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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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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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