Battery fusing and disconnect switch

piphomer · February 2013

Hi all!

I am building a hybrid system using a 1100Ah, 48V battery bank with 3 Sunny Island 5048s.

I have been using this fuse and this switch, connecting with copper busbar and 3 x 50mm2 wire to run to the SIs.

During some extreme load testing (at loads which will rarely or never occur in practice), I had sustained current of over 400A through the switch. The fuse did not blow: however it got extremely hot which seemed to conduct heat through the busbar to the plastic switch, causing the plastic of the switch to become soft and the spring mechanism of the switch then extruded itself through the back of the switch leading to total failure.

So my question is, I clearly need a larger form factor fuse, and preferably also a different kind of switch better able to deal with high temperatures. Does anyone have any experience in this area and could point me in the right direction? Also, I have sized the fuse to protect my battery wiring, but it leaves me little overhead above my normal operating current. Should I be sizing the fuse at a much higher amperage, given that what I am trying to protect against is short circuit currents which on this large battery bank would be in the thousands of amps?

Thanks for any help!!

-Phillip

westbranch · February 2013

Re: Battery fusing and disconnect switch

Any details as to the loads? size, type, etc.

Cariboocoot · February 2013

Re: Battery fusing and disconnect switch

Welcome to the forum.

Definitely the wrong kind of fuse. You want to be using the T type: http://www.solar-electric.com/infubr.html These are available up to 400 Amp.

Why are you pulling 400 Amps? Each SI should have its own fuse and disconnect; one of them will drawn less than 200 Amps. Trying to pull full current for all three through that once switch is not a good idea, as you've seen. They're just not meant for that much current.

How heavy a wire are you using? If it is too small it will make matters worse. As 4/0 AWG would be over-current @ 400 Amps you've got a bit of a problem here. Some rethinking of the over-all design may be in order to eliminate these potential problems.

inetdog · February 2013

Re: Battery fusing and disconnect switch

piphomer wrote: »

Hi all!

I am building a hybrid system using a 1100Ah, 48V battery bank with 3 Sunny Island 5048s.

I have been using this fuse and this switch, connecting with copper busbar and 3 x 50mm2 wire to run to the SIs.

During some extreme load testing (at loads which will rarely or never occur in practice), I had sustained current of over 400A through the switch. The fuse did not blow: however it got extremely hot which seemed to conduct heat through the busbar to the plastic switch, causing the plastic of the switch to become soft and the spring mechanism of the switch then extruded itself through the back of the switch leading to total failure.

So my question is, I clearly need a larger form factor fuse, and preferably also a different kind of switch better able to deal with high temperatures. Does anyone have any experience in this area and could point me in the right direction? Also, I have sized the fuse to protect my battery wiring, but it leaves me little overhead above my normal operating current. Should I be sizing the fuse at a much higher amperage, given that what I am trying to protect against is short circuit currents which on this large battery bank would be in the thousands of amps?

Thanks for any help!!

-Phillip

What value was the fuse? This type of fuse (ANL) is designed more to protect a battery charger than to protect a battery itself (having no deliberate time delay in http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/Data%20Sheets/Bus_Ele_DS_2024_ANL.pdf) So it is not ideal for use on the load side of the battery where short term higher currents are common.
Since all fuses will generate some heat at high currents, and the contact resistance of the switch itself may also generate substantial heat at currents like that, I think you may have to provide more thermal "distance" between them or provide some sort of heat sink. The fuse will also get very hot if the nuts holding it to the fuse holder are not solidly tightened. Same for the terminal nuts on the switch.

The switch, although it has a 5 minute rating of 500 amps is only rated for 300A continuous, so there may be an issue there too.

I think that you have to choose between protecting the battery against short circuit using an even higher amp fuse while protecting the charger and the inverter separately with appropriately sized fuses or breakers, or using a lower amp fuse and risking unnecessary fuse blows under heavy load.

piphomer · February 2013

Re: Battery fusing and disconnect switch

Thanks for the replies so far! I can see I made a good decision to come to this forum.

I guess in the interests of keeping my request simple, I left out quite a few details. So to give a bit more of the story, we are actually using 9 Sunny Islands in 3 clusters with a multicluster box. Each cluster is fed by 7 strings of 4x 12V 162Ah batteries to give 7x 48V mini-strings (for want of the correct term). We have paralleled these mini-strings in groups of 3, 2 and 2 using 50mm2 (1/0) wire. We bring join these wires at a busbar then run solid copper bar (25 x 4mm) to the switch and fuse, then more copper bar to a second busbar where we split out again to 3x 50mm2 wires which feed each Sunny Island. Here's a schematic of what we're doing for each cluster:

Attachment not found.

