Do I need a fuse/breaker and what size

[Rockinfigs]

My current setup is two 200amp(400amp total) lithium batteries in parallel... Positive going to a blue sea switch then to a positive buss bar..

Do I need a fuse/breaker between my batteries n the switch? If so what size?

Thks

 

[FilterGuy]

My current setup is two 200amp(400amp total) lithium batteries in parallel... Positive going to a blue sea switch then to a positive buss bar..

Do I need a fuse/breaker between my batteries n the switch? If so what size?

Thks

There should be a fuse as close to the battery as possible.

The size of the fuse is determined by the larger of the load current and Charger current. (The load current is usually the larger).

 

[Rockinfigs]

There should be a fuse as close to the battery as possible.

The size of the fuse is determined by the larger of the load current and Charger current. (The load current is usually the larger).

Thks for the reply.. Sorry I'm a newbie so with my 400 amp lithium Batts installed in my toyhauler how do i find out the load current... My hauler has has a 50 amp service... Is 50 amp fuse big enough? Thks

 

[FilterGuy]

What will be powered off the batteries?

An inverter?
DC loads?

 

[Rockinfigs]

What will be powered off the batteries?

An inverter?
DC loads?

I have an 1800 watt inverter. N the toyhauler for all the lights n water pump.. Etc. Typical toyhauler stuff.

 

[FilterGuy]

Make a list of items that will be on the system.
For each item, measure the current while it is on.
Add up the current for all of the items to get your load current.

The max current for the inverter can be calculated. (I will assume 90% efficiency)
The inverter will draw 1800/.9=2000W from the battery.
When the battery is low the voltage will be around 12V so the current from the inverter will be 167A..

You will end up with a list

Inverter - 167
lights - x
Water pump - y
fan - z
etc -n
-------------------
Total ??????

The fuse should be rated for 1.25x the total load.

 

[Hedges]

I'm going to say 300A fuse, just for the inverter:

1800W / 10V / 90% x 1.12 x 1.25 = 280A fuse indicated

That's assuming 10V low battery; maybe lithium wouldn't get that low? But include IR drop across cables.
90% efficiency (probably generous)
1.12 is "ripple factor", something I've come up with; RMS/average current in battery cable supplying sine-wave inverter - it isn't DC.
1.25 is 25% higher to avoid nuisance trips.

The fuse is something that should never blow except in case of a fault.

 

[Rockinfigs]

I'm prob looking at a 400 or 500 amp fuse then..

Is a fuse mandatory between the batteries n the battery cut off switch or can i run it without the fuse

 

[FilterGuy]

'm going to say 300A fuse, just for the inverter:

1800W / 10V / 90% x 1.12 x 1.25 = 280A fuse indicated

That's assuming 10V low battery; maybe lithium wouldn't get that low? But include IR drop across cables.

12V is about as low as LiFePO should ever go.

1800/12/.9=167A = Max continuous current.
Your calculation is more conservative than mine with the extra 1.12 multiplier. In my experience that is not needed (I don't see fuses blow).

For simplicity, normally I put the 25% margin in after I have the total for all the loads. However, if it is just the inverter it would be 167 x 1.25 = 208A. That would mean a 225A Class T