battery and system fusing size question

Firstascent

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Hey all, looking for some input on a couple items. I attached a quick rough diagram to help show a portion of my setup I'm working on and where my fusing questions are.

I'll have a Lynx Shunt with a Distributor on each end, the battery side and the load side. the fuse for the Lynx Shunt takes a CNN fuse and Victron calls this the System fuse. makes sense because all the loads go through this location.

Victron recommends a 400A fuse on the multiplus II for the DC connection so that's what I'll use there. The BatteryProtect is a 220A model so I'll go with a 250A fuse there. I have the 150/100 MPPT so I chose a 125A fuse there. Thats the gist of it for my load side..
On the BatteryProtect, I don't expect to be running 220A continuous. This is on my RV which is mostly LED lights and smaller items, the only larger DC loads are my leveling system (which wouldn't be used while everything else is running) and my Generator start which is only a momentary load up to 95amps while it's starting.

I'm looking for some input on fuse size for the Lynx Shunt "system fuse" as well as my individual battery fuses. I'm thinking whatever my Lynx Shunt fuse is, should my batteries each be half of that? They are the 280aH EVE cells. each battery will be a 12v 560aH battery config, and I'll have two of those in parallel but they'll each have their own fuse.
 

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HRTKD

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What is your BMS rated for? If the BMS is rated for only 120 amps, you don't need more than about a 150 amp fuse for each battery.

A 125 amp circuit breaker may work out better on the output side of the solar charge controller. If you need to work on the PV side of the system you may want to have the battery power to the solar charge controller off also.
 

smoothJoey

mumble...
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As I said before when you first floated this idea in another thread, the battery fuses should be directly off the battery positive terminal of each battery and should absolutely come before the disconnect switchs.
You should be using a Victron power-in instead of a lynx distributor.
 

Firstascent

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What is your BMS rated for? If the BMS is rated for only 120 amps, you don't need more than about a 150 amp fuse for each battery.

A 125 amp circuit breaker may work out better on the output side of the solar charge controller. If you need to work on the PV side of the system you may want to have the battery power to the solar charge controller off also.
I have the Batrium BMS so it doesn't care since the loads don't go through the BMS directly. So in my case the BMS isn't a factor on fuse size.

and yes, I'll actually have a circuit breaker between PV and MPPT as well as between MPPT and DC distribution, not a fuse :)
As I said before when you first floated this idea in another thread, the battery fuses should be directly off the battery positive terminal of each battery and should absolutely come before the disconnect switchs.
You should be using a Victron power-in instead of a lynx distributor.
Hi Joey, yes the diagram isn't an exact representation and changes are still being made and I'll probably do something as you just mentioned (like you told me before).

But that wouldn't change the fuse size in either scenario
 

Firstascent

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If I have two fuses, one for each "battery", would I want them to both add up to my "system" fuse?

just as an example, 300A fuse on each battery which would be in parallel so a total of 600A. So should my system fuse be at or slightly less than that, say 550A?
generally speaking that is.
 

smoothJoey

mumble...
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If I have two fuses, one for each "battery", would I want them to both add up to my "system" fuse?

just as an example, 300A fuse on each battery which would be in parallel so a total of 600A. So should my system fuse be at or slightly less than that, say 550A?
generally speaking that is.
Where does this "system" fuse fit in your topology?
Why is it needed?
If the charge sources are fused at source and the loads are fused at the busbar what else is required?
 
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mrzed001

Voice of reason
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Hey all, looking for some input on a couple items. I attached a quick rough diagram to help show a portion of my setup I'm working on and where my fusing questions are.

I'll have a Lynx Shunt with a Distributor on each end, the battery side and the load side. the fuse for the Lynx Shunt takes a CNN fuse and Victron calls this the System fuse. makes sense because all the loads go through this location.

Victron recommends a 400A fuse on the multiplus II for the DC connection so that's what I'll use there. The BatteryProtect is a 220A model so I'll go with a 250A fuse there. I have the 150/100 MPPT so I chose a 125A fuse there. Thats the gist of it for my load side..
On the BatteryProtect, I don't expect to be running 220A continuous. This is on my RV which is mostly LED lights and smaller items, the only larger DC loads are my leveling system (which wouldn't be used while everything else is running) and my Generator start which is only a momentary load up to 95amps while it's starting.

I'm looking for some input on fuse size for the Lynx Shunt "system fuse" as well as my individual battery fuses. I'm thinking whatever my Lynx Shunt fuse is, should my batteries each be half of that? They are the 280aH EVE cells. each battery will be a 12v 560aH battery config, and I'll have two of those in parallel but they'll each have their own fuse.

Here is how I design fusing:
https://diysolarforum.com/threads/4-0-cables-250anl-fuse-heating.27153/post-323078
 

Firstascent

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Where does this "system" fuse fit in your topology?
Why is it needed?
If the charge sources are fused at source and the loads are fused at the busbar what else is required?
Technically the system fuse is not needed since everything else is fused, that’s correct. But the Lynx shunt requires a fuse and everything goes through the Lynx shunt. The Lynx shunt is rated at 1000A for what it’s worth.

in my diagram you’ll see my note on the CNN fuse next to the Lynx shunt, that’s what that is referring to
 

smoothJoey

mumble...
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Technically the system fuse is not needed since everything else is fused, that’s correct. But the Lynx shunt requires a fuse and everything goes through the Lynx shunt. The Lynx shunt is rated at 1000A for what it’s worth.

in my diagram you’ll see my note on the CNN fuse next to the Lynx shunt, that’s what that is referring to
Ok understood, the battery fuses should be...
$max_continuous_current / .8 fuse_headroom = $fault_current

If each battery+bms can handle the full load they should be wired and fused for the full load.
That way you can run the full load with only one battery in the case that the other is offline.
If each battery+bms can't handle the full load they should be wired and fused to...