The maximum sustained current per cluster would be 15kW / 48V = 312A. of course it could go as high as 18kW/48V = 375A for 30mins according to SMA's specs, and much higher in fact for short periods but we don't expect to see that as our generator would kick in if load ramped up that high.

We could conceivably use a separate switch and fuse for each Sunny Island but that would give us 9 of each which is a lot of expense and failure points, as well as making it laborious to switch the system off (not that we expect to be doing that often).

The fuse and holder pointed to by Cariboocoot looks like just what we need, so thanks for that. I'd still like to try to find a heavier duty switch that will handle the 400A total of each cluster. I'm thinking there may be some kind of knife switch, or maybe even a fused DC disconnect that I can route all three clusters through?

Thanks folks.

Cariboocoot · February 2013

Re: Battery fusing and disconnect switch

I just have to ask: why have you got all that DC connected together? In my opinion you'd be better off giving each SI its own battery bank with appropriate fuse & shut-off. Any one of those may draw 200 Amps, but normally would be below 100 Amps. You're trying to funnel up to 600 Amps through equipment not capable of handling it. Such equipment would be rare, and expensive.

The 1/0 wire isn't really sufficient for handling the peak expected 200 Amps per unit, much less 400 Amps. And it could potentially see that from any one battery bank given that wiring configuration: bad connection to one battery set and the three inverters will try to pull full power from the remaining connected sets. Each of these battery banks should be fuse protected to prevent this, and given the 1/0 wire size now in use that would be a 200 Amp fuse per bank (Blue Sea terminal fuses on each string would be a good idea).

The weakest link in your design is trying to force all the current for three inverters through one switch and fuse. At the very least you should change that to one switch and fuse per inverter. Otherwise that part of the circuit is subject to too much stress even an nominal full power operation (approx. 300 Amps @ 48 Volts - the limit of the switch). At this one point even 4/0 wire would be stressed at full power. The simplest, safest solution here is to go with one switch and one fuse per inverter.

vtmaps · February 2013

Re: Battery fusing and disconnect switch

I think that 7 parallel strings of batteries is a very poor design for a system. Its also dangerous, if you don't have a fuse in each string. After a while, as the batteries diverge in electrical characteristics, you will have one string discharging into another.
read: http://forum.solar-electric.com/showthread.php?14674

-vtMaps

piphomer · February 2013

Re: Battery fusing and disconnect switch

That's the way SMA dictate the system should be laid out. Primarily because each SI in a cluster is supplying one phase in a three phase system. As the phases will never be perfectly balanced, having individual battery banks for each unit would result in one bank being drained when another is full. By sharing the bank across the three phases, unbalanced phase loads is not an issue. SMA also have designed the architecture such that if one cluster's battery bank is lower than the other(s), then less load is drawn from that cluster on discharge and more current is pushed into it on charge, so as to keep them balanced over time.

I see your point about a bad string connection resulting in overload on the remaining strings and I'll need to give that some thought.

Thanks for your help!

piphomer · February 2013

Re: Battery fusing and disconnect switch

I'm not sure how else to do it other than parallel strings. I have to have 4 batteries in series to give me my 48V, so how else do I get the total Ah capacity I need other than putting strings in parallel?

Cariboocoot · February 2013

Re: Battery fusing and disconnect switch

Two things:

The line from the battery positive bus bar to the inverters can be replaced with one line, switch, and fuse per inverter. They will still have common DC connection points but the parallel wiring will negate the current handling problem.

The battery bank itself could be made from fewer strings of higher Amp hour capacity batteries. Right now you've got seven * 162 Amp hours totaling 1134 Amp hours. If there is no need for AGM's this bank could be constructed from 2 Volt cells like these: http://www.solar-electric.com/repoba2vo13a.html Thus you'd have 24 of these all in series so the current sharing would be even. At the very least you could use four parallel strings of eight 6 Volt batteries like these: http://www.solar-electric.com/repoba6vo325.html That would eliminate some of the current sharing problem.

westbranch · February 2013

Re: Battery fusing and disconnect switch

Can you explain why there are 2 banks with 8 cells and 1 bank with 12 cells?

There is something not quite right here...

