What do our experts think of this of this?

[Ray-Vic]

Hmm... I'd seen that and somehow forgotten it along the way. I know it's overkill, but I could use 2 x 50mm (which I've already got) rather than buying 70mm...

I'm guessing you keep the cable lengths the same, use the same lugs etc so that they're as 'equal' as possible.

If you have 50mm already on a single Fogstar Drift 300Ah, then that is fine, you wouldn't need 2. The cable lengths only need to be identical if you are adding another battery so both charge and discharge at the same rate.

Edit: I should caveat this by saying I'm referring to hi-flex battery cable, of which the 50mm version is rated at 345A.

 

[Raul]

I have started planning out my new system and it will possibly be 48V and 5KWh capacity so Megafuses would not be suitable.

At 48v nominal the mega will not be suitable for lithium. Pylontech with smaller 48v modules, has in their small print recommendation for HRC fuses, due to high fault current.

 

[Raul]

I had forgotten about MRBF. Thank you.

Initially I was not sure for the MRBF max kA rating at lower voltages. Thanks to Ray-Vic in post 51 he shows the data for eaton fuse sold by blue sea. Thats reassuring.

Also be aware that MRBF are not suitable for lithium at 58v, absorb voltage.

 

[treblid]

What are the experienced views regarding decent quality silver plated ignition protected ANL fuses in 12V lithium systems say up to 300Ah? For example:

Blue Sea Systems Silver-Plated ANL Fuses

Blue Sea Systems ANL fuses, ignition protected with high interrupt capacity for protecting large battery banks. Silver-plated contacts for corrosion resistance.

www.12voltplanet.co.uk

 

[Raul]

Hmm... I'd seen that and somehow forgotten it along the way. I know it's overkill, but I could use 2 x 50mm (which I've already got) rather than buying 70mm...

I'm guessing you keep the cable lengths the same, use the same lugs etc so that they're as 'equal' as possible.

Here is a table for current carrying capacity based on method of installation. Bear in mind that no other derating factors have been applied, like grouping, ambient temperature etc. At 30 degC the rating is 1:1 and grouping is one single circuit per containment, as in trunking or conduit. Third column is what I look at, because the cables are in trunking fixed on a wall. If is in free air, no containment, then look at column 5, method C.

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[Ray-Vic]

As this thread is about fusing, below is a picture I took when I was inspecting a lift in London and when returning it to service the large starting current caused a fuse to fail in the TP&N switch-fuse. Nice smell, lots of smoke, so I opened it up to find this!!! The holder in the middle with the HRC fuse actually has copper wire behind it, so the fuse is doing nothing!

And this was on a lift which was in service and being used by the public!

Some scary things out there.

 

[AdrianChen]

Cable to small for a 12.8v to 3000w, add another one and fuse with 250A.

On the boats, if for some reason we used twin cables each was fuse separately, in case one failed leaving one good cable remaining on too big a fuse....

Sounded good when the sparky lectured me!

 

[fishplug]

Here is a table for current carrying capacity based on method of installation. Bear in mind that no other derating factors have been applied, like grouping, ambient temperature etc. At 30 degC the rating is 1:1 and grouping is one single circuit per containment, as in trunking or conduit. Third column is what I look at, because the cables are in trunking fixed on a wall. If is in free air, no containment, then look at column 5, method C.

Thanks Raul - extremely helpful.

There won't be any containment as such, although some of it will run through the vehicle cavities.

All I really know is not to run data and power in the same containment!

I'm very much at the 'little knowledge' and dangerous stage! But at least I know it and try not to do anything that might kill me or those around me.

This is why I really appreciate the help that people like you and Ray-Vic and others who actually KNOW what they're talking about offer people like myself.

Without your generous sharing of knowledge I (and others I'm sure) would be installing terrible systems which we thought were fine because some idiot on YouTube told us it would be 'fine'.....

 

[bigtwin]

I'm very much at the 'little knowledge' and dangerous stage! But at least I know it and try not to do anything that might kill me or those around me.

You, clearly, are bright enough not to be a victim of the Dunning Kruger effect so have sufficient awareness of the extent of what you don’t know. There are many who over-estimate their own knowledge/capability.

