# What size fuse for Chennic DC-DC charger?



## m38mike (Dec 27, 2008)

I've got a Chennic 400w charger on board to keep my 12v batt in good shape. But as I looked at the instructions, no-where does it mention what size the fuse should be in the fuse holder. The fuse holder is on the high voltage input line. I put a standard 20 amp fuse in there, and it blew right away. I'm wondering if a 35 amp fuse might be better. (400w/12v=33.3a)

Anyone with a Chennic 400w charger figured out the fuse size? Care to share???


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## dtbaker (Jan 5, 2008)

I'll look at mine again, but I think its a 40amp slo-blo. gotta be slow to avoid the momentary voltage sag draw for vacuum pump and headlights.... I also added a little tiny battery in parallel.


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## m38mike (Dec 27, 2008)

40 amp sounds reasonable. Not sure if I need the slo-blo style. this charger maintains a 12v batt that runs lights and wipers and relays. No vacuum pumps on my old Willys. But the slo-blo would reduce the problem with startup spikes taking out the fuse. I like it.


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## Overlander23 (Jun 15, 2009)

m38mike said:


> Anyone with a Chennic 400w charger figured out the fuse size? Care to share???


I've got a 500W Chennic DC-DC converter that uses a 15 amp rated fuse in that holder. But I imagine it would have something to do with the input voltage. You're not fusing for the 12v output. Mine's designed for 192vdc nominal, but will go up to 250vdc. The stock 15 amp, therefore, is pretty overkill (192v * 15A = 2880w), but probably accounts for increased current draw at low pack voltages, and possibly startup spikes(?)

I was snapping fuses when my motor controller's ripple voltage (noise) was causing problems. Never manifested itself at "idle", but would always blow fuses on acceleration. Solved temporarily with a rectifier, but as the good citizens here point out, should be filtered through a choke.

I do know that HV fuse will blow instantly on any polarity reversal.


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## m38mike (Dec 27, 2008)

I'm thinking that I need to manage about 400 watts with this charger, so I'm going to experiment. I picked up some 40amp 12v fuses to try. They should take about 480 watts before they blow. I also picked up a 30amp circuit breaker that'll fit the fuse holder. I'll try that one first, to see if it'll keep up with the 12v demands. That should allow up to 360watts before it opens. And the advantage of that one is that it'll reset automatically. 

Now I've got to send it all through an ammeter to see how many amps it's really pushing into the 12v system battery.


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## dtbaker (Jan 5, 2008)

When I had lead batteries in a 96v system thru a Curtis dc-dc I am sure I had more sag, but never blew the 12v side or saw headlights dim when vacuum pump, headlights, or heater fan were turned on. after upgrade to lithium I went to 120v chennic dc-dc, and would suffer momentry brown out of headlights when vacuum came on. I reduced it by putting a little 12v rechargable battery inline on the output like a mini-aux battery. I think it only has 1.2ah, but seems to be enough to fill in when the dc-dc sags under inrush load.


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## m38mike (Dec 27, 2008)

I still need to do a test drive, but I've got the Chennic DC-DC wired through an ammeter now so I can see how many amps it's providing. I put the 30 amp circuit breaker into the fuse holder and it's working. I turned everything on and the Chennic was pushing about 15-17 amps into the 12v battery. Once I'm out on the road on a typical commute and see how much I draw down the 12v battery, that'll tell me for sure if the Chennic is big enough, and if the 30 amp breaker will work for me.


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## Overlander23 (Jun 15, 2009)

Unless I'm comprehending what you've written incorrectly, I'm confused by your description of your setup. You said that you've got the inline fuse holder on the HV input side (which would match my own Chennic's setup)... not the 12V output side. Or have I missed something?

If this is the case, a 12v rated fuse (or breaker) isn't the best idea in the HV line. 15-17 amps into the 12v circuit seems about right for a partial load (180-200 watts). But you shouldn't be drawing anywhere near that kind of amperage on the input side... unless your HV side is 12v. Take a common conversion, 120v. The converter should be drawing 2A or less of HV current at that output wattage.

My Chennic pulls less than one amp under normal conditions when my pack is reading 215v. That's with pretty much only the Soliton-1 as a 12v load, and whatever topping up the Chennic is doing to my small 12v AGM.


