Repair Your Vehicle Battery Charger





Introduction: Repair Your Vehicle Battery Charger

UPDATE: Originally I published this tutorial for repairing my Bosch C3 vehicle battery charger, but 2 days after publishing this instructable, a friend of mine called me that he got the very same problem with another type of charger, and I fixed it for him the very same way. So I renamed the title to be more general, as it could solve these kind of problems possibly on several types of vehicle battery chargers.

I have a Bosch C3 intelligent charger that soon after the warranty time just stopped working.
It's a hundred-dollars intelligent charger and I really love its simplicity so I tried to search the net for a possible fix.

Unfortunately I only found dozens of complaining customers, who also bought this device and having the same problem: The device turns on, blue ring around the Mode button is illuminating, but nothing happens if you press it. (See the link about this issue at Amazon customer reviews on Bosch C3 Charger)

The illuminating blue led circle around the Mode button showed that it is powered on, so I was pretty sure that only the switch itself gone away. Being an electronic hobbyist I thought that I try disassemble and see if the switch is broken. The device is already dead, so I can't make it any worse. :) Finally I did it! The fix was successful and was relatively easy, it took 1,5 hours for me including taking the pictures. I am so happy that I could fix it, that I thought it may worth creating my 1st instructable, and help you guys out there, fixing your own Bosc C3 charger! It is a typical problem with this charger but please be advised that I cannot assure if your device has the same problem or something else!

Warning! Only try this if you are having experience in repairing electronic devices. You will need to have soldering skills to replace the switch. You will open up a circuit that can cause serious injury or even death if powered on, so as the step no. Zero, make sure you pulled out the plug from the wall outlet!!! I hope it's needless to say, detach the charging cables from the battery as well. :) Only try this instructable at your own risk! I will not take any responsibility for any possible damage you may cause!!!

Step 1: Open Up the Case

Again, please double check that you have unplugged the power chord and detached the battery from the charger terminals!

First you need to open up the case by removing 6 screws that holding the case top and bottom part together.

Each of them is hidden behind a rubber plug. Just pop out those plugs with a small flat-headed screwdriver.

For disengaging the screws you will need a special tool to unscrew the 6 screws I made it from phillips head, grinding down its edges with my Dremel until I got a nice triangular shape tool that fits in the heads of those nasty special screws.

Once you have removed the 6 screws, the bottom of the box should come off with some wiggling.

Step 2: Remove the Sandwitch Board From the Top of the Case

Unfortunately the swith is on the very top, so you will need to lift out the boards from the case-half.

You will see 5 small bolts on the panel, but only 2 of them are holding the panels inside! Only remove these 2 philips bolts as shown in picture. The other 3 are holding the two boards together, so you don't have to remove them yet.

After removing those 2 bolts you will see that the wires are rubber-fitted through the case and their connection wires inside the box are very short.

So you won't be able to remove the panels completely but you can turn it out from its place so you will access to the other side of the board.

Once you have flipped the boards out, you will see the micro switch!

Then you will say: OMG! :) This microswitch is a very common piece, in my country (Hungary) it costs about 0.1 USD. Yes, you are reading it right, zero point 1 USD is the total cost of this repair (and a reason for a failure of a hundred-dollar device).

The switch is a 6x6mm 4-pins push-to-close type switch in SMD case, again, very common type.

It has about 3.5mm height at the top and the correct height is important, so if you are unsure, just take it out and ask for the same in your electronic store. :)

Fortunately I have couple of these switches laying around so I just take one out from my stock and replace it.

Before you go any further, you can check the switch with a multimeter to make sure it is indeed went away.

For this, just grow a 3rd hand or use something like a crocodile clip to hang your multimeter set to short-circuit-check function (when it beeps on contact) on any two pins of the switch. Two of the pins are shorted by default on both sides.

When the switch is pushed, it makes contact between the 2 pair of pins. So if you are keeping the button pushed with your 3rd hand :) the multimeter should beep between any 2 of the 4 pins. Simple as it is!

I checked mine and I was so glad that my switch was faulty!

Let's get to soldering!

Step 3: (Optional Step: Separating the Boards for Ultimate Protection)

As you can see, the pins of the switch is soldered onto the top of the board, so if you are in a hurry you can simply replace the switch at this point and proceed to the next step.

However I was thinking differently...

The case of the Bosch C3 is very solid and robust. Marked as IP65 at the bottom sticker, so it should be weatherproof. As you have seen the case is almost airtight, the power inlet and charging outlet wires are rubber molded, there is a rubber seal between case halves and even the screw-heads are plugged in heavily.

As you can see on picture 3, I found traces of some kind of a corrosion on several soldering points and on the board. Without knowing the reasons of these kind of corrosion exactly, I'd take a wild guess on that the reason why microswitch failure happened might also be a result of some kind of this corrosion on the metal contacts of the switch. Maybe condensational moisture, I don't know.

