DC Breaker / Battery Cable Fried

[perholmes]

Hi,

My DC Breaker for 20 kwH of LiFePo burned through one of the terminals, which happened over a couple of months. Something smelled funny, but I couldn't locate it, until it burned through the breaker completely, and then I could localize it.

I'd love if anyone could help explain how this might have happened. Three of the terminals and cables are in perfect shape, but the output on the + side burned the breaker, and oxidized the cable about 10 cm into the cable.

My father, who's a molecular biologist, theorized that if there's some dirt on the connection, then the resistance goes up, which creates heat, which oxidizes the copper faster, which increases the resistance, in a vicious circle.

Pictures are below, but basically, I need to figure out what to do:

• Is it a good theory, that dirt causes resistance -> heat -> oxidation -> heat -> etc?
• Would it be a good practice to re-seat the battery connectors at least once a year, i.e. take them out of the breaker and put them back in?
• Is there a known way to clear the copper ends? My father suggest vinegar or citric acid, but is worried about stopping the process.
• Is there's a known good way to make a stronger connection?

I have this fixed with a new breaker and a shortened cable. But I feel like I'm sitting on a time-bomb now.

Per

 

[TomC4306]

Loose connection.
You need a small torque wrench and screwdriver bit holder.

I had the same issue with a PV combiner box circuit breaker. When I actually torqued the connections, I was amazed at how loose they really were. This after I had tightened them as tight as possible with a regular screwdriver in my hand.

 

[Mattb4]

I am not a fan of the screw type connection on a breaker for battery cables. I prefer a MCCB breaker that has bolts to fasten a crimped on ring lug.

 

[Hagger]

I recommend to use cable Ferrell. That will avoid that the fine strands move, shift during slightly heat up and cool down. With each cycle it become more lose and loses more tension from the screw. That’s create a higher resistance that lead even to a faster increase of the process und finaly the resistance is that high that it will create a scenario like yours.
As mention, use a cable ferrel and tighten the screw. It’s also a good practice to checkt e.g. once a year If screws till tighten.

So far my guess what cause the problem and how to avoid it.
Never mind you are not the first one who face that issue.

Edit: by just see what others wrote at the same time.
Yes it’s recommend to use a larger (Philicaly larger) DC breaker that is not polarized (!) if you plan that it shall act
as in case of overcurrent or a short. As well if you plan to operate it under load.

If it will serve you just as a Disconect switch / Isolator under no load and you have an additional fuse in place than indeed you can use that small devices. Someone correct me but for load of 200 A+ I have ‘t seen even brand breakers in that smal from factor that will extinguish an arc in both direction of current flow.

 

[Brucey]

Did you ever retorque the breaker terminals after initial install? They can tend to loosen over time.

 

[Q-Dog]

I ALWAYS recheck torque on new connections after an hour, then again a day or two later. And I check for hot spots the first time I power it up and again when I can put max power through the connection and stress test it. But I am paranoid and an electrical chicken.

 

[Backyard]

I like to lube the threads of the screw terminal. Helps a lot in getting good torque. Aluminum can gall and give a false torque.

 

[robbob2112]

SO - as others have said - loose cable.

Proper way
insert the cable
tighten it down
loosen it and wiggle it
tighten it down again this time with a torque screwdriver or wrench
wiggle again and check it once more
Come back in a week and check it again.

And note - the type screw that is used inside most of these breakers that looks like it will take both Phillips and slotted is not actually that... It is ECX according to Milwaukee ... but if you don't have that type bit it is a square drive or slotted.... not Phillips at all. Below is a picture and a link to a driver set that includes the correct insulated bits.

Milwaukee 8-in-1 1000V Insulated Slim-Tip Multi-Bit Screwdriver 48-22-2218 - The Home Depot

Complete your drilling and nailing task effortlessly with the selection of this wonderful Milwaukee 8-Insulated SlimTip MultiBit Screwdriver.

www.homedepot.com

 

[perholmes]

Thank you for your beautiful answers!

• I've had trouble finding unpolarized DC breakers. Do you have any examples I could start a search with?
• No, I had never retorqued them, but I'll take your advice and use a proper torque wrench.
• What do you think of the idea of re-seating the cables periodically?

Quick idiot check, the polarized DC breakers expect the positive side to connect to both positive on the inverter and battery, correct? I'm wired with plus on the left side and minus on right side for this particular breaker (left image). It's only that that wiring diagram on some DC breakers themselves suggest the wiring on the right (plus to minus and minus to plus), which feels wrong.

 

[robbob2112]

I like to lube the threads of the screw terminal. Helps a lot in getting good torque. Aluminum can gall and give a false torque.