$bms_max_continous_rating / .8 fuse_headrom = $fault_current
 

smoothJoey

mumble...
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Since the shunt and both busbars are rated for 1000 amps I would put a big honkin fuse in there.
Its not really protecting anything and usually the higher the fuse rating the lower the resistance.
 

Firstascent

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Ok understood, the battery fuses should be...
$max_continuous_current / .8 fuse_headroom = $fault_current

If each battery+bms can handle the full load they should be wired and fused for the full load.
That way you can run the full load with only one battery in the case that the other is offline.
If each battery+bms can't handle the full load they should be wired and fused to...

$bms_max_continous_rating / .8 fuse_headrom = $fault_current
I like it, somewhat along the lines I was thinking but I like your formula. Thanks!
 

Firstascent

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Since the shunt and both busbars are rated for 1000 amps I would put a big honkin fuse in there.
Its not really protecting anything and usually the higher the fuse rating the lower the resistance.
*searches big honkin fuse* bam! Haha thanks, and makes sense. I’ll just get the biggest CNN fuse available as close to 1000A
 

mrzed001

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Thanks that’s some good info in that single post. I won’t be fusing my negative battery lead, only positive, but great and helpful info, thanks for sharing that!

If you use it on an RV and if you will ground the negative line to chassis ... then I suggest fuses on both; in the battery side.
Remember the RV fire
If your chassis comes in contact with the + side (a wrench, a (car)accident ... ) before the fuse ... firework.
But if you have a fuse on negative too, then fuse blows, no firework.
 

Firstascent

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In a car accident ?
yes. pull over, remove cable, proceed with accident. problem solved. 😁

This is an area I need to look into further. Right or Wrong, and for whatever reason, it seems it is NOT common practice to ground the batteries to the chassis in RV's. This is just based on random searches while looking into what others have done on their builds.

But if you look at any typical vehicle the manufacturers still ground the battery to the chassis. Not as a return path though as each device/component still has it's individual negative return/wire back to the battery.

I haven't decided yet if I will have a floating ground system or not.
 

time2roll

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In a car accident ?
If you are performing battery service I would also advise pulling off the roadway to a safe place.

ALL RVs that I am aware have the house battery negative bonded to the frame with no fuse. All of them.
 

mrzed001

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yes. pull over, remove cable, proceed with accident. problem solved. 😁

I would like to record that for youtube :LOL:

This is an area I need to look into further. Right or Wrong, and for whatever reason, it seems it is NOT common practice to ground the batteries to the chassis in RV's. This is just based on random searches while looking into what others have done on their builds.

But if you look at any typical vehicle the manufacturers still ground the battery to the chassis. Not as a return path though as each device/component still has it's individual negative return/wire back to the battery.

I haven't decided yet if I will have a floating ground system or not.

You know what is the reason for grounding? To have a path for error. So you do not get a shock.
That is why you have a fuse box. In any line going from there has a short that fuse will blow (literally that is its job).
BUT it only protects from the place it is on the + line.
If your chassis has a direct contact with any of your cables (or battery main) BEFORE the fuses .... thats a bummer
 

Firstascent

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If you are performing battery service I would also advise pulling off the roadway to a safe place.

ALL RVs that I am aware have the house battery negative bonded to the frame with no fuse. All of them.
I should clarify an important detail :) I agree with your statement on the house battery negative bonded to the frame in RV's from the factory. I meant from aftermarket/self installed builds where people have upgraded to solar and lithium batteries. Could be oversight, no knowledge of it, or 1000 other reasons. But most builds I've seen didn't have the negative attached to the frame in any way.

This is more meant of just an interesting note I've seen, not any type of recommendation or advice for anyone that see's this :)

I would like to record that for youtube :LOL:



You know what is the reason for grounding? To have a path for error. So you do not get a shock.
That is why you have a fuse box. In any line going from there has a short that fuse will blow (literally that is its job).
BUT it only protects from the place it is on the + line.
If your chassis has a direct contact with any of your cables (or battery main) BEFORE the fuses .... thats a bummer
ha, that'd be good one. I'll let you know if I ever get the opportunity, or will, to do so. Someone, somewhere, would think that actually makes sense in this world hahaha

grounding aside, my latest design, which is not shown in my original image in post 1, is that each "battery" (2p4s) will have a Class T fuse basically directly attached to each main battery positive. Not quite Directly attached to the terminal, I'm still designing the specifics, but I'm thinking attaching it to a short bus bar or similar. basically it will be within a few inches of main battery positive.
I already have the Lynx distributor, so although not cost effective. I'll be using my first distributor as a Lynx-In and not using a separate fuse for my battery paralleling.

since my fuse will essentially be attached to battery positive terminal it makes it more highly unlikely something would come in contact with with chassis and any wire BEFORE my fuse.

With all that being said, it still doesn't mean I won't attach a chassis ground.

[edit] I just saw that thread with the bus fire, wow! good thing nothing majorly bad happened. I made my decision before seeing this but just that much more of a reason to do what is recommended on installing the fuse as close to main battery terminal as possible
 
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