Using a low votage switch. ok with pre-charge?

[DanGT86]

I'm wanting to use a 100-200 amp rated switch/circuit breaker on my battery box. I would also use a precharge resistor in parallel with the switch. The idea would be to connect the pack, allow the controller caps to pre-charge for a few seconds, then flip the switch. A breaker would be a nice additional safety feature. The cheap marine switch/breakers I'm finding are rated at 48v max. Ive been using a pre-charge loop in my wiring but its sloppy looking and annoying to explain to people. I'd rather just have one clean xt-90 connection and a switch.

So im wondering if i can use 48v rated parts at 84v (20s lipo) since they will not be switched under load. The pre-charge resistor brings the system to 66-70v before I make the main connection so the switch is only seeing <20v when the connection is made right? Isnt the drawback to low voltage switches the sparking at the contacts? Is there some drawback to the low rating once the switch is closed?

Something like this.

Or this

 

[amberwolf]

When the switch is closed, then it's just a wire, as far as voltage is concerned. Current is another thing.

BUT:

If you have a *breaker* that isn't rated for the voltage it's used on, then if something happens to cause it to pop, it could arc across the contacts while trying to open (and it *will* be under load then, or else it wouldn't have a reason to pop). This might do nothing, or damage the contacts (so they don't make good contact when reset), or could weld the contacts together.

But it could also keep arcing across the contacts, which means
--it won't break the connection, so whatever it was going to protect won't be protected.
--it could catch fire from the heat of the plasma in the arc.

It's not that likely, but it *could* happen, and that would suck.



That said, I've been using a breaker like this:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=67833&p=1263618&hilit=breaker%2A#p1263618

on my CrazyBike2 for years, and it's popped a couple times from overload during some experimentation, without issue. I also used it as my power switch, so it saw at least hundreds of cycles, probably more like thousands (at least twice a day almost every day for years).


I'm using the actual breaker in that pic on SB Cruiser, and havent' had it pop yet.


The pack on either one is 14s NMC, so around 58v full. The breaker is off a 24v powerchair, and is rated 63A, though I regularly pull well over 100A thru it on SBC, for a few seconds at most, without popping it. 80A+ on CB2 for the same few seconds. Continuous is only a few A on CB2, and at least double that on SBC.

However, it is an AC breaker; commonly these are stated to not work at all on DC circuits, however if they didnt then it wouldn't have popped successfully when I was doing the CB2 experiments. (and it probably couldn't be used on the powerchairs they come from, which is a LOT of powerchairs).

So, just in case, I also have a 125A fuse at the actual battery terminal.

 

[DanGT86]

Thanks.

Thats a good point about the arcing during the breaker tripping. That would be exactly the type of high load situation i'm trying to avoid when already outside of the rec voltage range. I could see that being really hard on contacts not designed for it.

Probably better to use a propper fuse and the high amp switch instead. At least the fuse doesnt wear out.

You do have me curious about why an ac breaker would or wouldnt work on dc.

 

[dustNbone]

AC cycles through 0V every 1/60 of a second (or 1/50 if you're on 50hz power), so any arc will be extinguished at the 0V point within 1/60 of a second of it's initiation.

DC doesn't do that, so when an arc is initiated it can keep burning even as the contacts are pulled apart, because plasma (which is what an arc is, burning plasma) is fairly conductive. At high enough voltage it can maintain the arc across a considerable distance, burning up things like brass contacts pretty quickly.

DC breakers/switches (at least ones designed for higher voltages) have some kind of mechanism in them to break/extinguish any arcing quickly before anything terrible happens.

An AC breaker is not required to handle this situation, so it may or may not be successful at it.

 

[DanGT86]

I see. Makes perfect sense.

So then dc breakers could be implimenting something as simple as a a heavier spring to get the contacts further away from eachother or broken faster?

Perhaps my pre-charge resistor would stop it from arcing accross the contacts druing a trip. I bet the current would murder the resistor but the temporary existance of an alternative conductive path might prevent the plasma arc.

 

[dustNbone]

I think most of them employ a magnet to pull the arc away from the air gap, lengthening it enough to snuff it out.

 

[fechter]

You might try looking for a breaker made for solar systems. I've seen some that are reasonably priced.

 

[teklektik]

See: https://endless-sphere.com/forums/viewtopic.php?f=28&t=56742&p=846166#p846166