What is the best combination for diy battery protection, class T fuse/battery disconnect/dc beaker?

While i am learning more and more i come up with new questions. In a 16 cell 48v lifepo4 battery build, what hardware would you think is best for protection?
Right now i am putting just a 160 amp dc breaker. Could a class T fuse in combination with a simple battery disconnect switch be better?

Aridom82 said:

While i am learning more and more i come up with new questions. In a 16 cell 48v lifepo4 battery build, what hardware would you think is best for protection?
Right now i am putting just a 160 amp dc breaker. Could a class T fuse in combination with a simple battery disconnect switch be better?

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A class t fuse on a 48 volt system is very important as far as the switches or breaker make sure they can handle more than 48 volts because your system will exceed 48 volts

imo a class t is mandatory

I feel the same way, there is lot of community consensus on the pack design but once you get out into the disconnect, OCP, and precharge resistor circuit there is a real lack of specific recommendations. People will describe conceptual protections but are skittish with exact product recommendations. I guess because there's just a lot of uncertainty, and not a lot of confidence from anyone that their setup is ideal.

A T Class fuse as close as possible to the battery terminal is the only consensus.

Depends on money you want to spend. You can substitute a high quality MCCB breaker for a class T fuse. They can even have higher interrupting current then a class T fuse and also perform as a disconnect/switch.

I have already 3 class T fuses, and the DC breaker is high quality. Do i need to add another T fuse in the green box?

The danger with some breakers & fuses is that under a dead short, they could become a welded mess, and not break the circuit. The Class-T has very high current interrupt capacity and is nearly gospel for battery protection here on the forum. Setting it as close as possible to the battery makes the best sense, but how, the server racks are not designed with a convenient location for the Class T to go. I feel that some smaller-form-factor fuses may be a solution, but didn't go this way.
Most server rack batteries will come with a breaker built in, but the question is: will this melt/fuse together solid and not break the circuit under a dead short like say a rachet gets dropped across pos-neg at the battery terminals and boom?
I decided to set my ESS up with 150A class T on the pos between each server-rack battery and the main pos bus, as well as a DC rated Single-pole breaker at 125A between the Class-T and the battery. This is not really ideal, It would be better to put the Class T right on the battery terminal - but I don't see any way for this to fit properly. Cover the battery terminals after connection of the first cable, and a lockable door to the rack is my solution.

The main pos bus and neg bus feed a large main disconnect 2-pole DC breaker, the outfeed cables from the main breaker feeds two 350A Class T that each feed two shorter buses (one pos one neg) that allow me to connect each inverter (currently two, soon to be three) each with their own stud, to allow easy removal of just one, while the others run - if this were ever needed. If I could change anything, it would be to move the Class T closer to each battery, but how this would be mounted remains a mystery, and I see that the factory server racks although they have main bus bars, do not supply a convenient location for each server rack to connect with a Class T.

Again, the T fuse is the gold standard for protection but it does not have switching capabilities. A high quality dc breaker will switch with no problem but failure is a possibility in a short. Would it be the ideal to use a T fuse in combination with just a battery disconnect switch?

Aridom82 said:

I have already 3 class T fuses, and the DC breaker is high quality. Do i need to add another T fuse in the green box? View attachment 163489

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Move the class t fuse to the battery.

DIYrich said:

Move the class t fuse to the battery.

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What do i put there instead if anything?

200+ kilowatt hours of LFP DIY. I have dp circuit breakers on every battery, oversized wiring, T-class fuse between cells eight and nine and used busbars to connect them. All batteries surrounded with quarter inch ceramic fiber matting to contain heat in case of one bad cell. All areas of PowerShed near electrical equipment or batteries covered in 5/16” concrete board. Nothing on either my home or shop solar are within 90 feet of the house. DIY CO2 and halon fire extinguishing system. Interconnected smoke alarms with one in the MBR. ?

RV10flyer said:

200+ kilowatt hours of LFP DIY. I have dp circuit breakers on every battery, oversized wiring, T-class fuse between cells eight and nine and used busbars to connect them. All batteries surrounded with quarter inch ceramic fiber matting to contain heat in case of one bad cell. All areas of PowerShed near electrical equipment or batteries covered in 5/16” concrete board. Nothing on either my home or shop solar are within 90 feet of the house. DIY CO2 and halon fire extinguishing system. Interconnected smoke alarms with one in the MBR. ?

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Ok so your solution is both, 2pole dc breaker + T fuse. The fuse between cells 8 and 9. Got it. I will research firehazard next.

Most "disconnects" are not rated to break a fault. Personally I would opt for a breaker for a single battery, and fuses for groups of 6 batteries or ~900Ah, whichever is lower. The equipment DC circuit breakers should be adequate for that end.

