I have LiFePo4 battery and 3500W inverter, do I need to use fuses?

Hi,
I intend to buy a LiTime 200-300Ah LiFePo4 and I have a 3500W inverter.
if BMS can handle over discharge, short circuits, why do I have to use any fuse between battery and inverter?
My use of this setup is for regular appliances at home, peak would by the use of my vacuum cleaner (2600W surge).
I have several smaller LiFePo4 batteries to cover my few days spent in my off-grid location.

thank you

Not all about the batteries or the inverter… think about the wire and problems that could come from them if they are over taxed.

Short answer yes, if it was me. Protect the wires, buses, connectors, lugs from shorts and other unforeseen problems.

JohnnyBravo said:

if BMS can handle over discharge, short circuits, why do I have to use any fuse between battery and inverter?

Click to expand...

The BMS is a semiconductor switch when in protection mode. Any BMS switch failure usually results in an internal SC and allows maximum current flow , ( In the region of several thousand amps), from the cells, to flow through the cables to your equipment. This has happened. Read the section on this forum, ' up in smoke...'

JohnnyBravo said:

Hi,
I intend to buy a LiTime 200-300Ah LiFePo4 and I have a 3500W inverter.
if BMS can handle over discharge, short circuits, why do I have to use any fuse between battery and inverter?
My use of this setup is for regular appliances at home, peak would by the use of my vacuum cleaner (2600W surge).
I have several smaller LiFePo4 batteries to cover my few days spent in my off-grid location.

thank you

Click to expand...

Cars have brakes, why do we need seatbelts, airbags and crumple zones? The car can be stopped before it runs into things using the brakes.

Under normal circumstances, the BMS will take care of itself. When it doesn't, hopefully you have a tclass fuse or high AIC DC breaker installed.

They told you didn't they

So, MRBF 25% larger than the discharge current of each battery, even if that is only one. And a class T near the bus bar or inverter that is 25% over max expected current.

Bms tend to become a dead short when they fail. Inverters tend to fail shorted when they die. Either one will pull a tremendous amount of current.

You need a fuse with high AIC to kill any arc...yes that can happen even at 12v... welding is usually at lower than that.

The MRBF has 10ka AIC at 12v and the class T has 50ka at 12v.

If you are wondering why fuse both ends of the same wire, if it is longer than a foot or two what happens if something cuts the insulation and causes a short? At high amperage you can have a red hot wire with melting insulation melting off in under a second before a fuse blows.

ok, so I should use a fuse and to not rely on BMS.
As the fuse is supposed to protect the wire, why do you go for class T fuse that can sustain (as I have read) 10k-20k amps?
Why not picking a fuse that is matching the max surge (+ margin) you have from your appliances?

Perhaps another option is to purchase a battery with integrated breaker.

JohnnyBravo said:

ok, so I should use a fuse and to not rely on BMS.

Click to expand...

Enjoy the BMS when it works. Have a fuse for if it fails.

JohnnyBravo said:

As the fuse is supposed to protect the wire, why do you go for class T fuse that can sustain (as I have read) 10k-20k amps?

Click to expand...

You can overwhelm fuses/breakers with too much current to where it just arcs across it. LFP batteries are very low resistance and can produce many thousands of amps in a dead short. A fuse with insufficient AIC rating will not stop the current flow.

JohnnyBravo said:

Why not picking a fuse that is matching the max surge (+ margin) you have from your appliances?

Click to expand...

the 10-20K amps is the AIC rating. You want to size the fuse appropriate to the wire.

I can find Victron Mega fuse rated as 300A / 32V, this 32V what it means? does it fit my 12V system?
Can you also tell what AWG I should use for the battery to inverter wires for my 3500W inverter (7000W peak)? My overall consumption is not bigger than 2800-3000W as surge (when I start my vacuum cleaner).
Many thanks

JohnnyBravo said:

I can find Victron Mega fuse rated as 300A / 32V, this 32V what it means? does it fit my 12V system?
Can you also tell what AWG I should use for the battery to inverter wires for my 3500W inverter (7000W peak)? My overall consumption is not bigger than 2800-3000W as surge (when I start my vacuum cleaner).
Many thanks

Click to expand...

Here its all explain

Thanks for the video! Always nice to have a visual!

(3500w/12v)*1.2% = 350amps

This is the size you need to aim for wire wire between the battery and the inverter. You are looking at 4/0 of 105c welding wire. This is a snip from windy nation. They can make the cables for you if you can't or don't want to crimp them yourself. At this amperage you want it perfect since any tiny little amount of resistance will cause large amounts of heat. Just measure carefully. If you go with standard 75c 4/0 from the hardware store you would need 2 runs and it is hell to bend.




When picking fuses you want one with the lowest resistance and the best ability squash a short. Were it me I would probably use class T at the battery and the inverter. They all produce heat but not all of them contain sparks when they blow at high amps.

Also for holders the ignition protected ones from Blue Sea Systems/Eaton are a good idea. Easy latches to get at them, well designed and can take the heat.

The high amperage is why many people go with 24v or 48v. I reduces component size and amperage by 1/2 or 3/4. If you haven't ordered anything you may want to consider it.

well, I jumped into this inverter 12V/7000W (3700W continuous use) because I already had 12V / 100Ah and 12V / 80Ah LiFePo4 batteries.
I know the inverter is oversized for my daily use so I was thinking to get 300A Blue Sea System Class T fuse to match my battery type as LiFePo4 and to match my peak consumption that shall not exceed 3000-3200W (2800W vacuum cleaner start consumption + Starlink router + fridge + laptops/TV/phone chargers and some bulb lights).
As my current batteries (+ one 1300 EcoFlow) could not support my weekend stay on my off-grid location, I want to buy 200-300Ah Litime LiFePo4 that I will run with the inverter I have (so far I was using my EcoFlow for 220V output and my LiFePo4 batteries to load the EcoFlow).

You can downsize your fuse to 300amps if you want to at the risk of randomly blowing it.

The class T fuse current/time curve lets it run almost 10 minutes at double the current. I would match the wire to the invert and just downsize the fuse if that is what you are after. This way if you ever want to go back to the 350a fuse you can without replacing wire.

BUT - if you already have everything wired up and are just adding a 3rd battery in parallel the existing wiring and fuses should be fine, assuming you have everything scaled to the inverter you have.

no, I have just the inverter and its provided cables, 2 red and 2 black wires AWG 6 or 4, the seller told me I can connect them all to the battery, I don't know if you can scale up to fit the required amps resistance by using multiple cable wires :D

About the risk to blow the fuse, it should be low since my peak consumption is less than 3000-3200W.

Regarding the proper sizing of the battery cables to sustain 300A, I found 4/0 AWG on amazon on windynation, I can't find thicker than that..

Windy Nation is good stuff.

Their 2/0 welding wire is good to 325amps - 105c rates. So if you fuse at 300 and wire at 2/0 you should be good. Just get a spare fuse when you buy the first so you won't be tempted to bypass it

Their 4/0 is good to 440amps and would be a better choice if the run is long. Probably worth pricing the difference between it and the 2/0.

Once you get past 4/0 the wire is crazy thick and hard to bend. Usually measured as kcmil or MCM. IF you ever think you are getting to that in the DIY space it is time to up the voltage or split the install

From different tables I saw that for 300A I need 4/0 wire or even thicker, indeed Windy Nation ampacity chart show different figures.

About their cables, is there a difference between this

and this


or it's the same cable?

Different tables are for different types of wire sheath.

See this in my answer page

Battery cable is typically more corse than welding wire. The sheath is typically oil resistant.