Never ever ever this - lubricating threads will do two things --- first - it will let you over torque and possibly strip the threads... Second it can let the screw back out on its own.

They do make a special anti-seize compound that doesn't, it is used between aluminum and steel - commonly sold for installing spark plugs in a car ... that is the only thing you might consider as it won't make things smooth enough to over torque it.

 

[perholmes]

SO - as others have said - loose cable.

Proper way
insert the cable
tighten it down
loosen it and wiggle it
tighten it down again this time with a torque screwdriver or wrench
wiggle again and check it once more
Come back in a week and check it again.

And note - the type screw that is used inside most of these breakers that looks like it will take both Phillips and slotted is not actually that... It is ECX according to Milwaukee ... but if you don't have that type bit it is a square drive or slotted.... not Phillips at all. Below is a picture and a link to a driver set that includes the correct insulated bits.

Thank you, Rob, I'll get exactly what you suggest.

 

[robbob2112]

Thank you for your beautiful answers!

• I've had trouble finding unpolarized DC breakers. Do you have any examples I could start a search with?
• No, I had never retorqued them, but I'll take your advice and use a proper torque wrench.
• What do you think of the idea of re-seating the cables periodically?

Quick idiot check, the polarized DC breakers expect the positive side to connect to both positive on the inverter and battery, correct? I'm wired with plus on one side and minus on the other. It's only that that wiring diagram on some DC breakers themselves suggest the wiring on the right (plus to minus and minus to plus), which feels wrong.

View attachment 282427

positive doesn't mean exactly what you think it does... It means the most positive potential not positive and negative.

So you could used a polarized breaker between the PV panels and the MPPT....but you can never use a polarized breaker between the batteries and the inverter since the most positive flips depending on if you are charging or discharging the batteries... you either use a fuse and a disconnect switch or a non-polarized breaker.

 

[perholmes]

Hi Rob,

Got it. Just to make sure I understand what I'm looking for, please let me know if the following is an example of a non-polarized DC breaker. I wouldn't buy this one, it's too cheap. I only want to understand if I have the right concept.

Amazon.it link

Also, my batteries are two BMSs which already have built-in breakers. Isn't it actually redundant that I have a breaker on the way to the inverter at all? Maybe just a disconnect switch and being done with it?

 

[SomethingAppropriate]

isn't it actually redundant

Redundancy is a good thing here.

Protects both sides of the wire

 

[Brucey]

Hi Rob,

Got it. Just to make sure I understand what I'm looking for, please let me know if the following is an example of a non-polarized DC breaker. I wouldn't buy this one, it's too cheap. I only want to understand if I have the right concept.

Amazon.it link

Also, my batteries are two BMSs which already have built-in breakers. Isn't it actually redundant that I have a breaker on the way to the inverter at all? Maybe just a disconnect switch and being done with it?

Best practice is to never rely on a BMS to work, they can fail and have full battery current available at the terminals.

 

[Backyard]

Never ever ever this - lubricating threads will do two things --- first - it will let you over torque and possibly strip the threads... Second it can let the screw back out on its own.

They do make a special anti-seize compound that doesn't, it is used between aluminum and steel - commonly sold for installing spark plugs in a car ... that is the only thing you might consider as it won't make things smooth enough to over torque it.

False. Lubing the threads can get you to the right torque whereas aluminum and copper threads can prevent it due to galling.

I've done this thousands of times from large MCM gauges down to 30+ AWG. With most being between 14 awg to 4 awg. I've had far less failures when I started lubing them. Stripped threads and loosening of the wire became less of an issue too. Aluminum and copper should not be compared to steel or traditional fasteners. It behaves quite differently

 

[Crowz]

This is why I use these breakers :



Midnite Solar 250amp breaker

Lugs beat screw clamps any day !

 

[SomethingAppropriate]

This is why I use these breakers :

View attachment 282433

Midnite Solar 250amp breaker

Lugs beat screw clamps any day !

Is it generally recommended to leave the terminal posts on that breaker?
Does it matter? Can I use one and not the other?

 

[robbob2112]

I didn't see anything in that product description indicating the breaker was polarized... no marks on the outside of the breaker for polarity... so it would work

If your batteries have DC breakers already built in they are sufficient and you can skip another one.... If you are talking about using the BMS to turn things on and off that isn't a good choice..... Pictures are worth a thousand words...

Some batteries have switches on them that look just like breakers but if you look close at them they don't have an amperage rating stamped on the handle or near it. In that case it is just a switch.

The large breaker @Crowz pictured is one choice - it has a large gap and arc chutes for when it opens to break the arc....

False. Lubing the threads can get you to the right torque whereas aluminum and copper threads can prevent it due to galling.