So, if you had 12x100Ah batteries and two inverters I would rely on breakers for the individual batteries and inverters, and a set of fuses where you parallel the two 600Ah busses together.

Not sure how to make the Class-T fuse fit onto a battery or cell terminal, - as far as I know they can't direct fit. The only way is to have a short section of cable/bus bar. Ditto on a server rack terminal, the class T will not directly fit on any terminals I have tried. One thing to keep in mind, the class T fuse has a holder - and part of the reason for this is to physically hold the two halfs of the fuse apart if it does trigger.

OffGridForGood said:

Not sure how to make the Class-T fuse fit onto a battery or cell terminal, - as far as I know they can't direct fit. The only way is to have a short section of cable/bus bar. Ditto on a server rack terminal, the class T will not directly fit on any terminals I have tried. One thing to keep in mind, the class T fuse has a holder - and part of the reason for this is to physically hold the two halfs of the fuse apart if it does trigger.

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The same busbars you connect each cell with can be connected to the ends of a T class fuse. I just bent the bus bar at about a 45° angle. Use proper size bolts with washers/lock washers. My 2000 pound battery sets in a power shed. I am the only one in there. Ever. Once the T class fuse blows and I have blown one. It just sits there, you hear a little pop, that’s it. Even if it did blow apart the two busbars are not gonna come together. The main reason I put it between cells eight and nine are now you have 2 24 V batteries instead of 1 48 V battery. Then I have a bigger class T fuse and another bigger circuit breaker going to my inverters, along with a 350A shunt on the negative. It’s your money, your battery, your PowerShed, and your house. Build it as you wish.

Aridom82 said:

What do i put there instead if anything?

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You can put a breaker there.

I use 2”, 4” & 8” polyamide high temperature tape. It prevents my BMS wiring from shorting out against busbars. I then cover the entire top of battery with the 8” wire to help prevent anything from falling into battery. Ensure proper torque before covering. Use thermal camera during high charge/discharge to check for hot spots monthly.

RV10flyer said:

The same busbars you connect each cell with can be connected to the ends of a T class fuse.

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I am looking at it.
In my original set up, I only had two factory built server rack batteries, so the location for the class T ended up exterior of the pack.
Later when I built my first DIY pack, I just carried on with the same set up (batter terminal to DC disconnect to class-T to main pos bus)
I like your solution: answers my original issue: "each rack T-class fuse should be as close as possible to the battery" - in your approach it splits the battery pack into two 24v sections, and effectively cuts the circuit. I am looking at the way I built my own DIY packs, It may be there is enough space to fit a 125A class-T between the two rows as you have done, by cutting out a section of the bus-bar and adding in the class T.
I am working on battery #6 in the next couple weeks, I am going to try this out, see if it will fit. - Thanks @RV10flyer.

OffGridForGood said:

I am looking at it.
In my original set up, I only had two factory built server rack batteries, so the location for the class T ended up exterior of the pack.
Later when I built my first DIY pack, I just carried on with the same set up (batter terminal to DC disconnect to class-T to main pos bus)
I like your solution: answers my original issue: "each rack T-class fuse should be as close as possible to the battery" - in your approach it splits the battery pack into two 24v sections, and effectively cuts the circuit. I am looking at the way I built my own DIY packs, It may be there is enough space to fit a 125A class-T between the two rows as you have done, by cutting out a section of the bus-bar and adding in the class T.
I am working on battery #6 in the next couple weeks, I am going to try this out, see if it will fit. - Thanks @RV10flyer.

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It’s all about learning, experimenting and seeing what works for you and what doesn’t. I’ve blown molten aluminum/copper across the dining room with 3000A+ short circuit (now keep my wife’s hot red fingernail polish handy to mark everything), overcharged/ permanently enlarged 4 ea expensive 100aH Fortune cells to 4.2V/cell. Have fun. Wear eye protection and 8mil nitrile gloves at a minimum.

RV10flyer said:

It’s all about learning, experimenting and seeing what works for you and what doesn’t. I’ve blown molten aluminum/copper across the dining room with 3000A+ short circuit (now keep my wife’s hot red fingernail polish handy to mark everything), overcharged/ permanently enlarged 4 ea expensive 100aH Fortune cells to 4.2V/cell. Have fun. Wear eye protection and 8mil nitrile gloves at a minimum.

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In my design it barely fits, but it does fit. The cell rows are sepated by 30mm. I am going to use two small pieces 6mm or 1/4'' thick copper flat bar to bridge it. I will just drill and tap the holes and screw the t fuse directly into it.

I haven't blow up anything yet, but that doesn't mean it will not happen tomorrow.
Similar to your set up, my solar-battery-inverters are all far away from the house, for good reason, and peace of mind.