how to disconnect battery

[salve]

Hi. I have a really basic question. I have my battery connected to the inverter. But I have to disconnect it to install the fuse box. Is there a safe way to disconnect it? Like the positive wire first or something? Last time I took it off (the positive, to install a bolt on fuse for the charge controller) when I put it back on there was a spark. I did what will said when I initially connected the battery to the inverter (using the resistor) so I know about that. How do I disconnect it and reconnect it so there's no spark/harm to the components?

 

[snoobler]

 

[SolarQueen]

Use a breaker instead of a fuse.

 

[Zil]

If the inverter has no load and disconnected for a few minutes, the inrush should not be a problem. With battery it is good practice to disconnect the negative first and connect the negative last. Maybe you need a buss bar to have multiple connections that do not interfere with each other. I like fuses on DC circuits.

 

[Kugel8x57]

Use a breaker instead of a fuse.

I think I read somewhere on this forum about using a double-pole 240vac breaker as a disconnect switch. Connect/disconnect both positive and negative at the same time. Will that work well for 24vdc? If so, I have a small panel and some spare breakers that I can use for building an "on/off switch box". I would still use fuses for system component protection.

What say you?

Many Thanks and God bless!

Kugel

 

[Zil]

no.

 

[Kugel8x57]

no.

OK.

Because...? Are they inefficient? Get hot? Won't handle the amperage? 30 amps is 30 amps, for the most part, whether it is DC or AC.

I'm referring to using them as a disconnect, not as an overload breaker.

I want to understand, because this adds to my knowledge base, and helps to prevent error in the future.

Many Thanks and God Bless!

Kugel

 

[RickP]

I think I read somewhere on this forum about using a double-pole 240vac breaker as a disconnect switch. Connect/disconnect both positive and negative at the same time. Will that work well for 24vdc? If so, I have a small panel and some spare breakers that I can use for building an "on/off switch box". I would still use fuses for system component protection.

What say you?

Many Thanks and God bless!

Kugel

I have a regular Bussman weatherproof breaker on my positive battery side, very first connection from the battery pack. It’s a super easy disconnect when needed. I still do the resistor thing for capacitors if it’s off a long time because the inrush current can trip the battery protection in my Battle Borns. It resets right away, but why push it if it’s unnecessary? Mine is a 24V system, and it works fine. This particular breaker is 200A, but you can go smaller if your system doesn’t require that.

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[Kugel8x57]

Thank you!

Best Regards and God Bless!

Kugel

 

[Liam M]

I have a regular Bussman weatherproof breaker on my positive battery side, very first connection from the battery pack. It’s a super easy disconnect when needed. I still do the resistor thing for capacitors if it’s off a long time because the inrush current can trip the battery protection in my Battle Borns. It resets right away, but why push it if it’s unnecessary? Mine is a 24V system, and it works fine. This particular breaker is 200A, but you can go smaller if your system doesn’t require that.

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Is this a lithium only issue?
Is the use of a resistor recommended in the case of a lead acid battery that has a breaker as a disconnect?

 

[RickP]

Is this a lithium only issue?
Is the use of a resistor recommended in the case of a lead acid battery that has a breaker as a disconnect?

The resistor is to slow the inrush of current to the capacitors, which will (for the briefest moment) take about as much power as they can get. My particular system has a Victron Multiplus that can draw a lot in that instant, and the BMS in the BattleBorns is apparently fast enough to read that as an amount more than permitted by the BMS settings, and cuts the load off. It resets in a couple of seconds, so it's not a major inconvenience and shows that part of the BMS is doing its job! In most scenarios, it really isn't a lot of actual drawdown on the battery so I don't think it would cause any problems, but I am no expert! For a couple of dollars, the resistor removes the spark and the concern for me.

 

[gnubie]

The main issue is related to lithium batteries because they have a BMS that can be damaged by very high current, ie components fail before the BMS can react and disconnect under high current. If no damage happens the BMS will still trip and disconnect the inverter etc. There is also a side issue of the capacitors in the inverter. Excessively large charge / discharge rates do degrade them. It won't happen straight away, but the damage does accumulate.

A lead acid system doesn't have a BMS (not of this type anyway) to worry about. For the breaker, so long as it has the right trip characteristics it'll be OK too. There is more to a break than just the straight out amps rating. There is also another spec, often denoted with a letter - A, B, C, D, that tells you how quickly the breaker reacts. A breaker with D rating (this is made up, check what the letters mean in your country) may be rated for 100A continuous but be quite happy to supply 150A for 30 seconds, while an A rated one may trip at 110A immediately.

 

[Liam M]

The main issue is related to lithium batteries because they have a BMS that can be damaged by very high current, ie components fail before the BMS can react and disconnect under high current. If no damage happens the BMS will still trip and disconnect the inverter etc. There is also a side issue of the capacitors in the inverter. Excessively large charge / discharge rates do degrade them. It won't happen straight away, but the damage does accumulate.

A lead acid system doesn't have a BMS (not of this type anyway) to worry about. For the breaker, so long as it has the right trip characteristics it'll be OK too. There is more to a break than just the straight out amps rating. There is also another spec, often denoted with a letter - A, B, C, D, that tells you how quickly the breaker reacts. A breaker with D rating (this is made up, check what the letters mean in your country) may be rated for 100A continuous but be quite happy to supply 150A for 30 seconds, while an A rated one may trip at 110A immediately.

Thanks.
Will said in his video it's a good practice for any inverter over 1700-2000W when using drop-in lithium, would you say it's a good practice at that size inverter, regardless of battery chemistry?