So to the point, I decided to separate the boards of sandwitch panel, to be able to clean it properly. Additionally, the switch can be replaced easier if you have access to the bottom too. And again, setting switch height properly is important here!

If you like me prefer to separate the panel boards, remove the 3 screw that you didn't remove previously, and set them aside in a safe place. Be careful you have not only the bolts, but also the nut, spring washer and plastic separator that keeps the distance between the two panels. One of these 3 screws can only be grab through a hole from the bottom panel side.

Once you get all the bolts out, you can de-solder the pinline that connects the 2 boards together. Be careful and patient, don't heat it very long, use your desoldering pump and heat it in shorter bursts.

Now here comes the switch change, but if you are reading this, then I'm sure I don't need to tell you how to do this.
Easiest part of the project, but if you are interested in how I did it, I will tell you in the next step. :)

After soldering, I cleaned both boards with a diluent and applied a layer of Urethan 71 coating insulator to protect the circuit from later corrosion.

When it's clean, dry and shiny, you can put the 3 bolts, plastic spacer, washer and nut back and tighten the bolts. Only when the panel is in its final place, only then solder back the pins of the pinline. I also applied the Urethan spray on the pinline after soldering.

Step 4: Change That Nasty Micro Switch!

Fortunately the factory-installed soldering points of the switch are relatively big, so you will have enough room for soldering. Apply medium temperature for according to your soldering iron, which for me was about 270 degrees Celsius space.

Heat the point in shorter bursts, do not heat one point for long! Doing so can heat up the board and components too much and may kill you C3 charger board.

Heat up the solder until it became liquid, then suck it up with your desoldering pump. Keep it going again, until you can see a the gap between soldering pins and the board. As it frees up, you will notice when you can move it straight up and away from the board.

When you removed, you can check the failure again with your multimeter, and check the replacement switch too for proper operation. Insert the switch's legs into their apropriate holes.

Don't worry about the orientation of the new switch, it is quite easy: the distance and the shape of the pins are not the same on all sides, so there are visually 2 'longer' and 2 'shorter' sides of the switch regarding the overall dimensions including the legs. You can put it in 2 equivalent ways into its place legs, but you can't really put it in a wrong direction unless you force the legs to fit in the wrong holes. :)

Put it the new switch in place and push it down fully. Bend the other ends of the legs to fit the switch tight, and solder the four pins to their posts. Solder in the new switch and clean up the area. Use a diluent to clean up everything on the board.

Now, with the new switch in place you are ready for cleaning and packaging your repaired C3! :)

Step 5: Wrap It Back Up

Now you are ready with the tough part, just need to repeat the steps of disassembly in reverse and you are ready for a charge-up! :)

Carefully turn the panel back into its place, in the top section of the case. Be careful, and don't forget to insert that black round plastic spacer right onto the Mode button top foil before you put the board back into its place. Without that plastic spacer, you wont reach your new microswitch when pushing the Mode button, and it won't work. (Ask me how I know... :) )

You will need to play-in the board to its place. Take care about the correct alignment, the bigger board should touch the holes of the 2 screws, and the high voltage cable (right side of the case) should be aligned correctly in their guides just as it was before.

If succeeded, you can tighten back the 2 screws, put on the back cover, and fasten the 6 case-screws with your triangle-shaped special tool. Don't forget to insert the 6 small rupper plug above the case screws!

Congratulations, you made it, and let's hope that you made it well! Try to charge a battery!

No suprise, it is live again, and charges just fine! :D

I hope you enjoyed my first instructable, please feel free to ask any questions!



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26 Discussions

I'm happy too that it helped you, thanks a lot for your feedback! :)

Greetings. I have a question about the Bosch C3 Charger. I was also annoyed with a switch, but it started to funfill a few times. The charger seems to charge, the appropriate diodes are on, and the charging diode flashes, but the charger DOES NOT KEEP. Do not you know where the fuse is 1.6 A, or if there could be a problem in the switch?
Many thanks in advance.

1 reply


How do you mean "funfill" and that "it does not keep"? The charging LED starts blinking then shuts off? Other leds are on? Can you send a picture? Not sure about the fuse, but if it were a blown fuse, then it wouldn't power on at all... If it reacts on the button (you can start charging by pushing it) then it is not the switch.

I would try it on a known good battery, however bad battery does not start charging. Also if left the car with a light on overnight or something, the battery can have very low voltage. Regular car batteries need time to recover. You need to disconnect from the car and wait at least half an hour until real voltage can be measured. If you don't wait enough (or if its dead) the battery is disharged very much, then it might be recognized as 6V battery, and c3 starts to charge it as it were 6V. In this case, just press the button again until LED jumps from 6V to 12V LED and wait for charging. However if a battery is toast, it wont start charging at all. Measure the battery voltage with a multimeter after waiting an hour after disconnecting from car, it should read more than 10V. If you left it disconnected overnight and it is only 5V then you probably need a new battery.