Ian

 

[pappajohn]

Maybe @Lenny HB can comment
Apparently ordinary mega fuses under a dead short of the battery, can weld themselves back together after blowing.

Then surely, if they do weld back, they will once again blow if the short still exists.

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[Raul]

There won't be any containment as such, although some of it will run through the vehicle cavities.

You're welcome,

Still follow third column as thats the closest scenario, its like in a containment, not in free air, but, not covered by insulation, or, through an insulated wall.

 

[bigtwin]

Then surely, if they do weld back, they will once again blow if the short still exists.

Only if they are the weakest link in the circuit and there is no guarantee of that.

Ian

 

[Gromett]

Then surely, if they do weld back, they will once again blow if the short still exists.

It;s not so much that they weld back. When the fuse blows, if there is a high enough current flowing through it then it will arc. This will ionise the air and allow the arc to be sustained.

Same reason you shouldn't use an AC rated switch on a DC circuit as I posted on a previous thread.
Check this out. The two videos are very informative.

Changing leisure battery.

Just got back from a trip. We were on hook up so really didn't notice an issue. At home I checked battery levels and the leisure battery quick sank to below 10v, so pretty dead. I just need to know if there is anything specific I need to do when I change it. Below is a pic of the connections...

www.motorhomefun.co.uk

 

[Raul]

Then surely, if they do weld back, they will once again blow if the short still exists.

If the arc it's not extinguished, the current will carry on, even with the fuse blown in the middle. Only when the arching stops the current stops.

 

[fishplug]

Sent emails to Fogstar and 12Volt Planet today to see what the 'official word' was on the subject.

Fogstar have yet to reply, 12Volt Planet replied within 58 minutes and told me that Class T fuses were really mainly for 3000va/w and upwards inverters and that a MRBF rated below the max amperage of the selected cable would be fine for my needs. (single 12v 300Ah LiPo battery)

So, no need to spend the £100+ that Urban Vanlife tried to scare everyone into buying.

I'm grateful to them for raising the question in my mind and for prompting me to find a safe solution, but it wasn't quite as they described it.

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[bigtwin]

I'm grateful to them for raising the question in my mind and for prompting me to find a safe solution, but it wasn't quite as they described it.

It rarely ever is from those chasing likes/subscriptions.

Ian

 

[fishplug]

agreed

That said, it's prompted me to do some more learning and to invest in an extra level of safety.

I'll keep the MCB as an isolator switch and will probably rate my MRFB low enough to protect the MCB as well as the cable. I don't have much in the way of high loads. Cooking will be with gas and water heating will be primarily from diesel heater / heat exchanger. That leaves the glow plug on the heater and the compressor on the fridge as the most power hungry items.

I've yet to buy the water system and one of the potential options is a 12v heating element that is set to come on when the leisure batteries are full so it just uses 'excess' solar to heat the water. If I do go down that route then I guess I need to calculate that draw and factor it in.

I've seen that a few people have started discussing MRFBs in the comments of the video now, and they do seem to have been reasonably well accepted by Urban Vanlife - which is fair enough.

 

[Raul]

Class T fuses were really mainly for 3000va/w and upwards inverters and that a MRBF rated below the max amperage

Bear in mind the inverter is NOT the source of the high current, the battery is. Even if you have just 12v lighting, the supply ( battery) has the potential that needs to be interrupted by the fuse. Saying that, I do think a MRBF fuse is adequate, on a 300Ah.

Anyway, today was a rainy day, and had some time to finalise some of the temporary jobs. I managed to measure the internal resistance on 305Ah envision cells, packs, very close to EVE cells that are used in fogstar batteries. Surprising results. I will post later on my findings, if there is interest for it.

 

[fishplug]

I don't have an inverter, so it's not a consideration for my setup, but your comments about the potential coming from the battery make perfect sense.

Interest in your findings? You bet there is!

I've learned so much from this thread already - but more knowledge is always beneficial.

 

[Cush]

Ok, so head above the parapet time, no question’s a stupid question and all that and trying to better myself…

I get and trust that a LiFePO4 can dump a great deal of energy (too much) very quickly with potentially disasterplus consequences. I get that the T and mrfb type fuses negate the potential for arcing and voiding mega fuses.