 

[gnubie]

If you are removing DC from the inverter input frequently, yes. If not, the capacitor degrade issue won't really be a problem so you'd only be working around BMS issues on lithium or other types that have that sort of BMS.

 

[Zil]

".....a double-pole 240vac breaker as a disconnect switch. " In general a 240VAC breaker is not suitable for DC circuits. I'm sure there are exceptions. If you just want a disconnect switch, this is what I use. https://www.bluesea.com/products/6010/m-Series_Mini_Dual_Circuit_Battery_Switch_-_Red

 

[YellowBuddy]

I think I read somewhere on this forum about using a double-pole 240vac breaker as a disconnect switch. Connect/disconnect both positive and negative at the same time. Will that work well for 24vdc? If so, I have a small panel and some spare breakers that I can use for building an "on/off switch box". I would still use fuses for system component protection.

What say you?

Many Thanks and God bless!

Kugel

no.

Just to clarify, it’s the usage of a Vac breaker that’s the problem. Using a Vdc breaker is fine. It’s exactly what commercial UPS system do:




Just make sure you use a DC breaker, or one that handles both AC and DC like the one above and not just an AC one.

 

[Leaking Acid]

The reason you cannot use AC breakers is that they are designed to break a current flow with a zero crossing. 60 or 50 (whatever your standard frequency is) times a second there is zero potential across a switch on an AC circuit. This makes it much easier to break the circuit without massive arcing. AC Switches don't have to open as wide a gap to break the circuit. However with DC because the current is not alternating it takes a more sofistcated switch to break the circuit.. That is also why they tend to be more expensive. AC breakers in a DC circuit could potentially arc and burn up.

 

[Hedges]

I think I read somewhere on this forum about using a double-pole 240vac breaker as a disconnect switch. Connect/disconnect both positive and negative at the same time. Will that work well for 24vdc? If so, I have a small panel and some spare breakers that I can use for building an "on/off switch box". I would still use fuses for system component protection.

What say you?

Many Thanks and God bless!

Kugel

Square D QO branch circuit breakers are UL listed to 50VDC (or was that 48VDC?)
Manufacturer says the 2-pole breakers, when both poles are used (i.e. interrupt both positive and negative terminals) are "Square D Certified" up to 125VDC. So each pole handles 62.5VDC.
I think it is the breakers up to 70A which have this rating.
They also have interrupt ratings up to several thousand amps (couple different levels, depending on which breaker.) That would be the maximum battery short-circuit current they could handle and successfully interrupt.

Fuses - some are "current limiting", really they are time-limiting, but deliver fault current to a victim device for a short enough time to avoid causing it to explode. Typical main breaker in a house can interrupt 22kA, so even though branch breakers are rated 10kA, the system can safely be used on a utility drop fed by a transformer up to the larger figure. Suitable fuses perform similarly, so a "bonded fault", a dead short, opens the fuse rather than causing the lower rated breaker to fail catastrophically.

 

[Zil]

Fuses protect wires. Any device that explodes is not the fault of the fuse that is protecting the wires to that device. needs repeating. Fuses protect wires.

 

[jvbutter01]

Like the positive wire first or something? Last time I took it off (the positive, to install a bolt on fuse for the charge controller) when I put it back on there was a spark.

back in the day, we were always told to put take negative off first, and put back on last when swapping out batteries in cars. I assume your inverter has no power switch?

 

[Hedges]

Fuses protect wires. Any device that explodes is not the fault of the fuse that is protecting the wires to that device. needs repeating. Fuses protect wires.

As far as overload goes, yes, fuses and circuit breakers protect wires. For instance a 70A breaker will allow > 70A for a limited time before tripping, perhaps several minutes at 100A. Short enough to not overheat the wire. Somewhere around 300A it will trip instantly by means of magnetic action.

I'm setting up a circuit breaker tester, will be able to deliver 230A steady-state, maybe twice that short term. I've tested fast and low trip of smaller breakers so far. Once I have some transient protection installed I'll connect my scope and automate it somewhat.

But what if a fuse/breaker has to interrupt 5000A or 50,000A? In that case it's job is to prevent things downstream (switches, branch circuit breakers) from exploding. It does that by interrupting the current in a short period of time. Too high a current available and it won't interrupt at all, and it is the one that blows up.

Circuit Breaker: Techniques and its Principle - C&S Electric

A circuit breaker stops the current flow in an electric circuit as a safety measure. Read more techniques and principles about circuit breakers at C&S Electric.

cselectric.co.in

The problem arises if utility source can deliver more current than your breakers are rated for. 22kA interrupting is sufficient for residential locations but not some commercial/industrial. Fuses, even the size of my little finger, can be rated 100kA or 200kA.

 

[Zil]

well you are way over my head. I have no plans to ever fuck with 50,000 amps. You are on your own. I will max out at 350 amps.

 

[gnubie]

350A is your limit but if the system is shorted you will briefly see massively more current than that before the fuse goes away and the fuse has to be able to successfully interrupt that.

BREAKING CAPACITY: Also known as interrupting rating or short circuit rating, this is the maximum approved current which the fuse can safely break at rated voltage. Please refer to the interrupting rating definition of this section for additional information.

CURRENT RATING: The nominal amperage value of the fuse. It is established by the manufacturer as a value of current which the fuse can carry, based on a controlled set of test conditions (See RERATING).

source document

*edit to include more information*

 

[Zil]

that is why god made Class-T fuses.

 

[gnubie]

A fast acting fuse will still have the same issue and needs to be rated to break a hell of a lot more current than the continuous pass rating.