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Old 05-31-2018, 11:09 AM   #1
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Magic 50A Breakers

Hi

Ok, I'm in the middle of rewiring this and that. I've dropped in a nice new 60A converter / charger. I'm putting in wires and notice a weird looking bus bar down connecting things to the battery..... hmmm ..... look at the manual .... tiny note says the gizmo is actually a set of circuit breakers. Dig a bit more and there is a 50A breaker from the converter to the bus bar and another 50A breaker from the bus bar to the battery.

First off this seems a bit weird with a stock converter / charger that is rated at 55A. One would *think* that a 50A breaker isn't what you put on a 55A converter output. Now I'm looking at 60A through the same setup.

I suspect I'm not the first to run into this. Pretty much everybody and his brother seems to put 60A converter / chargers into their Airstreams. What happens? Do people run into trouble. Is there some magic sub note on these breakers that says 50A really is 60A? I'm not having a lot of luck finding a data sheet on these gizmos ...

Bob
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Old 05-31-2018, 11:14 AM   #2
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The converter will not be able to put that much current to the batteries. By the time it gets there, the actual current will be more like 30 amps max, no matter how good the converter. Generally, it would be more like 15-20 amps. Only a portion of the converter output goes to battery charging.
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Old 05-31-2018, 11:18 AM   #3
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Remember that the breaker is NOT there to protect the electronics/electrical stuff. It is there, and sized for protecting WIRING.

In your case, the wires may only be able to handle 50 amps maximum. The breaker is sized to open the circuit if the current gets to be too much for the wiring...
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Old 05-31-2018, 12:12 PM   #4
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Hi

Indeed fuses and breakers are there to prevent fires. You don't want the wire to overheat and burn. Also figuring that 55A out of the converter will charge a 100AH lead acid battery in under two hours is wishful thinking ( to say the least ).

One other note, the 50A magic breaker also is in series with a number of loads on the trailer. A few things Tee off ahead of the breaker, but not everything.

The wire is number 6. The insulation is rated at 105C. That puts it into the "over 75A" category and thus not a big deal in this case. It's a two wire run so that helps a little as well. A 50, 60 or 70A breaker or fuse would protect the wiring just fine and still have some margin.

The cable run from the converter is maybe 15 feet, so 30 feet round trip. That should be about 0.012 ohms. At 60 A that would be a 0.72 V drop. If the converter is putting out 14.6V, the battery will see 13.88V. Indeed, the new batteries will quite happily slurp up 60A at that voltage.

If indeed you had stock lead acid's at .... errr .... 12 volts .... ( not that *anybody* would ever go that low ) the stock setup should source full converter output into the batteries. It would do that long enough to get a breaker interested in tripping. How interested, I have no clue. All the data I can find goes back to SAE specs. I have no desire to spend $78 just to look at a copy ....

So, I'm still confused.

After doing some digging, the largest breaker you can get in an SAE Type II trip seems to be a 50A device. I can see the value of a Type II in this case. I'm wondering if they just tossed i the biggest they could get and moved on .... yikes ...

Of course, maybe the magic stock converter never puts out 55A under any conditions no matter what.

Bob
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Old 05-31-2018, 01:16 PM   #5
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It's always a good practice to run oversize on wire and undersized on breakers...if it makes sense for a particular circuit.

The other way round can lead to fires and overheated wiring...
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Old 05-31-2018, 01:23 PM   #6
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Quote:
Originally Posted by rmkrum View Post
It's always a good practice to run oversize on wire and undersized on breakers...if it makes sense for a particular circuit.

The other way round can lead to fires and overheated wiring...
Hi

I certainly will not argue with that. The first thing I did was check the wire insulation and gauge.

One thing you normally want to avoid on a breaker is cycling. You get into weird stuff like welding "shut" .... that's not a good thing. I really am wondering what SAE did on the spec. I"m not quite $78 curious yet ....

Bob
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Old 05-31-2018, 02:08 PM   #7
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I'm not a truster of low voltage DC breakers, unless they are the manually reset type, preferably aircraft/marine quality.