My question is… if the fuse is to protect the cable, should the fuse be rated to protect the smallest gauge wire between the battery and charger / solar controller which are attached to the battery?

TIA of the schooling, Cush

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[Gromett]

Ok, so head above the parapet time, no question’s a stupid question and all that and trying to better myself…

I get and trust that a LiFePO4 can dump a great deal of energy (too much) very quickly with potentially disasterplus consequences. I get that the T and mrfb type fuses negate the potential for arcing and voiding mega fuses.

My question is… if the fuse is to protect the cable, should the fuse be rated to protect the smallest gauge wire between the battery and charger / solar controller which are attached to the battery?

TIA of the schooling, Cush

Raul will probably have a more comprehensive answer than me.

BUT. The main fuse protects the big chunky cable going from the battery to the busbar or fuse distribution panel or Inverter.
Then each smaller wire should have a fuse at it's source to protect that wire.

For instance. A big fuse on the battery which supplies the inverter and the fuse panel. The fuse panel then has say a 5 or 10A to protect the wires to the lights.

I hope that makes sense?

 

[Raul]

Ok, so head above the parapet time, no question’s a stupid question and all that and trying to better myself…

I get and trust that a LiFePO4 can dump a great deal of energy (too much) very quickly with potentially disasterplus consequences. I get that the T and mrfb type fuses negate the potential for arcing and voiding mega fuses.

My question is… if the fuse is to protect the cable, should the fuse be rated to protect the smallest gauge wire between the battery and charger / solar controller which are attached to the battery?

TIA of the schooling, Cush

In the entire system, you should have more than one fuse, if you have several circuits: one big main fuse from (supply) battery supplying a distribution, then each circuit has its own, serving various outputs/inputs; lighting fused for the lighting wire, charge fused for the charger wire and so on. Downstream this smaller fuses can be ordinary fuses as the load is fixed and can't draw more than what is connected to it. but you fuse to protect the wire for that load. If a charger output is 20A, your wire should be big enough to take 20A indefinite time, and fuse with 20A.
like on your house, your supply may be a industrial HRC 100A fuse, then your lighting is a 6A for that circuit alone.

 

[Raul]

While I was tidying up some wiring and had to fit another breaker, I thought this is a good opportunity to dig out the IR meter for lithium cells, and take some readings. I used this meter in the past to measure the IR consistency, as I was connecting cells in big banks. I gave up measuring as consistency was impeccable from fogstar.

I used as a guinea pig one of my house storage bank, and took various measurements. Individual cells, are 0.18-0.22mOhm,




I have taken several combination across 4 cells all connected as in a 12v pack, lowest 1.18mOhm and highest 1.36mOhm, this is without the bms, as the meter shoots a 100v AC at 100hz to get a reading, so I was worried about damaging the bms. All 4 cells groups gave a 13.19v
To add the bms in equation, I found the data on fogstar site as a download for JK bms. The 200A bms has a IR of 0.47mOhm, the 150A 0.65mOhm, and 100A 1mOhm.
What I got is, 1.36 highest reading + 0.47 for the 200A bms = 1.83 mOhms
13.19v / 0.00183Ohms = 7207A a max short circuit current of 7.2kA
That represents a 305Ah battery with a 200A BMS.

Now for those interested in house batteries:
For the 48v pack is even worse, the pack IR is 3.6mOhm the highest reading, and at 52.8v
Again 3.6+0.47=4.07mOhm 52.8/0.00407=12972A, 12.97kA of PSCC prospective short circuit current.

I understand that may be errors, temperature, etc. but, it is a good indicative of what we are dealing with. the cells are at 47% SOC so close to the middle for a balanced IR value. Yes at 50% soc would been better and 20C outside, but it was about 12C.

 

[Cush]

Thank you both Gromett and Raul so bloody obvious when you say it out loud. The big fuse protects everything after it. It’s not as if the battery gradually rises to the catastrophic point. In the unlikely event of it happening it will be a sudden surge that won’t get past the first big fuse and then all the other downstream fuses will protect those wires and attached devices.