It might be better if you had larger cells to reduce by 1/2 the number of cells.

piphomer · February 2013

Re: Battery fusing and disconnect switch

Sure. I used HOMER to calculate the optimum total battery capacity and it came out to around 80 2kWh batteries for my entire system. That worked out at 84x 12V, 160Ah batteries across three clusters. That gave me 28 batteries per cluster, or 7 strings of 4 batteries which all need to be in parallel. My reasoning was that all these strings are ultimately in parallel at the busbar so while physically it looks like two lots of eight and one of twelve, electrically it's just seven parallel strings.

Does that make sense?

We've been using C&D VRLA batteries, and also experimented with Fullriver. 200Ah is about as big as these cells get (which is actually 160Ah at the C/10 rate that SMA say the capacity should be calculated at).

-Phil

Cariboocoot · February 2013

Re: Battery fusing and disconnect switch

What you've got may look equal electrically, but it isn't necessarily. Different total length of wires from any given battery string to the bus bar means different resistance and thus performance on that string. This applies to current going in or out. If you were to have seven parallel strings the only hope of keeping performance equal is for each string to have the same length wire to the bus bars. As per the attached diagram, but with seven strings instead of two.

Better yet would be three parallel strings of eight 390 Amp hour 6 Volts. Fewer batteries, fewer connections, and less current-sharing trouble.

inetdog · February 2013

Re: Battery fusing and disconnect switch

piphomer wrote: »

I'm not sure how else to do it other than parallel strings. I have to have 4 batteries in series to give me my 48V, so how else do I get the total Ah capacity I need other than putting strings in parallel?

The standard answer to that question is to use lower voltage batteries, even if they are the same size.
Instead of 7 strings of four 12 volt batteries, use 24 2 volt batteries in series. That would be almost the same capacity, and if you needed more you would go to a larger heavier 2 volt battery.

westbranch · February 2013

Re: Battery fusing and disconnect switch

piphomer wrote: »

We've been using C&D VRLA batteries, ........... 200Ah is about as big as these cells get

-Phil

see my sig line.... the C&D's are 2 v per cell 900Ah (20hr)
the Absolytes are also 2 v pc, and range well above 1000Ah

David and Laura · February 2013

Re: Battery fusing and disconnect switch

We're also in the process of building a Sunny Island system (not three-phase, sadly), and SMA indicated that to us that while all Sunny Island inverters in a cluster MUST share the same battery bank, you should have independent fusing and disconnection switches on a per inverter basis.

In a multi-inverter configuration, if one of the inverter DC bus trips or there is an inverter failure, the remaining inverters will continue to run. For three-phase with just three inverters, that results in loosing one of the phases. When you have a cluster, (six or twelve inverters), you just loose capacity on that phase.

There are many fault scenarios (not just in the inverters) that can result in loosing a phase, so protective relaying is a must for all loads that are sensitive to a single phase fault.

piphomer · February 2013

Re: Battery fusing and disconnect switch

Dear all,

Thanks for all your replies, I've learned a lot!

We are going to go ahead and put fuses on each of our parallel strings now, and even out the cable lengths as much as we can. We are still stuck with our 3-2-2 arrangement though. In future, we will definitely look at lower voltage, higher Ah batteries to avoid parallel strings altogether.

One more question at this point: our fusing is really intended to protect our battery wiring in the event of a short in the battery bank somewhere. We don't consider it necessarily as being able to protect the Sunny Islands in case of a fault inside them. Considering that the Sunny Islands are actually capable of putting out 12kW each for 60ms or so, it really seems we need slow blow fuses for this case. Yet the Class T fuses seem to be mostly fast acting. I have found some slow blow Class Ts but they seem non standard to me. Any comments on this thought? There's also these Mega fuses which do seem to be slow blow and similar size to the Class Ts. Would these be something we could consider?

Many thanks again!!

-Phillip

Cariboocoot · February 2013

Re: Battery fusing and disconnect switch

Actually what the fuses protect is the components of a circuit that have not failed. If one the SI's shorts internally there is no protecting it; it's gone. Meanwhile it creates a dead short that can allow the batteries to pump everything they've got through the circuit. That's when the fuse should pop and stop the current flow so that nothing overheats and starts a fire.

You don't really want slow blow fuses if they are properly sized. You want them to cut out quickly if their current rating is exceeded because that means something is wrong somewhere.

BB. · February 2013

Re: Battery fusing and disconnect switch

Fuses are there to protect the wiring--So, at the very least, they should be sized to the wire/circuit they are installed on/in.

