a major drop or hit with something as the charging indicator was broken and the outer mesh that the voltage switch is mounted on was all bent.

In order to test it out, I took out my multimeter, and after plugin it in I measured the output voltage on both the 12V and the 6V output. At first, I thought that I might be getting a lower voltage because of the oxidation on the terminal clips but after some scraping, the output voltage was always half of what was supposed to be.

Something must be wrong on the inside so, in order to check, I proceeded to open the case and inspect the insides.


Step 1: Open the Case

A major challenge in opening the case was these security type screws that have the thread poking in the middle of the nut and the slot is on the outside of it. Since I didn’t have an appropriate screwdriver, I managed to improvise and use two smaller screwdrivers to push on the two slots at the same time.

Luckily, the screws were not very tight and I managed to open them.

The leather strap on the top was held in place with rivets, so in order to keep it intact I wiggled the case and I managed to open it without removing the strap.

On the inside, I immediately saw that the charging indicator was completely broken off and some of the posts that keep the components in place were also broken. However, regarding the electrical components, nothing seemed visually damaged and the transformer was looking excellent without any signs of heat or impact damage.

Step 2: Inspect the Circuit and Principle of Operation

To learn how the charger worked, and how it was built, I used my multimeter to trace out all of the connections, and also I did some voltage measurements in different parts of the circuit.

The transformer is wired directly to the mains input on the primary side of the windings and the secondary winding has a middle tap for the 6V output so the secondary winding has three wires coming out of it.

One of the wires goes directly to what seems to be the bridge rectifier and the other two are soldered to the voltage select switch that was mounted on the top.

The switch is then connected to the other input on the bridge rectifier where on its output, the positive wire was directly connected and the negative wire was connected through a piece of a metal strip that acted as a current shunt.

The charging indicator was connected across this shunt and was basically measuring the voltage drop across it when a lot of current was drawn to charge the batteries.

My initial suspect was the broken charging indicator as I thought that it might be somehow shorting the circuit and thus reducing its output voltage so in order to test that I desoldered it from the circuit and measured the voltages again but nothing changed.

Step 3: Test the Output of the Transformer

To verify that the transformer was working properly, I completely removed it from the circuit and I measured the output voltage on the entire winding and on the half tap as well. The voltage measured about 8V on the half winding and close to 16 on the entire winding which meant that the transformer was good and something else is the issue.

Step 4: Bridge Rectifier Wiring

Now, the only thing left to check was the bridge rectifier so I measured each diode separately and it seemed to be working. However, its construction is a bit exposed so over the years the material that makes the diodes have developed a relatively large voltage drop, cutting the output at half on the charger.

I decided to replace it, so, I sourced a new bridge rectifier, that can handle up to 50A and got to remove the old one by desoldering all of the wires.

Since I’ve never worked with such a style of a bridge rectifier, I made the mistake to assume the placement of the connections since one of the AC and the positive output connection were marked. I made the assumption that the other two are on the same side as the marked ones but in fact, they were diagonally opposite.

Unfortunately, when I first connected the charger, with a wrongly connected rectifier, the diodes shorted the output and the magic smoke was released. Baffled, I opened the datasheet and learned about my mistake, so I bought a new rectifier and replaced it again.

So, note to self: make sure to always check the datasheet first, when faced with a new part.

Step 5: Install New Bridge Rectifier

The installation of the new bridge rectifier was really simple as it was a lot smaller than the old one. Since I don’t have wire crimps to add connectors I added solder directly to the pads on the rectifier and soldered the wires.

One lead from the transformer is soldered to one of the AC pins on the rectifier, while the other two are soldered to one side of the switch. The other two contacts on the switch are soldered together and then they are soldered to the diagonally opposite side of the rectifier to the other AC connection.

The output leads are soldered on the other pins where the positive wire is soldered directly to the rectifier and the negative goes first to a resettable fuse and then to the negative connection on the rectifier.

Step 6: Assemble the Charger

With all of the electrical connections made, I returned everything to the case and I wiggled with the wire mesh on the outside in order to get it flat and sit properly in the dedicated slot. Once that was done, I’ve returned the screws in place and I first tightened them with a regular screwdriver until I could, and used my two smaller screwdrivers technique for the final tightening.

Step 7: Replace the Alligator Clips

As a final step, I removed the old rusty alligator clips and I’ve added new ones by first flooding the clip with solder and adding the wire to that solder joint. For some extra relief on the wires, I’ve bent the outer flaps over the wires and I made sure to add the cover to the clip and call it a project done.

Step 8: Test and Enjoy!

When I measure the output now, it shows 8V for the 6V battery type and 14V for the 12V battery type, making it right on the spot where it should be.

I hope that you managed to learn something from this Instructable and if you did, I encourage you to hit the like button, subscribe to my YouTube channel if you want to see me do more such videos, and check out some of my other projects as well.

Cheers and thanks for reading.