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Tom Ranson avatar image
Tom Ranson asked

Battery fusing recommendations

Hi all,

Refering to the diagonal parallel wiring diagram below. I have 4x UltraMax 100Ah 24V LiFePo4 batteries with their own internal BMS's ("drop in" type batteries which do not speak Victron) which have replaced a flooded lead acid bank.

My question is, for this new LiFePo4 bank, should I install fuses in-between each individual LiFePo4 battery on their positive leg? I have been unable to clarify if this is a requirement or not, or if it's safe to rely on the internal BMS for overload protection etc. I have a fuse on the main positive leg coming from the bank going to the load.


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Lithium Batteryfuses
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3 Answers
Alexandra avatar image
Alexandra answered ·

@Tom Ranson

Call me a total victron fan, but you can use the lynx system and individually fuse onto a bus bar.

There is even a mod for the lynx power in to take the fuses if you dont want to use the electronic mod to power up the lynx distributor without the lynx bms.

That is the ultimate best way.

As for it being a requirement, usually fusing is for wiring, not equipment protection. So in that department you will be fine to use one main disconnect for the bank.

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Tom Ranson avatar image Tom Ranson commented ·

Thanks @Alexandra. Unfortunately, I'm not going to be going down the Lynx road, as much as one might like to.

What would be the advantage to fusing each individual battery in my scenario? Bearing in mind I'm planning to use the diagonal wiring and not busbars.

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Alexandra avatar image Alexandra ♦ Tom Ranson commented ·

@Tom Ranson

Not too much of a stress.

In between batteries (unless regulations require) it is not necessary. Fusing is for wiring not so much equipment protection. The bms takes care of the battery as you said.

There is a decent product that acts a a rapid disconnect and fusing, look up the mersen fuse disconnect. There is quite a range of them for different amperages needed.

What is the bank powering? What size wiring as well? Make aure your links are just as beefy, seen may banks let down by the interlinks.

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Alexandra avatar image Alexandra ♦ Alexandra ♦ commented ·

Too many protections can actually cause issues as well. ... Too much of a good thing.... Not so good.

Extra resistance etc also being a factor in your proposed wiring setup.

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Tom Ranson avatar image Tom Ranson Alexandra ♦ commented ·
Understood. Just would rather be safe than sorry.
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Tom Ranson avatar image Tom Ranson Alexandra ♦ commented ·
It's powering an MP ii 3000VA. Using 50mm2 wire and fused to 300A between the battery and busbar, and busbar and MP ii. There are also MPPT chargers appropriately fused to the busbar.
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Alexandra avatar image Alexandra ♦ Tom Ranson commented ·
are you using 2x50mm² - neg and pos? Half the fuse rating then for each one ise the mersen or something like it with 125/150A on each cable.

If you are only using 1 then the voltage drop will be crippling for the inverter.

The batteries protect themsleves, the wiring cannot, hence the fusing.


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Alexandra avatar image Alexandra ♦ Alexandra ♦ commented ·
It is better to individually bus bar if you want to individually fuse, the extra resistance between the bank with the way you propose to wire it will mean the end batteries will work harder than the inner ones and are more likely to have issues.


Ohms law is a major factor here, then you get current stacking on the terminals, check your batteries can actually handle that and are rated to run that many in your setup.

When you do finally set up, tong test between batteries under a large load and you will see what I am saying. Then think about what will happen if the first on in the series switches off due to over current.

3000va ÷ 12v is 125A, it is alot of current to handle if it does go wrong.

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Tom Ranson avatar image Tom Ranson Alexandra ♦ commented ·
Understood that individually busbaring makes the most sense (and makes individually fusing easier), however I was hoping to get away with a diagonal setup. Do you think this is actually a bad idea?


I actually only cap the inverter output at 1000W, and it's a 24V version. Everything is 24V throughout.


Understood re. current stacking on the battery terminals at the ends of the diagonal wiring.

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Tom Ranson avatar image Tom Ranson Alexandra ♦ commented ·
@Alexandra I'm using 50mm2 throughout for negative and positive with 300A resettable fuses- one between the battery bank and positive busbar, and then another 300 between the positive busbar and the MP ii. I was following the wire sizing and fusing guide for the MP ii to make these decisions.
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klim8skeptic avatar image
klim8skeptic answered ·

Another thing to consider, is if the batteries BMS has overcurrent protection already.

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Tom Ranson avatar image Tom Ranson commented ·
@klim8skeptic yes they do, so is fusing them also even necessary? This is what I'm struggling with.
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klim8skeptic avatar image
klim8skeptic answered ·

@Tom Ranson My thoughts are that if the batteries BMS has overcurrent protection, that is sufficient, and that any extra fusing to combine the batteries may not give any extra protection. Do confirm with Ultramax re overcurrent protection in the BMS.

batteries-in-parallel.png

Proper paralleling of the batteries is essential. Diagonal is good for 3 batteries, only OK for 4 batteries. Wiring Unlimited gives 4 options to parallel 4 batteries. The "Halfway" method gives correct current balancing, with the only draw back of having 2 different battery interconnecting cable lengths. Smartguage goes into detail regarding battery paralleling, well worth the 15 min read.

Wiring Unlimited is a good source of info, essential reading.

Fusing for a 12v high current system is costly. Dont cheap out. A proper 300a circuit breaker will cost half a weeks wage. Decent fuses and fuse holders come in much cheaper.

Above all KISS. Every inch of cable, and all the "stuff" between your batts and inverter will introduce voltage drop. Less is better in this case.

Remember, fuses are there to protect cabling.


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Tom Ranson avatar image Tom Ranson commented ·

@klim8skeptic many thanks for making such a detailed response, it is highly appreciated.


I have read some portions of Wiring Unlimited and have indeed found it useful.


For the avoidance of any doubt and to try and do it "the best way possible", I have decided to wire the 4x batteries using the busbar method and I am going to individually fuse them and install individual isolators. It will help me sleep at night, even though given the BMS protection, it may not strictly be necessary.

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