Battery Isolator switch question

[steve yates]

What is the minimum continous current rating I should be looking at for battery switches to
a) isolate 2 314ahr lithiums in a house bank, wired independently with 25mm cable each battery has a 150a classt fuse.
(They will each have their own isolator, usually left always on, with the bank turned on and off via this main isolator)
B) isolate a 90ahr agm engine start & windlass battery, wired with 50mm cable to starter and to windlass, 100a breaker for windlass, no fuse between battery and starter.
C) act as an emergency combine between the two (hopefully never used)

I am confusing myself with the options and thinking it cannot be as simple as just a wee bit higher than the fuse rating?

Thanks

 

[PaulRainbow]

What is the minimum continous current rating I should be looking at for battery switches to
a) isolate 2 314ahr lithiums in a house bank, wired independently with 25mm cable each battery has a 150a classt fuse.
(They will each have their own isolator, usually left always on, with the bank turned on and off via this main isolator)
B) isolate a 90ahr agm engine start & windlass battery, wired with 50mm cable to starter and to windlass, 100a breaker for windlass, no fuse between battery and starter.
C) act as an emergency combine between the two (hopefully never used)

I am confusing myself with the options and thinking it cannot be as simple as just a wee bit higher than the fuse rating?

Thanks

BEP701 Contour switches are rated at 275A continuous, they will be fine for all of the above.

 

[Boater Sam]

I would go for at least 3 times the fuse rating. Fuses often take twice the rated current to melt.

 

[Trident]

Just a side note - whilst 25mm cable is rated up to 150 amps in theory (it can be as low as 100), if the loads are likely to be high I'd go to 35mm as things will get quite hot with 25 if say running an inverter or water maker etc for long periods. If its smaller loads like lights and fridges then 25 is fine but do take maximum amp ratings on cables with some caution as to how they will be used and how they are clipped in etc.

 

[Boater Sam]

With low voltage installations the so called current rating of the cable is not the important factor. Rather it is the tolerable volts drop incurred at the current drawn which is very dependant on the length of the cable as well as its cross sectional area.

 

[PaulRainbow]

Just a side note - whilst 25mm cable is rated up to 150 amps in theory (it can be as low as 100), if the loads are likely to be high I'd go to 35mm as things will get quite hot with 25 if say running an inverter or water maker etc for long periods. If its smaller loads like lights and fridges then 25 is fine but do take maximum amp ratings on cables with some caution as to how they will be used and how they are clipped in etc.

I don't like using 35mm on the battery or BMS, just looks wrong on 6mm fixings. I use a pair of 25mm cables from the battery to the BMS and BMS to negative busbar.

 

[Trident]

I don't like using 35mm on the battery or BMS, just looks wrong on 6mm fixings. I use a pair of 25mm cables from the battery to the BMS and BMS to negative busbar.

I agree and do the same if its a double cable BMS on the negative (not all are as you know) but I run 35mm off the positive - there are plenty of good 35/6mm cable ends . Either way a single 25mm may not be enough for the OP but if he's using two on the positive to the fuse and cut off then of course that's fine

 

[steve yates]

I agree and do the same if its a double cable BMS on the negative (not all are as you know) but I run 35mm off the positive - there are plenty of good 35/6mm cable ends . Either way a single 25mm may not be enough for the OP but if he's using two on the positive to the fuse and cut off then of course that's fine

I’m running one pos cable from each battery to its own isolator, then one cable from each isolator to a master isolator, and from there to the distribution panels via a power post. The neg cables got to a busbar, which is cabled to the shunt. There are no inverters or watermakers in the system, nor ever likely to be. The biggest loads will likely be the autohelm, fridge and when the deisel heater starts up, there is nowt else of consequence really. On the charge side there is a 50a mppt, (eventually 2) and a dc to dc and shore charger of 30a each.

 

[Trident]

I’m running one pos cable from each battery to its own isolator, then one cable from each isolator to a master isolator, and from there to the distribution panels via a power post. The neg cables got to a busbar, which is cabled to the shunt. There are no inverters or watermakers in the system, nor ever likely to be. The biggest loads will likely be the autohelm, fridge and when the deisel heater starts up, there is nowt else of consequence really. On the charge side there is a 50a mppt, (eventually 2) and a dc to dc and shore charger of 30a each.

Sounds like you have spec'd it perfectly then - probably no more than 20 amps load at any time and for the LFP low draw and low charge rates for the size of pack so should last you decades