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## m38mike (Dec 27, 2008)

You're right, I didn't describe what I'm doing very well. So let me describe my set up again. I have a Chennic 400 watt DC-DC charger. On the HV + input wire there's a place for an automotive fuse. Knowing that the charger puts out 400 watts, at 12 volts that's 33 amps. That's what goes into the battery. On the other side of the charger, my pack is providing 144 volts. So if the charger only needs 400 watts, at 144v then it uses less than 3 amps. 

An automotive fuse will blow when the wattage it's designed for is exceeded. A 12v 30a fuse will carry up to 360 watts. That same fuse at 144v will carry up to 2.5 amps. 144v x 2.5a = 360w. This is why an automotive 12v-style fuse can be used in this application. 

I'm currently using a 12v 30a circuit breaker in the HV fuse spot to see how well a circuit breaker will work. I'd like to replace it with a 40a circuit breaker, but I have not found a 40a plug-in cb. So I'm experimenting with a 30a cb for now. It seems to be working OK. I've got some 40a standard fuses to replace the cb if it should fail.


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## Joe Jackman (Dec 6, 2010)

Hi Mike
I'm sorry but you have a serious error in your thought about what causes a fuse to blow.
A fuse blows at its rated current. Watts do not enter into this. A 30 Amp fuse blows when something over 30 Amps is drawn through it.
The voltage rating tells you how wide the gap will be when the fuse blows. The higher the voltage rating, the wider the gap. If you use a 12 Volt fuse on a 144 Volt circuit, then the gap, when the fuse blows, will probably not be wide enough to prevent some serious arcing from the kick-back from any inductance in the circuit, and that could cause some serious damage to the circuit you're trying to protect as well as other components connected.
Joe.


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## m38mike (Dec 27, 2008)

Joe Jackman said:


> Hi Mike
> I'm sorry but you have a serious error in your thought about what causes a fuse to blow.
> A fuse blows at its rated current. Watts do not enter into this. A 30 Amp fuse blows when something over 30 Amps is drawn through it.
> The voltage rating tells you how wide the gap will be when the fuse blows. The higher the voltage rating, the wider the gap. If you use a 12 Volt fuse on a 144 Volt circuit, then the gap, when the fuse blows, will probably not be wide enough to prevent some serious arcing from the kick-back from any inductance in the circuit, and that could cause some serious damage to the circuit you're trying to protect as well as other components connected.
> Joe.


OK, I've got 4 questions and one observation about your line of reasoning. 1st question, why would Chennic put an automotive fuse socket in the HV+ line if an automotive fuse was not suitable? 2nd question, Where would I find an automotive-type fuse rated for 144v and 3a? 

My comment goes to the gap created in the fuse. When the 20 amp fuse I originally put in the fuse holder blew, the gap looked just like it would have for a 12v break. It was open very wide. I'm not saying your wrong about the gap, just that my experience so far has not supported your statement. 

If your statement that a 30 amp fuse blows when 30 amps are drawn through it is true, then what role does the voltage play in that equation? If voltage played no role in the current calculation then why did a 20 amp fuse blow when only 2 amps were drawn through it? Even if the 2 amps were at 144v the voltage shouldn't matter should it?


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## EVfun (Mar 14, 2010)

m38mike said:


> If your statement that a 30 amp fuse blows when 30 amps are drawn through it is true, then what role does the voltage play in that equation? If voltage played no role in the current calculation then why did a 20 amp fuse blow when only 2 amps were drawn through it? Even if the 2 amps were at 144v the voltage shouldn't matter should it?


Voltage plays no part in a fuse blowing. A 20 amp fuse didn't have only 2 amps flowing through it, unless the fuse itself was defective. If this is powering your DC to DC converter the likely problem is that it has a capacitor bank on the power input so there is a large surge of current when power is first applied.

Fuses have voltage ratings because there is a limit to how much voltage they can safely stop when they blow. AC and DC ratings are often different and need to be respected for safety. That is all the voltage rating if for.


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## dtbaker (Jan 5, 2008)

is your dc-dc 'always on', or does it come on/off with main contactor keyswitch?


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## Overlander23 (Jun 15, 2009)

Startup current, or... if the voltage has somehow reversed. The Chennic's fuse will blow instantly if voltage is reversed. I'd wager that their "reverse-polarity" protection has more to do with that fuse than actual current protection. Doesn't necessarily make sense.

