Hi, in the RV camper I have 2 115Ah AGM batteries in parallel.
I would like to replace the cables with 35mmq and install the smart shunt.
Currently 4 cables depart from the batteries (2 positive and 2 negative) after 1.5 meters of cable the positives enter fuses and then are connected to the pin of the battery release creating the parallel.
I would like to protect the entire section with 2 MRBF fuses connected directly to the batteries.
Since the batteries are not easily accessible, can the 2 fuses be left at 1.5 meters replacing them with 2 mega of the Victron even if they are in series with MRBFs but accessible and from less Ampere?
Is it correct to make the parallel like this compared to doing it directly in the batteries and then having a stretch with only 2 1.5 meter cables and then a single mega fuse?
Thank you.
Well, the reason for a fuse is to protect the cable and the battery.
Just imagine what will happen, if there is a short circuit on the cables before the fuse.
Then you have very high currents unfused that could lead to cable fire, exploding batteries etc…
So the fuse has to be as close as possbile to the battery pole.
I know, most people would argue, that such a short cicuit is impossible, but you never know.
The cables may chafe and the isolation might get broken, or even a crash of your RV could harm the cables and lead to much bigger trouble then the crash itself. 
Hard to answer if the amps rating is ok without knowing what your loads are. (It can theoretically be max amps ÷ 2)
You do need to fuse right by the battery if possible.
Hello, exactly, I add the MRBFs to protect the entire stretch from the battery connector but I wanted to understand, not being very accessible if to insert in series at a distance of 1.5 meters another mega fuse victron (2, one for positive) with a lower ampere value to be more easily reached and verifiable in case of downstream failure..
Now originally the manufacturer has installed 2 50A fuses (one for each positive) and I was thinking of replacing them with the victron mega which is 60A minimum and instead the 80/100A MRBF so that if there are failures they jump the mega first.
I think the maximum absorption now can be 30/40A peak but I will measure it when I install the smart shunt.
Can it be corrected?
And can installing these 2 mega in series to mrbf for greater accessibility, is correct as a method?
Thank you.
The megas are now larger than the MRBF so are likely to blow after.
What type of fuse is in the MRBF? Slow blow fast blow? Megas are slow blow.
Yes, I would install the MRBF with a larger amp for a later intervention only if severe and I still have to buy them, do you have any advice on the fuse holder and the MRBF fuse?
I was looking at the most famous brands, but they all have a hole for M10 or rather from a design of about 9.5 mm, but the AGM batteries are M8 (hole and screw M8), do you use a washer? Only above?
In any case, is the use of double fuses allowed for accessibility and subsequent parallel and correct practice?
Thank you
No. Sorry. Not much personal experience with them.
For the most part in installations it is wise to install at current sources. Extra fusing or disconnects in accessible places makes working on the system much safer (with less grunting and choice words)
Thank you, so I install the MRBF on the battery poles then 1.5 meters of cable, 2 mega fuses in a more comfortable and inspectible place and then parallel on the pin of the battery release correct like this?
For the length of cables both perfectly the same, the negatives are the same but in addition to the positive there is the mega fuse, negligible?
The negative and positive don’t need to be the same. But all the battery cables need to be the same length on each.
Thank you. After taking accurate measurements, the negative cables would each need to be 2 meters long to reach the input of the Victron SmartShunt. Is this a problem, or does the length not matter as long as there are no loads connected before the shunt? Are 2 meters too much?
The distance will not matter for the SmartShunt. You are correct, no loads before the shunt.
Ideally the shunt is right by the battery, since it is supposed to measure battery voltage and current.
But sometimes needs must. As long as you are confident (as in you have checked thoroughly with a meter) there is no voltage drop under load or rise under charge over the cable which would influence the end result then you are ok installing it there. (This is important otherwise you will end up with undercharged or over discharged batteries.)
So the only problem that could be there is only as an incorrect reading but at the level of electric circuit operation everything is ok right?
They are double 35mmq cables I think the voltage and resistance drop is minimal in 2 meters for the absorption I will have.
The issues will arise in high current movements. Whether in or out.
It mattered more for lead acid. With lithiums a little less so.
I should have explained better above. The extra wire length will not affect the current measurement and SOC as current is constant throughout a length of cable. It will affect the measured battery voltage, more so at higher current. If the shunt is used in DVCC it will not compensate for voltage drop as well as if the shunt is next to the battery.
Ok, MRBF fuses have a hole for M10, someone used them on M8 I read about using a washer, because my AGM batteries have M8 hole and screw for M8 ring terminal.
In case I want to change the type of fuse, which one do you recommend to protect the batteries very close?
The batteries are in a compartment as I said not very comfortable to be reached and when they have to be removed they have to be pulled out and therefore the cables are about 1 meter long.
I read that the fuses must be fixed and for this I had chosen the mrbf.
Thank you