To remove T Fuse or not with JK BMS

[callmeburton]

I am adding two packs to the system and opted not to get a T fuse for them as I am installing JK BMS's identical to the two now installed on the other packs.

This is really a question of resistance as I am "ok" with leaving the existing T Fuses blocks in place.
Right now I have added 1" of copper to the wire I plan to use between the pack and the switch for each pack to approximate the resistance of the T fuse on the other packs. But this had me thinking about removing the T fuses on the other packs and simply bolting the two ends of these wires together or making new wires to replace their length.

Thoughts ?

 

[chrisski]

I put a class 2 fuse on each of my 6.5 kWh batteries and then an additonal Class T fuse after the two batteries are combined.

I would not trust the FETs in the BMS should they fail shorted.

 

[callmeburton]

I would not trust the FETs in the BMS should they fail shorted.

I thought all the main rack mounted packs with BMS's included had FET based protection and not T fuse level protection these days?

 

[chrisski]

I thought all the main rack mounted packs with BMS's included had FET based protection and not T fuse level protection these days?

In addition to the DIY battery pack I mentioned, On my house build My Four Fortress 5 kWh batteries all have an internal class T fuse along with CB protection after they are combined.

I can't speak to the internals of the other batteries. I've seen some with external circuit breakers at the batteries.

I understand your reluctance to put additional class T's in. Probably $125 per fuse with the price of lugs, fuse holder, and fuse put in, not to mention about 6" X 2" X 3" of space per fuse. I would also understand a manufacturer would leave that cost to the user to lower his product price.

You will also find many DIY builds that have done exactly what you are asking and add a fuse only after the batteries are combined.

For me, if you have lots of batteries and the batteries are taken out of the system, sometimes a single battery and wiring can't take the current rating of all five in the system.

 

[callmeburton]

Would the preferred "fix" (to keep the resistances even in the packs) be to replace the current T fuses with the Class 2 fuses (which look cheaper) and bolt them in place with big heat shrink over them?

Then maybe move the other two or one T fuses near the combiner bus bar for the positive side of the inverter connection?

I currently have 2 magnum ($99 before shipping) T fuses .... already have the lugs. Packs shouldn't be "moved" as I have no way of easily doing so. The only thing I can do easily is disconnect one pack from the bus bar via a switch and 350A anderson connector.

 

[callmeburton]

Cut the wires as if there is a T fuse in place and bolted them together for now so I can commission the new batteries soonish ... I say that as 8 of them are damaged and I contacted vendor the 4th and am waiting for them to reply to either both my emails or text message :/

Not sure I want to use those 8 cells yet so at min will get one pack setup so we don't have to use the generator as much.

 

[Zwy]

I put a class 2 fuse on each of my 6.5 kWh batteries and then an additonal Class T fuse after the two batteries are combined.

I would not trust the FETs in the BMS should they fail shorted.

Yes, this is correct. I run a 175A Class T on each 200A JK BMS. I prefer the fuse blows before the FET's weld shut.

I wouldn't sleep in a house that didn't have a Class T on every battery pack.

 

[Zwy]

Would the preferred "fix" (to keep the resistances even in the packs) be to replace the current T fuses with the Class 2 fuses (which look cheaper) and bolt them in place with big heat shrink over them?

Then maybe move the other two or one T fuses near the combiner bus bar for the positive side of the inverter connection?

I currently have 2 magnum ($99 before shipping) T fuses .... already have the lugs. Packs shouldn't be "moved" as I have no way of easily doing so. The only thing I can do easily is disconnect one pack from the bus bar via a switch and 350A anderson connector.

Don Rowe- Class T with holder $59.99 any day of the week.

 

[callmeburton]

I think I rather stick to 300A fuses but the Don Rowe is slightly cheaper than the magnum by $20 right now. The JK's are 200A continuous with their short 4awg equivalent cross section silicon wire runs. I think the over current protection has a 1.5s delay as well as we have seen when inrush currents to inverters affect the units.