The original fuse in my 500W Chennic is a 250V-15A fuse. But I'm almost positive it's actually a 250VAC rated fuse. Technically, that's not good enough for high DC voltages, which is why I back it up with a 600VDC rated fuse... which is not small enough to fit in an automotive-style holder.

But then again, the Chennic is cheap... which may account for certain design decisions.

Here's the relevant point regarding fuses and voltage from Wikipedia: http://en.wikipedia.org/wiki/Fuse_%28electrical%29#Rated_voltage


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## MN Driver (Sep 29, 2009)

Where do you find 600vdc fuses? The ones I find require me to order a large quantity. I'm not running anything high voltage DC yet but have been planning out my system and haven't been sure where to look for good fusing for both lower amperage auxiliary stuff and the high amp drive fuses.


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## m38mike (Dec 27, 2008)

dtbaker said:


> is your dc-dc 'always on', or does it come on/off with main contactor keyswitch?


My dc-dc only comes on when the contactor closes. The contactor controls the dc-dc on/off switch, and the power supply to the dc-dc.


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## EVfun (Mar 14, 2010)

m38mike said:


> My dc-dc only comes on when the contactor closes. The contactor controls the dc-dc on/off switch, and the power supply to the dc-dc.


... and I take it the fuse blows right when switched on?


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## m38mike (Dec 27, 2008)

EVfun said:


> ... and I take it the fuse blows right when switched on?


I'm not sure. I didn't have any way to tell if the dc-dc was working when I first wired it in. Now I have an ammeter in the 12v+ line to the battery so I can see what the dc-dc is producing. I suspect I must have had a spike in demand to cause the fuse to pop. I've got a 30a circuit breaker in the HV+ wire now and it's working fine. I get variable readings on the ammeter so I know I have power going from the dc-dc to the 12v battery. With the headlights on the ammeter is reading about 15 amps. without the headlights it's down at 2-4 amps. Not bad since I run 8 fans, the controller, and a water pump all the time.


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## Joe Jackman (Dec 6, 2010)

Hey Mike
Sorry I didn't get back to answer your questions right away, but I see that many of the knowledgeable people on the forum did, so that's good.
There are many causes for the fuse in your Chennic to blow. You might want to refer to the thread, started in Technical Discussions by Overlander23, regarding why his fuse was blowing too.
Incidentally, a 30 Amp fuse might not blow on a high current failure in the DC-DC converter and cause the HV wires to catch fire. When the converter is operating at 500 Watts (~40 Amps at 12.6 V) it will be drawing about 3.5 Amps from your 144 Volt pack.
When Overlander23 says that the fuse in his was 15A @ 250V, that sounds about right.
Take close note in the Technical Discussions thread about Tesseract's solution for his problem, because it solves a whole plethora of reasons for the fuse to blow, other than the obvious failures.
Hope this helps.
Joe.


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## Overlander23 (Jun 15, 2009)

MN Driver said:


> Where do you find 600vdc fuses?


I picked mine up from McMaster-Carr... you can buy them individually for about $10-13. I'm using CC Class fuses. Actually, I was mistaken... these are 300VDC fuses (600VAC). They do, however, list a whole bunch of 600VDC fuses, as well.

http://www.mcmaster.com/#fuses/=dgrb6i
Check out the UL Class CC Midget Fuses


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## m38mike (Dec 27, 2008)

So it sounds like the right answer is to cut the low voltage fuse socket out of my high voltage line, and replace it with a class G fuse block like one of these: http://www.mcmaster.com/#class-g-fuse-blocks/=dh1pq6Then add a class G fuse which are rated 600/170 VAC/VDC, probably rated at 3 amps. That should allow the 400 watts the dc-dc needs for maximum charging of the 12v battery, but still provide protection to the dc-dc unit itself. Here's a link to the fuses rated at 3 amps. http://www.mcmaster.com/#fuses/=dh1nao The class G fuse is about half way down the page. 

Looks like I got some bad advice from a friend who said he knew about using the low voltage fuses in this application. What he said seemed to make sense. I can see now that it was ignorant thinking. 

So from the guys that DO know, Would a 3 amp class G fuse and holder on the HV input line be the right answer to protecting my dc-dc unit?


