Leisure battery planning - advice needed

[t0mb0]

Now I've learnt about the unsuitability of Midi fuses for Lithium batteries, so need to buy a MRBF fuse. 100A one on 100A battery?

A 100A fuse is fine as long as

a) Your anticipated max draw is sufficiently under that.

and

b) The cable is rated sufficiently over that.

The MRBF is a good design as it means the whole cable length is protected. An MRBF with a single sufficiently chunky cable to a distribution fusebox which then feeds all the other (individually fused) circuits is an excellent way of doing it.

EDIT: Just to add to this - you would also need to make sure that the fusebox itself that you’re feeding is rated up to the full 100A in this scheme - not sure how common that is.

 

[ben5560]

Thanks mate that's really useful, It's only going to be the standard 12v fridge, usb sockets, usb lights and a diesel heater - so way under 100A. And planning to use 16mm² cable.

 

[WillC]

Fuse size is dictated by cable size and vice versa, if you want a cable to carry 100A then you need to decide how long the cable run is going to be as that will decide the CSA, basically the longer the cable run the fatter the copper needs to be.
Also just because a battery is rated at 100Ah this doesn't mean it won't pass many hundreds, possibly thousands of Amps into a short circuit (lithium especially with its low internal resistance), the 100 Amp Hour rating roughly means that the battery can supply 1A for 100hrs or 100A for one hour and is the capacity of the battery.
Put another way you could connect two 100A fuses for two separate supplies no problem but empty the 100Ah battery in 30 minutes hence the attraction of larger batteries.
You shouldn't have a problem using midi and maxi fuses with lithium?

I hadn’t picked up on any issues with this, and have fitted midis in my setup. I await the outcome and thoughts of out trusted electrician…

 

[ben5560]

Fuse size is dictated by cable size and vice versa, if you want a cable to carry 100A then you need to decide how long the cable run is going to be as that will decide the CSA, basically the longer the cable run the fatter the copper needs to be.
Also just because a battery is rated at 100Ah this doesn't mean it won't pass many hundreds, possibly thousands of Amps into a short circuit (lithium especially with its low internal resistance), the 100 Amp Hour rating roughly means that the battery can supply 1A for 100hrs or 100A for one hour and is the capacity of the battery.
Put another way you could connect two 100A fuses for two separate supplies no problem but empty the 100Ah battery in 30 minutes hence the attraction of larger batteries.
You shouldn't have a problem using midi and maxi fuses with lithium?

Apparently it was because of the risk of midi/maxi welding themselves together with the extreme high amps and therefore T Class fuses or MRBF were recommended?

 

[WillC]

So after Googling, the MRBF attaches to the battery terminal directly, so protects the whole cable. A trad setup has the fuse immediately after the terminal. What’s the crack here? To protect the short cable section before the fuse? Or something weird about Li batteries?

 

[ben5560]

So after Googling, the MRBF attaches to the battery terminal directly, so protects the whole cable. A trad setup has the fuse immediately after the terminal. What’s the crack here? To protect the short cable section before the fuse? Or something weird about Li batteries?

Something about the cable/extremely high amp surge, from AI:

Risk of “failing closed” under extreme faults - Under a brutal lithium short, the fuse element can vaporize and form an electrical arc instead of opening cleanly; with MIDI/MEGA types the arc can sometimes weld the element or terminals so current keeps flowing rather than going open‑circuit. That defeats the whole point of the main safety fuse and can escalate to fire.

 

[WillC]

Yes, that’s it, just found this blurb on the roamer site… Battery protection fuses for LiFePO4
Sounds like the first fuse needs to be capable of handling the huge current the Li can push under short circuit.
I might need to find me a marine fuse then…

 

[Salty Spuds]

Something about the cable/extremely high amp surge, from AI:

Risk of “failing closed” under extreme faults - Under a brutal lithium short, the fuse element can vaporize and form an electrical arc instead of opening cleanly; with MIDI/MEGA types the arc can sometimes weld the element or terminals so current keeps flowing rather than going open‑circuit.�� That defeats the whole point of the main safety fuse and can escalate to fire.

You’re correct. Lithium has the potential for huge short circuit currents. What most people don’t appreciate is that fusing isn’t just about the current rating of the fuse at which it blows, it’s also about the maximum current it can safely and reliably break. With Lithium batteries, that short circuit current can run to thousands of amps. So the application may only call for a piddly 20A fuse, but that fuse must break at its rated capacity and time and also be capable of handling and breaking the potential SCC. Old school lead acid batteries had a natural internal resistance that limited the potential SCC, Lithium has no such natural resistance and can runaway. Lifepo is a slightly different case as it does have some self limiting properties, but it can still get messy.

