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Auto Battery Charger Blowing Fuses

DougB

Oct 13, 2012
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My trusty battery charger recently stopped working because the internal fuse blew. Since it has been a good device and now is not available to purchase, I would like to make it work again.

Here's pics:
photo1a.jpg

Photo2a.jpg

Without a circuit diagram, I am at a loss on what to do (not that my pay grade is high enough to use the circuit diagram anyway!). The fuse in the lower right is a fast acting 6.3A and I have replaced it 2 times. It blows immediately upon applying power. I see no component that looks like it failed. When a battery is connected to the cables (without plugging the unit into 120V), the display will show the % capacity - that still works. So I figure the problem is in the power supply side. I get a reading of 590 ohms between the positive and negative power input side. It seems that that means there should only be 0.2A (120V/590 ohms) going through it at least initially. Don't know if that is logical but it doesn't seem out of place to me. Otherwise, I took the Safety Capacitor (the yellow component in the lower right side) out and it seemed to test OK (with volt meter set to 2000Ohms, it showed open first then low ohms then higher, then open so it looks like it is therefore functioning). I took out the black disk component in the upper right corner because it had some semblance of damage. The board says it is a TH1 which implies it is a thermistor. It measures about 7ohms but does not change with heat.

Note: Picture does not show a 25A auto type fuse in the left half that I removed.

That's all I've got. Any suggestions on what to do?
 

Bluejets

Oct 5, 2014
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The unit is essentially a switch mode power supply (smps) and unless you have a certain level of knowledge with electronics then you are facing a rather steep uphill battle.
Blowing the primary fuse can mean rectifier diodes shorted or large caps faulty, or perhaps the switcher ic faulty or output mosfet/mosfets shorted.
Rather complex for a beginner, especially without a circuit diagram to refer to. Latter not always essential to those with experience though.
 

DougB

Oct 13, 2012
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The unit is essentially a switch mode power supply (smps) and unless you have a certain level of knowledge with electronics then you are facing a rather steep uphill battle.

I am a Mechanical Engineer with some training in EE so I sometimes can fumble though something like this and make headway. The unit doesn't work anyway so no loss if I screw it up.

Blowing the primary fuse can mean rectifier diodes shorted or large caps faulty, or perhaps the switcher ic faulty or output mosfet/mosfets shorted.

Do the experienced make an educated guess and remove a component and test it such that I might stumble in to it that way?
Thanks for your reply.
 

kellys_eye

Jun 25, 2010
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It's the primary fuse.
There are only a few components in the primary circuit that 'regularly' blow (short) to cause a fuse failure. These are (in no particular order) the bridge rectifier, the switching transistor(s) and/or the switching controller (IC). If you can identify those parts and find replacements then replacing them ALL will give you a good chance of it being fixed (but no guarantees). The total cost for the parts shouldn't exceed $10-$20.
Some close up pics of the right-hand side of the board might help identify the bits mentioned - include pics of the rear of the board if there are any parts on there too.
 

Bluejets

Oct 5, 2014
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I am a Mechanical Engineer with some training in EE

Yep...seen quite a few of those in my time.
 

DougB

Oct 13, 2012
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Actually, this happened when I was charging my RV battery with a portable generator at the same time as the in board battery charger was charging. If it was caused by that rather than just happened to fail at that time, could either using the generator or connecting the two chargers together have caused it?
 

kellys_eye

Jun 25, 2010
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could either using the generator or connecting the two chargers together have caused it?
The generator, certainly. If you have an inverter-generator you'd be ok - a much cleaner and safe power source - but most none-regulated generators have 'dirty' i.e. lots of harmonics on their voltage output and this is known to cause damage to SMPS circuitry.
 

DougB

Oct 13, 2012
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If you have an inverter-generator you'd be ok - a much cleaner and safe power source

My generator is a Honda 1000i which is an inverter-generator. But are you assuming it's good IF I charge the batteries with the inverter (which I don't) instead of plugging a battery charger into the 120V source (which I do)? Or are you just saying the output of the generator in general is better?
 

kellys_eye

Jun 25, 2010
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IG's are far better for powering electronic devices and your genset shouldn't have damaged the charger device BUT that's assuming all is well with the genset..... this can only be confirmed by using a 'scope on the output to test for 'cleanliness'.

Assuming it is ok then you've just suffered a random SMPS failure - see my previous post on the parts that are most likely to be the cause.
 

Martaine2005

May 12, 2015
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I was charging my RV battery with a portable generator at the same time as the in board battery charger was charging
Was the RV battery connected in circuit with both chargers connected to it?
Not a good idea for the future.


Martin
 

kellys_eye

Jun 25, 2010
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The way the fuse has blown indicates a dead short. This can be in the switching transistor(s) or the driver chip is 'stopped' and forcing the switching transistor to remain on. Either way one or both are dud and need replacing. The driver device is the 16 pin chip KA7500 and the two driver transistors are located behind the metal clip holding them on the heatsink.
 

DougB

Oct 13, 2012
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The driver device is the 16 pin chip KA7500 and the two driver transistors are located behind the metal clip holding them on the heatsink.


Thanks again for your help. Neither Digikey or Mouser have the parts in stock so will have to wait. In addition, I will be gone for over a month so will not be able to work on this for a while. Will let you know how it goes when I do.

Thanks!
 

DougB

Oct 13, 2012
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Oct 13, 2012
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Finally got the parts (KA7500 and the two PQP 27N25) and installed them - but no joy, fuse blows immediately upon turning it on. Interestingly enough, I tried as a last resort to disconnect everything leading to the main board (Fan and two cables to the display). When switched on, the fuse blew but not with as much fanfare (ie, no bright flash). It simply melted. Don't know if that's significant. Anyway, if there are no other suggestions, I guess I'll have to buy a new one.

Thanks for your help.
 
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