DC MCB vs Megafuse

[mfred68]

I am in the planning stage to build a victron off grid system. I have 6x 7kwh 48v 150Ah Lifepo battery banks which i want to connect it parallel.
I want to install a protection onto each battery before wireing them in parallel onto a busbar.

I an wondering what is acceptable, a megafuse or a resettable DC MCB breaker? I only need 63A types as the current should never rise above this per battery when used with a Multiplus 48/10000 140

I would prefer to use 2pole 63A DC MCB's 1 per battery, so is this possible, as I have only ever seen wire type fuses used such as mega fuses.

 

[scrubjaysnest]

Fuse or breaker should be 1.25 times Ah rating of battery to protect wires.
1.25*150=187.5. Round up to 200 amp fuse or breaker for proper protection.

 

[mfred68]

Fuse or breaker should be 1.25 times Ah rating of battery to protect wires.
1.25*150=187.5. Round up to 200 amp fuse or breaker for proper protection.

Surely a lower amp fuse is more protective than a higher amp fuse? My battery bms has a maximum discharge current of 100a so Surely a lower amp fuse can be used, as I don't intend to ever go anywhere near 100a current draw.

 

[mikefitz]

what is acceptable, a megafuse or a resettable DC MCB breaker?

You need a class T fuse, or since you are in the UK HRC fuses are easier to obtain, ( fuse has to withstand very high fault current in the region of several thousand amps)
Example,

https://uk.farnell.com/eaton-bussmann-series/100let/fuse-hrc-high-speed-100a/dp/1202322

Since its possible you won't get equal share of current and its also possible one or more batteries could be disabled by its BMS, a higher current than 63 amps may be taken from the battery. Suggest fusing at the battery limit or slightly over. 200 amps has been suggested. Ensure the cables to the positive bussbar from each battery can withstand this current, example, 35mm2

 

[edriz]

Surely a lower amp fuse is more protective than a higher amp fuse? My battery bms has a maximum discharge current of 100a so Surely a lower amp fuse can be used, as I don't intend to ever go anywhere near 100a current draw.

What if you're powering a machine that requires surge, won't the problem of the breaker shutting down?

 

[robbob2112]

Size cable to expected max current plus 20%. This can be max ah of battery or max possible current used. Size fuse as 25% over max current.

You will read it many times here.... fuses protect wire, they don't protect equipment.

Now, Victron they spell out the wire and fuse sizes in the manual. The mfg recommendations are what you should follow for the connection between the inverter and the bus bars. This fuse should be class T. Size recommended is 400amps and 70mm^2 (2/0 or 3/0) cable ( this should be high strand count like welding wire rated 105c)

Between each battery and the bus bar I highly recommend putting a fuse attached to the bus bar for each battery cable. These can be class T or MRBF fuses and holders or dc breakers as long as the AIC rating is 10k or higher. They should be directly attached to the bus bar. Windy nation 2 awg welding wire will carry 205amps. The fuse can be sized 175amps or as much as 225. The variation is because the batteries max out at 150amps.


This protects from a short on any battery cable by tripping for the affected cable. It also protects the other batteries if one decides to dump all current from a BMS issue. And by having the right sized cables and fuses they should never blow unless there is really a problem. And they WILL blow to protect the wires from melting.


I've done the math before with 4 batteries in parallel and a short halfway from the bus bar to the battery melts the sheath off the cable in 0.1 seconds and in 1 full second the cable is at over 900f. In 30 seconds the cable melts in two.


You don't mention your battery type or brand. Most of the larger ones have a breaker built into them to use as a switch to turn off.

P.s. all the cheap mega fuses, ANN, ANL and others do not interupt a DC arc and can start a fire when they melt.