The welded contact failure from repeated cycling of cheapo automotive ones is a real issue...IMHO they just make it a slightly longer delay time from short circuit failure mode to wiring fire...

Nice big bolt-in fuses on battery leads, right at the clamp, ATO fuses in the panel, with indicator LEDs built into them, now we're talking...as long as you carry plenty of spares "Justin Case"...

And, as a rather lazy type, I like to minimize the number of different spare parts, just to keep inventory simple. The hard to find slow-blow cartridge fuse on the tongue jack really bugs me...
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Old 05-31-2018, 06:21 PM   #8
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Hi

In this case by the glory of AS engineering, the power distribution is buried under a power recliner / couch. You disassemble that to get to the next layer. You then pull the sub woofer. Once that is out of the way, you can grab your multimeter to see if anything odd is going on with the gizmo that looks like a bus bar. I don't think you would want fuses in that location. Pretty much no practical way to check / swap a fuse. About the only choice for that location are self reseting breakers. With that goes a trust that they pretty much never ever will trip.

Again, all this makes running 50A breakers on a 55A circuit a bit of a puzzle ....

Bob
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Old 06-18-2018, 10:48 AM   #9
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Originally Posted by overlander63 View Post
The converter will not be able to put that much current to the batteries. By the time it gets there, the actual current will be more like 30 amps max, no matter how good the converter. Generally, it would be more like 15-20 amps. Only a portion of the converter output goes to battery charging.
Hi

Indeed it seems that a 60A converter is really only a 30A unit into a normal battery. That is a bit surprising, but I guess that's the way it goes in the converter business.

When you ask the question, you get an answer that talks about the "batteries can only take just so much". Well that's fine *if* you know exactly what the batteries are. One would *think* that a data sheet on a converter/charger would get into this if it is how they are designed to work.

In the case of a 400AH lithium install, the batteries will quite happily take 80A and still be on the low side of the charge rate specs. A high charge rate would be 400A ( yikes !!! ). Simply put, there is nothing a 60A charger could do that is "to much" for the batteries involved. There also is nothing in the wiring that would keep 80 or 100A from heading for the batteries ( other than the breaker ).

If you had a single 45AH flooded cell, then indeed 60A would be a bit to much. Not sure *why* you would buy a big charger for a small battery and then wonder why it was to big ....

Maybe the 60A is current into a dead ( 2V ) battery? Hmmm.... I don't think I'll take the batteries down that far to find out That's a strange way to spec something like this. Again, if it is the "way it's spec'd" - how come no note to that effect on the data sheet? It most certainly is not the way a power source is normally specified. The idea that "power is going someplace else" simply is not how the physics involved works ....

If you actually *do* want 60A, I guess you buy a 120A converter / charger and wire it for 120A even though it will almost never put out 120A. Strange business ....

Bob
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Old 06-20-2018, 09:19 AM   #10
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Hi

Ok, so wrong yet again. The converter most certainly *can* put out enough current to trip a 60A breaker. Of course it's now putting out 24V .... that didn't make the LED lighting very happy. Lots of them to replace .....

I think I'm in the market for a new converter after two weeks running this one ....

Bob
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Old 07-01-2018, 04:59 PM   #11
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Quote:
Originally Posted by overlander63 View Post
The converter will not be able to put that much current to the batteries. By the time it gets there, the actual current will be more like 30 amps max, no matter how good the converter. Generally, it would be more like 15-20 amps. Only a portion of the converter output goes to battery charging.
Hi

Well, the "all of them" does not include one of them

Put in a Victron Multi 2000 that is rated at 80A and you will indeed get just what the spec sheet shows. It does indeed put 80A into a low battery. So much for "power goes elsewhere" or "batteries will only accept so much". 80A is 80A.

Indeed, a gizmo that actually puts out the current on the label is bigger, heavier, and more expensive than the other ones .... hmmmm .... how weird is that

Bob
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