 

[WillC]

100A MRBF Fuse – Reliable Protection for Lithium Battery Systems Should do it.

 

[Salty Spuds]

100A MRBF Fuse – Reliable Protection for Lithium Battery Systems Should do it.

Does it need to be 100A? In a van conversion scenario, unless you’re running an inverter, 40A should be plenty. Also have a
Look at 12V planet, I think they might be cheaper.

 

[ben5560]

100A MRBF Fuse – Reliable Protection for Lithium Battery Systems Should do it.

That's what I found too,I messaged them just to double check as well earlier today

 

[ben5560]

Does it need to be 100A? In a van conversion scenario, unless you’re running an inverter, 40A should be plenty. Also have a
Look at 12V planet, I think they might be cheaper.

Is it about allowing the desired the current draw, but setting a ceiling on the current in case the lithium spikes it to crazy levels, therefore instantly protecting the cable?

16mm2 is enough right?

 

[t0mb0]

Thanks mate that's really useful, It's only going to be the standard 12v fridge, usb sockets, usb lights and a diesel heater - so way under 100A. And planning to use 16mm² cable.

Oh yeah, you can likely get away with a fair bit under 100A then. Your max current in practice is likely to be the charging current then rather than the draw.

 

[ben5560]

Oh yeah, you can likely get away with a fair bit under 100A then. Your max current in practice is likely to be the charging current then rather than the draw.

Which hopefully will be 50A if the Victron XS actually supplies that!

 

[Stay Frosty]

Is it about allowing the desired the current draw, but setting a ceiling on the current in case the lithium spikes it to crazy levels, therefore instantly protecting the cable?

16mm2 is enough right?

Yes and yes.
I think there's more than a bit of hysteria around the internet with lithium EV batteries and pyro fuses being added into the mix where we are working with much more modest installations.
My installation has a main 250A ANL fuse using 0 guage cable to the battery isolator then on to one of those Maxi input fuse blocks with 4 smaller midi fused ways fed from the busbar to provide lower current outputs.

 

[WillC]

Is it about allowing the desired the current draw, but setting a ceiling on the current in case the lithium spikes it to crazy levels, therefore instantly protecting the cable?

16mm2 is enough right?

That’s my thinking. We’re trying to stop a short circuit mega discharge from frying everything. So the 100 mrbf should stop that, even if the 30A midi further along has welded itself together.

 

[awillyfootball]

Someone has been tinkering under there as that doesn’t look OEM. At least I hope VW don’t just twist and tape cables together!

In essence what you suggest is fine but I’d want to get a multimeter on there and see exactly what is what. I’d also use a separate fused busbar for my leisure electrics rather than the VW one.

The cable from the SB doesn’t look that thick. Does it have the size stamped on it anywhere? Anything less than 10mm2 could suffer voltage drop issues. 16mm2 is ideal. That cable also needs to be fused at the SB end as per the Victron instructions (they also recommend the cable size). The output from the DC-DC is then best to the fused busbar rather than direct to the LB.

A 105ah Drift fits on its side under the seat. The OEM cables can be tucked to one side. The battery needs to be fused at, or very close to the battery, then cable to the busbar. All your feeds can then be taken off the fused busbar.

Here is a pic of my setup with 105 Drift, 30A Orion, Ablemail AMT12-2 etc under the passenger seat.

View attachment 305013

You can see the OEM cables tucked away n the top right corner (covered in bubble wrap).

Hi, nice setup,looking at fogstar as well with a Ablemail AMT12-2 did you just wire this in to the dc to dc charger mines in the same position but wonder if the wires will fit into the charger. Thanks

 

[WillC]

I unpicked the charging wire from the loom heading to the passenger seat, and curved it over to under the driver’s seat. There was enough length for this. Bit of gorilla tape to keep it in place, job done.

 

[ginkster]

Hi, nice setup,looking at fogstar as well with a Ablemail AMT12-2 did you just wire this in to the dc to dc charger mines in the same position but wonder if the wires will fit into the charger. Thanks

Sorry, just seen your question! Yes, the AMT is wired into the 3 terminals of the Orion with one of them via a switch so I can isolate if needed. If you don’t have room the AMT cables can intercept anywhere, as long as on is on the cable to the SB, the other from the LB and a negative to the chassis or any neg.