Mains charger fuse blown

[Tom Richmond]

What would cause an internal fuse in my mains charger to blow?

 

30A 'caravan and leisure' three stage charger, connected to 345AH 12v bank. Charge is from Kipor 2600, which recently has been on for some pretty long stretches, as I have been doing power tool work elsewhere. Fuse was one of two 25A blade fuses, couldn't get a good look at the back side of the PCB, but it seems like they are directly connected to the DC +ve and -ve out of the charger. If so I think it was the +ve that blew...

 

Nothing wrong with battery bank, fuses, cables etc... Nowhere I can see that a short could have occurred. The fuse was pretty melty, so I am guessing long exposure to high current, rather than something quick like a short.

 

Have replaced fuse and we're charging away, but would be good to know why it went, especially if it's likely to happen again!

 

Cheers

[b0atman]

charger is capable of putting 30 amps out whilst you have 25amp fuses ?

[tonyt40]

Tired fuse?

[Tom Richmond]

charger is capable of putting 30 amps out whilst you have 25amp fuses ?

 

Yeah, I thought that too... Fuses as fitted when it was supplied.

 

 

Tired fuse?

 

Can that happen? If so I'll relax some...

[WotEver]

 

Can that happen? If so I'll relax some...

Yes it can. Particularly if regularly run close to its limit.

[Taslim]

Yes it can. Particularly if regularly run close to its limit.

Yup. Same can happen with transient suppressors when run on a 'dirty' supply.

[by'eck]

A multi-stage charger in the initial constant current mode sometimes exceeds it's max output current for a short while after initial switch on. I have seen 108 amps from my Sterling Combi''s 100 amp rated charger, with highest figures ironically when batteries only partially discharged.

 

In OP's case 25 amp fuse is too low suggesting that manufacturer lacked confidence in the product. Suggest 50 amp replacement.

[Arthur Brown]

Well it seems that the fuse protected the charger, so stick with that rating. A blown charger is far more expensive to replace. Add some ventilation or a suitable fan to keep the kit cool.

[Chewbacka]

You said 2 fuses, with the assumption that one is pos and the other neg.

To enable the use of easily available and low cost fuses, some manufacturers will put two fuses in parallel. These fuses are happy to load share as if one fuse starts to take more current (talking near the fuse limit here, so under full load) it will warm up a bit, as it warms it's resistance will rise and so current will then tend to flow more through the other fuse. This will keep them balanced.

 

So if this is the case, if you uprate the fuses to 50A then you would need a fault current of 100A to fail, which effectively means you do not have a fuse.

 

If the blade fuses were very hot for a long time the female sockets may have oxidised or opened up a little, so I would confirm the insertion/removal load is reasonable and take the fuse in and out a few times to clean up the contact area.

 

Fuses to age, were they very old when they failed?

[alan_fincher]

You said 2 fuses, with the assumption that one is pos and the other neg.

To enable the use of easily available and low cost fuses, some manufacturers will put two fuses in parallel.

 

Yes, I think this is possible, so, without more evidence, I think the advice that 50A fuses can be substituted for the 25A is dangerous, and needs to be ignored.

 

If OP really is unable to inspect the back of the board to see what these fuses are doing, at least try disconnecting everything, removing both fuses, then use a multimeter to see if both fuse holders are connected in parallel. If there is zero ohms between the contacts at one end, and the same at the other, I think it is a fair assumption that they are actually two 25A fuses acting together to perform the role of a 50A one. If so the fact one blew should not be used as an excuse to fit a bigger one!

[Tacet]

If the two fuses are in parallel, how would you know if just one had blown?

[RD1]

...with highest figures ironically when batteries only partially discharged...

If you think about it, a fully charged battery will take very little charging current, a fully discharged battery will take a long time before it even accepts a charge, so a partially discharged healthy battery will take the highest current.

 

If a fuse blows in a kettle at home after say three years of normal use, then I am not that suspicious, I would check the security of the wires in the plug when replacing the fuse and walk away. If the fuse blew within a week or less, or just a few hours, then there is a serious fault and time to replace the kettle. If it works still after a year or more, then fuse fatigue was the cause.