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## Overlander23 (Jun 15, 2009)

I see no reason not to use the same fuse rating as the original fuse... just in a higher-quality, high-DC-voltage version. Three amps seems low to me, but I don't know your HV system details. As I mentioned, mine is 15A from the factory.

Say you have a 120v system that under acceleration and a weak charge state sags to 90v. If the DC-DC converter were drawing 400 watts, that's at least 4.4 amps right there.

The HV wires to the converter are certainly able to take more than 3 amps, and that's mainly what you're protecting. Leave yourself some headroom.


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## m38mike (Dec 27, 2008)

Overlander,
The problem is there was NO original fuse until I filled the auto-fuse-type-socket with an automotive, low voltage fuse. My pack is 144vdc, so the input wire for my dc-dc is not low voltage. Yet Chennic included a low-voltage automotive-fuse socket on the high voltage line.

I looked around the internet and did not find any high voltage fuse that would fit in that type of socket. What I've relearned in this thread is that I should have a fuse in line that exceeds my voltage and amperage needs. So I'm thinking that a 170vdc fuse should be adequate to handle my voltage. And I think 3 amps might be enough amperage to allow the dc-dc to work, but close enough to the limit to protect the dc-dc if it needs protection. 3a x 170v = 521w. I'm expecting that since it's a 400w dc-dc, that providing 521 watts capacity for input should be enough, if the dc-dc is efficient. If the 3 amp fuse blows easily or often then I'll go to a 4 amp, but I'll want to see if something else, like ripple from the controller, is causing the surge. 

Most of the time I don't expect much of a load on the dc-dc at all. The lights are LED except the headlights. The fans are all low amp fans. The cooling pump is low amperage. I don't have a vacuum pump or power brakes to provide power to. It looks like the Soliton Jr only draws a couple amps. My 12v ammeter tells me that I'm only drawing 15-18 amps when everything is turned on for the 12v stuff. So I'm well below the 12v 30 amp capacity of the dc-dc.


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## Overlander23 (Jun 15, 2009)

Yes, mine came with the automotive-style fuse holder, as well. And it had a 15A/250v fuse in it, but I'd wager that it was only good for 250VAC and maybe 32VDC, maybe even only 12VDC, which is why I backed it up with a proper high-DC-voltage rated fuse. But, I still have that less than ideal automotive style fuse in there. 

When I was having my problems, the automotive style fuse would blow first, so I tested for problems with cheap fuses knowing that my more expensive proper fuse would step in if the automotive fuse failed to break the connection.

In the end, I discovered that moderate acceleration under load was enough to cause enough ripple current to reverse its way through the converter and blow the fuse.

Keep in mind, though, that when fusing you want to look more towards the end of worst-case-scenario. While 170VDC may be your max pack voltage (and fusing for greater than 170VDC is the way to go), your pack could sag to well below 144VDC, under load or at the end of SOC, etc. The lower input voltage to the converter would raise its current requirements. So in essence, the only time you'd be able to draw 521 watts at 3 amps is when the pack is off the charger at 170VDC. As the pack voltage drops, the current draw would inversely grow with the same load. I know that the converter might not draw 500 watts all the time, it may not ever draw that much, but it just seems like 3 amps is cutting it closer than necessary. External fusing is really more for protecting the wiring than the device.


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## Joe Jackman (Dec 6, 2010)

Hi Overlander23,
I was wondering whether you would tell us what route you took with regard to your post at http://www.diyelectriccar.com/forums/showthread.php/dc-dc-converter-keeps-blowing-fusesi-59890.html 
I and possibly others could well benefit from what you did. After I had kicked myself around the room a bit, when i read Tesseract's solution, (figuratively and with a red face, since I'm Over 70) because I should have thought of that too, I wondered what you ended up doing.
I also thought it a useful thread for m38mike to look at, since he might have similar problems too, later on.
Thanks, Joe


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## Overlander23 (Jun 15, 2009)

At some point I'll actually go forward with the recommendation to use a choke as it will do the best of filtering out the transients. But for now, I'm using a 6 amp rectifier to block the reverse voltage. This seems to do fine, but won't actually filter out the oscillations which may ultimately lead to capacitor failure in the DC-DC converter.


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## Joe Jackman (Dec 6, 2010)

Thanks Jeff.

Joe.


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