Test the charger 1st on a good known battery and let me know the results!


7 months ago

Thanks for this valuable manual! I have C7 and the same issue with button :( I'm very dissapointed wth Bosch and "german quality"....

Btw, do you know the parameters for this switch? I checked online parts shop and they ask me for voltage, max current and push force. I didn't remove mine yet so can't check it. Thanks!

1 reply


Thank you, I'm so happy that you like it!

Only the size of the switch is important, I haven't measured voltage and amps but they supposed to be very low here. You can try measuring the voltage and amps with a multimeter though, if you are really curious, but there is a chance that you will only have momentary values that you can't actually read accurately, because it's an impulse switch, and its only for rotating the modes of the logic of the circuit.

For voltage: Set your multimeter to DC 20V setting and connect the two probes to one of both opposite pair of legs. (So the multimeter is connected in parallel to the switch)
For current: A little more complicated, you need to connect the multimeter in serial, so only one side of the switch (two legs) is not connected to the board, and the other two is connected to the first probe of the multimeter, and the second probe is connected to the board. So that the current can flow through the multimeter and can be measured. I think 20mA setting would be sufficient, but it doesn't hurt to start with 200mA or even higher setting, just for safety. :)

Thank you for this! I fixed mine in less than 15 minutes, i opened it up and just sprayed the switch with contact cleaner and the charger has worked for the past 2 weeks without a hitch

1 reply

Thank you, Glenn, I'm glad that you could make it work so quickly! :)


1 year ago

What if my C7 has no indicator lights at all when plugged-in, any chances of survival? I think someone dropped it while it's charging (pulled the extension cord).

3 replies

Hi Kirx,

I would open it up and take a look. You can't make it any worse when it's already dead. You can be lucky and have only a broken soldering point which you can easily re-solder. Pull out the wall outlet before opening! Check the soldering points at the high-voltage part of the circuit first (light brown color side of the PCB on mine), specially where the two main power supply wires connects to the board. Sometimes the gap at a broken soldering point is very thin. Check the continuity of wires/lines to the PCB with a multimeter. It should beep between the wall connector-end and the soldered point-end on the board. If it all seems to be ok, you can check each soldering points for a break with the multimeter, one post connected to the soldering point on board, the other touching the soldered part's leg. Any protective/insulation layer on the PCB can make you read a false positive , so make sure your multimeter's testing post is really touching metal. (My C1 didn't have any coating though)

Good luck!

Hi. Yea. Just opened my C7 and there's a component soldering point was disconnected but it's not easy an easy fix. Need to disabled everything on the board before I have room to do anything! Thanks for the howto anyway. Giving me the guts to open the box and see what's actually inside. Now it's time to look for someone capable to work on it.

IMG_2017-02-01 15:10:02.jpgIMG_2017-02-01 15:10:12.jpg

Oh yes, there it is! I'm glad that you found it!

Looks like only those soldered legs supported this transformer coil? Would be a poor design...

Maybe you can remove heatsink separator, to the right of the transformer coil, and then you can have access to the broken leg. I would add some hotgun support under the transformer to prevent breaking again in the future.

Looks an easy fix, but I really don't want to suggest doing something you are unsure about!

I hope you will make it or have it repaired. Good luck!

I was about to throw that C3 at the garbage when I decided to give the
Internet another shot. It failed on me couple of years ago prior to your
post. Couldn't find any quick fix back then. Now thank to you it works
like new. Love you Moire.

1 reply

I'm SO happy that it helps so many of you, thanks a lot for the feedback!

Love you too! <3

My Bosch C7 was the same,

thank you very much

greetings from Spain

3 replies

Hi Miguel,

Thank you for your comment, I'm very happy if it worked for you too!

I love Spain very much, will be visiting Fenavin in Ciudad Real in May! :)

Greetings from Budapest,


Good luck in the "sea of wine"
My father had business 30 years ago with Hungarians and gave us much palinka, you are a brave people.
I live in Zaragoza producing fruit.

Thank you for a great instruction

It worked - but I thougt I failed because the button didn´t work after I had changed the switch.........Until I tested it on the car battery !!!!

So it can be mentioned that it MUST BE connected to a battery before the switch works ! You can´t test it at your dining table.

1 reply

Hello Henrik,

Thank you for your comment, I'm glad that it worked for you too!

Of course, you need to connect the battery to be able to test the button by start charging. With no battery attached, pressing the button will do nothing as it is written in the user's manual of your charger. :)

The best is to connect the battery to the charger first, and only then plugging the charger power source cable into the wall outlet.