 

So with the charger, I would replace both fuses and wait and see, if ok after season one, put it down to fuse fatigue, again, if it fails sooner, there is another fault elsewhere.

If it has an onboard cooling fan, then consider that there may be a build up of dust which can cause components to heat up.

 

CAUTION :- not all fuses blow at the same current, due to poor quality and poor components, if you went for a good quality fuse then it will protect the circuit as designed.

 

Now compare that to a pack of 25 fuses from a pound type shop, they all probably work with a short circuit, but the most heating of the fuse will occur at 100 % load, and in fact they might continue to run at 120% full load for a few minutes, so in this time, some fuse holders might melt, or even catch fire, so caution when choosing values of fuses and quality of fuse holders. If in doubt, if you have the right test equipment, you could increase the load through a fuse and holder, to check the temperature rise of the fuse and damage to the holder just before the fuse finally blows.

 

Never increase the value of a fuse recommended by the manufacturer, unless you have their written permission.

Hope it helps,

Richard

[Tom Richmond]

Thanks all. Sorry, been looking the other direction for a bit...

 

The charger stopped working (well, stopped giving charge to the batteries) when the fuse was blown. So the remaining not blown fuse cannot have been in parallel. I think the comment above that the manufacturers did not have faith in the product is probably closest to the truth, but I am more interested in the answer that this fuse blowing successfully protected it.

 

Have replaced with like 25a fuse. If it blows again I will worry more, but if not happy to put it down to a tired fuse.

 

I might consider upgrading to a 30a fuse, but not 50! Point of note, 20cm down the +ve DC cable it is another 30a fuse...

 

Thanks for your comments all.

[WotEver]

I might consider upgrading to a 30a fuse...

I wouldn't. The manufacturer put a 25A in there for a reason.

 

You mentioned signs of heat around the fuse. When you put the new one in did the spring clips still have plenty of spring in them? Did the fuse snap firmly into place? I ask because fuse holder springs can lose their temper over time. (Cue the jokes about angry holders).

[Tom Richmond]

Nope, connections were fine. Good firm fit.

[RD1]

...

I might consider upgrading to a 30a fuse, but not 50! Point of note, 20cm down the +ve DC cable it is another 30a fuse...

Thanks for your comments all.

I think you need to reconsider that thought, as the power available goes up by the square law.

 

For example, 25 x 25 = 625, 30 x 30 = 900 values used to compare

 

so the power available goes up nearly 50% (well 44%) where is all that extra power going to be dissipated?

 

Oh a 50 amp fuse, the power will go up 4 times. hmmmm

Richard

[Tom and Bex]

We have a caravan and leisure 3 stage charger (albeit 40a) and the 2 25a fuses in ours are definitely connected in parallel.

 

I would be suspecting that the non blown fuse holder might have a broken internal connection or fuse just not making contact with the holder. This would explain the blown fuse showing signs of overheating and eventually blowing. Most fuses will take overcurrent for some time before eventually blowing.

 

Tom

[Chewbacka]

I think you need to reconsider that thought, as the power available goes up by the square law.

 

For example, 25 x 25 = 625, 30 x 30 = 900 values used to compare

 

so the power available goes up nearly 50% (well 44%) where is all that extra power going to be dissipated?

 

Oh a 50 amp fuse, the power will go up 4 times. hmmmm

Richard

25A at 12V is 300Watts

50A at 12v is 600W

 

So going from 25 to 50A fuse the power taken to overload the fuse only doubles.

 

 

I think you may be confusing having a fixed resistive load so at 12V it takes 25Amps (still 300W) but to increase the current to 50A through the same load would require 24V, so the power consumed by the load is 24*50= 1200W which is 4 times the power.

 

However with a fault the resistance is often very low and the fault current is limited by the fuse blowing. However the instantaneous current can be very high. just as the fuse blows. Though that depends upon the fault capacity of the supply.

Edited December 16, 2016 by Chewbacka

[WotEver]

Mixing up I²R with IE