I've done this thousands of times from large MCM gauges down to 30+ AWG. With most being between 14 awg to 4 awg. I've had far less failures when I started lubing them. Stripped threads and loosening of the wire became less of an issue too. Aluminum and copper should not be compared to steel or traditional fasteners. It behaves quite differently

It goes counter to my training when I was calibrating/repairing torque wrenches back 30 years ago..... as mentioned it depends on what you are lubing it with... and on the materials...

Yes, copper and steel and aluminum all play different with each other. We are generally using stainless steel bolts and putting them into either steel with copper between to force contact with a mating copper surface or we put them into aluminum and repeated torquing can cause the galling to tear out the threads...

I have a new set of cells just in and the bolts going in were galling a bit until I chased the threads with a tap to clean things up.... Now I am not worried about the galling.... But I do have some nickel based anti-seize compound to use if I think it is a problem. I also have the same that is based on copper depending on what I am bolting into

What I won't use is just plain grease.....

What is it exactly you are using on your threads?

 

[Crowz]

Is it generally recommended to leave the terminal posts on that breaker?
Does it matter? Can I use one and not the other?

This is how I use them.

 

[Backyard]

I didn't see anything in that product description indicating the breaker was polarized... no marks on the outside of the breaker for polarity... so it would work

If your batteries have DC breakers already built in they are sufficient and you can skip another one.... If you are talking about using the BMS to turn things on and off that isn't a good choice..... Pictures are worth a thousand words...

Some batteries have switches on them that look just like breakers but if you look close at them they don't have an amperage rating stamped on the handle or near it. In that case it is just a switch.

The large breaker @Crowz pictured is one choice - it has a large gap and arc chutes for when it opens to break the arc....



It goes counter to my training when I was calibrating/repairing torque wrenches back 30 years ago..... as mentioned it depends on what you are lubing it with... and on the materials...

Yes, copper and steel and aluminum all play different with each other. We are generally using stainless steel bolts and putting them into either steel with copper between to force contact with a mating copper surface or we put them into aluminum and repeated torquing can cause the galling to tear out the threads...

I have a new set of cells just in and the bolts going in were galling a bit until I chased the threads with a tap to clean things up.... Now I am not worried about the galling.... But I do have some nickel based anti-seize compound to use if I think it is a problem. I also have the same that is based on copper depending on what I am bolting into

What I won't use is just plain grease.....

What is it exactly you are using on your threads?

It's a different ballgame for studs with nuts or through bolting. Those don't generally need lube. It's the aluminum and copper lugs and screw terminals that need it. Even brass can benefit

For the simple stuff like plugs, smaller breakers, screw terminals, etc I will often just use oil like 3-1 or similar. For larger lugs and all aluminum connectors I'll use the anti oxide compound like Noalox. I rarely make dry connection if I can avoid it.

In my early days I spent a lot of time doing repairs on cords and plug ends. Lots of burned connectors on the plugs/receptacles. Even when it seemed like they where torqued correctly and not overload. Had a number of repeat issues. I remember a particular commercial espresso machine that went through several plugs and receptacles before I oiled the screw terminals. Then never had it burn up again afterwards. I was getting 6 months to a year before. Then it went several years without issues before I moved away

New job at a construction company had me repairing equipment, generator cords and temp power boxes. Those things are abused a lot. Thrown in the back of trucks. Dragged around on concrete and dirt. Ran over. Dropped. You name it.

Lots of loose connections, arcing and burning. Lost time of the job generally meant they where rushing out to buy new cords and power boxes. I started going through everything and torquing the connections. Didn't lube anything at first. Just thinking they are loose from the factory or prior person didn't tighten them properly. But the issues would still happen. Less but still there. Cost money with downtime. Then I decided to try the lube on those breakers, lugs and other wire terminals. Issues dropped to near zero.

I think the oil provides a measure of oxidation resistance to the wires too which helps keep the resistance and thus heat down. Then the galling and high friction on aluminum and copper threads is reduced so a more accurate torque can be applied.

Of course there's always those people who will over torque any fastener dry or not. It's not going to help those. But for anyone who has a good handle on the feel for proper torque can benefit from this

 

[Brucey]

Is it generally recommended to leave the terminal posts on that breaker?
Does it matter? Can I use one and not the other?

You can connect at the top or bottom of the post:

 

[SomethingAppropriate]

You can connect at the top or bottom of the post:

View attachment 282458

Ok cool. But you can't remove it?

 

[Brucey]

Ok cool. But you can't remove it?

Correct the lug bolt is part of the breaker:

 

[SparkyJJO]

I learned something new today. ECX screws? Dang it.