On battery fuse - advice please?

berni109

berni109

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I have very little space around my lithium battery and have to make some adjustments to the current wiring to install a small inverter.
The maximum discharge rate of the battery is 100amps.
1 of these
s-l1600~2.jpg attached directly to the battery would cut down on the number of wires I would need to fit into the space I have. BUT the fuses to fit all seem to be rated at 58volts - so what amperage fuse would I need ??????

Another question again to reduce the number of wires - the inverter will be 50 cm from the battery and it has an on/off switch so is another switch necessary at the battery end of the 50cm wire.

thanks if you can help with this.
 
Often the voltage specified on fuses is the maximum voltage they can handle, for example if you check the specs of the Blue Seas MRBF fuses, it stipulates that the max voltage is 58v: https://www.bluesea.com/products/5191/MRBF_Terminal_Fuse_Block_-_30_to_300A

It’s not essential to have an isolator switch between your inverter and battery. I almost always install an isolator, but as long as you have an appropriately sized fuse for the wire, it’s not essential to have an isolator switch.
 
I wouldn't have a single fuse far better to have a main battery fuse then seperate smaller fuses for inverter, distribution unit, B2B, solar etc.
Then if you get a fault that takes out a fuse you don't lose the whole system.
Can't give advise on fuses unless you give us details loads.

These are useful and save space.

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I've got a midi fuse holder distribution box. Means I can have 100amps as a main fuse to the battery, then distribute out to my solar, B2B, mains charger and the loads on their own fuses. The negative side is on a busbar to make more room.

Edit: exactly what Lenny suggested.
 
berni109 said:
BUT the fuses to fit all seem to be rated at 58volts - so what amperage fuse would I need ??????
Click to expand...
Most automotive fuses are rated for up to 32V, so they are suitable for 12V and 24V batteries in cars and trucks, but not for 36V and 48V systems as found in domestic battery storage systems and some high-power vehicle and marine applications. A fuse rated up to 58V is suitable for all these systems, for 12V, 24V, 36V and 48V systems.

The amps value of the fuse depends on several factors. Mostly the maximum likely amps of the loads connected to the battery, but also the wire amps capacity. So more info on your system would be needed to give you an amps figure.

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I thought I understood fuses but obviously not.
The maximum output from the battery is 100amps which I assume the BMS will control but I want a fuse that disconnects before it gets to 100 amps discharge.
The inverter is mainly to run a low power hairdryer occasionally 1000w divided by 12v = 83 plus 5% say 90amps
The wiring will be in 25mm copper so more than enough capacity and future proof of another battery added in future.

Just watched a YouTube of guy testing fuses and quite frighting how much excess amps it took for them to blow (victron were the worst over twice their rating before they blu).
 
Last edited:
berni109 said:
I thought I understood fuses but obviously not.
The maximum output from the battery is 100amps which I assume the BMS will control but I want a fuse that disconnects before it gets to 100 amps discharge.
The inverter is mainly to run a low power hairdryer occasionally 1000w divided by 12v = 83 plus 5% say 90amps
The wiring will be in 25mm copper so more than enough capacity and future proof of another battery added in future.

Just watched a YouTube of guy testing fuses and quite frighting how much excess amps it took for them to blow (victron were the worst over twice their rating before they blu).
Click to expand...
The max discharge for your battery will be 100 amps continuous, peak discharge will probably be well over 150 amps.

The fuse is not there to protect the battery it is to protect the cable so in the event of a fault the fuse cuts the supply before the cable gets too hot and starts a fire.

Diffent types of fuses have diffent rupture curves. A 100 amp fuse could blow in anything from milliseconds to 10 seconds at twice the rated current depending on the type of fuse and if over current by say 20% the blow time would be a lot longer.
 
Have I got this right?
Fuses blow when the heat in the fuse melts the connection caused by the resistance in that connection.
The size of the connection in the fuse is determined by the voltage and watts. So 1000w / 12v = 83.3 amps. BUT for my requirement 1000w / 52v = 19.2 amps ?
 
Two things you are missing what you are missing is the type of metal used in the fuse determines at what temperature the fuse will blow at.
The voltage will be a fraction of a volt 12v has nothing to do with it, the voltage is the volt drop across the fuse which will vary with the current flowing through it.

Nope you don't understand fues. :LOL:
 
berni109 said:
Just watched a YouTube of guy testing fuses and quite frighting how much excess amps it took for them to blow (victron were the worst over twice their rating before they blu).
Click to expand...
A 100A fuse is designed to be able to pass 100A of current through it all day without blowing. It will probably get a bit warm, but not very hot. The idea is that if there is a fault and the amps suddenly increases to a high value, say 200A or more, then the increased current will heat up the fuse. As it heats up, its resistance increases, so it will get even hotter. It's a kind of feedback effect, so that above a certain current the fuse will blow.

A 100A fuse has a resistance of about one milliohm, that's a thousandth of an ohm. At 100A, the power produced in the fuse is about 10W, which will make it warm but not hot. At 200A, the power produced in the fuse is about 40W, which is enough to make it get hot. When it gets hot, its resistance increases, so the power produced in the fuse will also increase, which makes it even hotter. This increase continues until the fuse melts, and the circuit is broken.

The voltage is not relevant to when a fuse melts, it is important only after it melts. When the fuse melts, there will be a spark. If the two ends of the fuse are too close together, the spark can continue passing electric current through the air across the gap. So it is important that the gap is big enough. The higher the battery voltage, the bigger the gap that the spark can jump. So a 58V rated fuse has a bigger gap so it can stop a spark from a 48V battery.

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Lenny HB are you describing the difference between quick and slow blow fuses and what are the technical terms I should look up.
 
I think I am starting to understand with the help of your comments. Thanks Lenny HB and autorouter for you patience and explanation.
Can you suggest anywhere I can find the technical specification for fuses so I know which ones to buy.
 
berni109 said:
Lenny HB are you describing the difference between quick and slow blow fuses and what are the technical terms I should look up.
Click to expand...
No I didn't mention them just pointing out using different materials effects the function of the fuse.

Quick blow, time lag (antisurge) & slow blow tend to be used mainly in electronics & consuler /industrial electricals.

The link in the post above gives good info on automotive fuses which is what I thought we were talking about.
 
Some electronic devices like radio speaker outputs are very sensitive to overcurrent, so quick-blow fuses are used which are extra-sensitive to overcurrent and will blow very quickly to protect the electronics. On the other hand, some devices like a fridge compressor motor or an inverter capacitor will take a long surge of current as it first switches on, then settles down to its normal value. A slow-blow fuse will avoid blowing during the switchon surge, but will blow if there is a sustained fault.

Then there's the ordinary standard fuse that is somewhere in between. So the choice of type depends on what device you are trying to protect.

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Can either of you confirm I am reading this graph correctly please.
terminal_fuse_delay_2.jpg

An 80amp fuse should work forever at 80amps before it blows.
At about 80 + 40 (+50%)= 120amps it should work for (say) 20'ish seconds before it blows.
At about 160 amps (+100%) it should blow in a little over a second.
All of this irrespective of vehicle battery voltage.

It's taken a wet afternoon at drax to get my head around this. Maybe it was all that talk about megawatts from the power station tour.;)
 
Yes, that sounds about right. Another thing that is specified, if you are going into the details of fuses, is the maximum current that it will stop. That is usually several thousand amps, and is not really something that is a problem. It's called the Rupture Capacity.
 

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