Next, slow blow fuses (in my humble opinion) are really designed for motor starting and other loads with heavy/short surges. For the most part, solar power systems will operate for hours at rated power (or at least minutes for edge of cloud effects).

So--The wiring and fusing should be heavy enough to carry the rated output of the the GT inverters (or loads) that are expected (x 1.25 for NEC derating).

There is a time when people operate with smaller wire/fuses--For example a 3 kW inverter with a 1.5 kW array--But even then, according to the NEC, there should be a 1.25x1.25 derating, or a 1.5 kW array should be wired for ~2.34 kW of energy.

After typing all of the above--Duh--This is an Off Grid inverter system. So, yes, you need to think about the output surge current too. However, even "fast acting" fuses are not what I would call fast acting (i.e., at 25% over current, blow in a few seconds).

See if you can find the data sheet for the fuse you are looking at. For example, here is a PDF Catalog from Cooper Bussmann:

http://www1.cooperbussmann.com/pdf/b829b267-b922-4fef-abbb-fd1237d3cc36.pdf

A 200 amp fuse will (on average) blow ~1 second at 1,000 amps. And take almost 1 minute at blow at 400 amps.

So--I would not worry too much about using slow blow fusing. And, perhaps you would want to up rate the wiring/fusing by one step if you either expect lots of heavy surge/near rated output operation--And/or a false fuse trip is a big deal (fuses are electrical heating elements--Running high current through fuses will cause thermal cycling and eventual failure).

Fuses and Breakers are not there to protect the Inverter from operating beyond its specifications--The inverter should have its own protective circuits to do that. The fuses are there to protect the wiring (and input wiring box) of the inverter (fuse fails before wiring/or cause input of inverter to start a fire on the wall of the structure).

Hope this makes sense.

-Bill

By the way, this document suggest using a fast acting fuse for 300% of FLA rating of a motor--To protect against shorts only.

inetdog · February 2013

Re: Battery fusing and disconnect switch

BB. wrote: »

Fuses and Breakers are not there to protect the Inverter from operating beyond its specifications--The inverter should have its own protective circuits to do that.

"Transistors and diodes are semiconductor elements which blow to protect fuses."

piphomer · February 2013

Re: Battery fusing and disconnect switch

Thanks David & Laura. Did SMA express an opinion on switches? Are the DC disconnects built into the units sufficient to disconnect them, or would I need three separate switches per cluster (nine in total)? Currently I have master disconnect switch per cluster, but it's underrated for the max current that it will see (albeit not sustained current).

piphomer · February 2013

Re: Battery fusing and disconnect switch

Thanks Bill & Cariboocoot...

Any opinion on the Mega fuses? They are a lot cheaper than the Class T's.

BB. · February 2013

Re: Battery fusing and disconnect switch

If this is the correct data sheet for the fuse family you are looking at:

http://www.littelfuse.com/~/media/Files/Littelfuse/Technical%20Resources/Documents/Data%20Sheets/Littelfuse%20Automotive%20Bolt%20down%20Fuse%20MEGA%2032V.pdf

Then, this will not work for you. These are rated at 32 VDC and are in the "automotive products" catalog (usually assuming 12 volt system with only 2,000 Amps Interrupt Rating).

-Bill

piphomer · February 2013

Re: Battery fusing and disconnect switch

Yeah that looks like it.

I should have been able to find that out on my own, thanks for the pointers Bill!

BB. · February 2013

Re: Battery fusing and disconnect switch

Not a problem, have been around long enough that for anything critical--Always find the data sheet first. For fuses, it is sometimes a bit of slog to find the right data sheet/catalog.

-Bill

David and Laura · February 2013

Re: Battery fusing and disconnect switch

piphomer wrote: »

Thanks David & Laura. Did SMA express an opinion on switches? Are the DC disconnects built into the units sufficient to disconnect them, or would I need three separate switches per cluster (nine in total)? Currently I have master disconnect switch per cluster, but it's underrated for the max current that it will see (albeit not sustained current).

We've never asked them about that specifically, given that, in our opinion, the DC disconnects built into the Sunny Island inverters are insufficient as they don't completely remove all power from the inside of the unit.

For example, in the scenario where you have an inverter fault, and you need to remove an inverter to send it back for repairs (or to replace it), you want to be able to isolate (and ideally lock out) both power feeds (AC and DC) before you open up the inverter and start disconnecting wires.

In a clustered configuration, you ideally want to be able to do so without having to take the entire cluster, or even worse, the entire system down.

Battery fusing and disconnect switch

Comments

Categories