Fusing of battery bank

Wilis · January 2010

Hi,
I have seen several references to fusing each parallel leg of a battery bank. At least I think that is what is being said. What device is being used to do this? I have looked on the NAWS store and don’t seem to find a large fuse with holder to do this. How would you decide what amperage of fuse to use? I know what the max load my inverter could draw, but don’t plan to ever get that large of draw. I also know I have 0/2 wire connecting my batteries with the inverter and connecting each leg, so would I fuse for that? What would be protected by this type of fuse would be if a battery cell shorted and tried to draw current from another leg. Just want to be safe and not have a fire. Have been running my system for several months now and have learned so much from this forum. Not off grid, just learning. Thanks for everyone taking the time to help make solar work for DIY people.

System: Flexmax 80, Magnum MS-AE, 4 Kyocera 210 ( soon to add 4 more), 12 T-105 batteries.

RCinFLA · January 2010

Re: Fusing of battery bank

You can use the class T fuses. If you can prevent too much stress from 2/0 cabling you might do it with lug fuses without holders. Usually easiest to prevent cable stress on fuse by putting in middle of series string.

As to amperage rating, this depends on your battery bank and number of parallel strings you have and inverter power level. The more parallel strings the more likely of a mis-match on current so more margin needed then just max inverter current divided by number of parallel strings.

The center to center hole spacing on the mounting tabs are 1.69 inch for 110 to 200 amp fuses and the 1.84" for 225 to 400 amp fuses so there are two different holders.

BB. · January 2010

Re: Fusing of battery bank

Wilis,

You have it exactly correct. Fuses/breakers are designed to protect the wiring so that is your maximum fuse/breaker size (you can go smaller if your loads/charging currents are less).

So, for 2/0 wiring, using this table, your maximum current rating would be 190-283 amps (distribution vs chassis wiring).

Here are some large fuses:

CFB-200T 200 amp fuse with holder, Cut Cable Connection

Midnite solar (available through NAWS) has some large breakers+enclosures like these 175 - 250 amp version.

The fuse block costs $40 and the breaker kits costs $190 or so... Difficult choices.

The breakers are nice since you can turn off a string for maintenance with just a flip of the breaker. But if you have a bunch of strings, that is an expensive set of breakers.

If you have just two strings, you might use a pair of fuses blocks plus an inexpensive 300 amp DC switch like this from the marine industry. Can switch between two battery strings for $30 each:

350 Amp Battery Selector Switch - Battery #1, #2, or BOTH

This is one reason I like to recommend that people stay with one or two strings and use large Amp*Hour rated batteries instead of a whole bunch of small battery strings in parallel.

I don't know how many people actually fuse/breaker their parallel battery strings...

For me, the "scariest" part of an off grid system is the battery bank (1,000's of amps of current when shorted and DC arcs which are very hard to suppress--actually more dangerous with respect to fire vs a 120/240 VAC wall outlet).

I think that fuses/breakers/switches on parallel batteries is the most important fire safety feature (short of not storing your generator gasoline supply next to the wood fired stove).

-Bill

RCinFLA · January 2010

Re: Fusing of battery bank

I consider the fuse in each battery string for a failure in the batteries.

You should have a main breaker switch going to the inverter. If snap, crackle, and pop starts coming from inverter you want a single battery line feed breaker switch you can shut off.

Wilis · January 2010

Re: Fusing of battery bank

Thanks again everyone,

RCinFLA I do have a main fuse in the epanel, between the batteries and the inverter. I think what I was talking about was for a different reason, but thanks for the good advice. BB as always you seem to have the answer. Thanks for the choices of fuses. I have 3 strings of 4 batteries and will be ordering one of the NAWS products soon. My battery box is wood, that is why I was afraid of fire. I placed a fire alarm above the system, but would like to correct the problem before I have to use it. My car and truck is parked in the other end of the basement and as you know a few years ago a car company had a problem with the + voltage on the caraberator. That was only one battery and I realize that 12 batteries have so much more energy. Wilis

peterako · January 2010

Re: Fusing of battery bank

Hi there Sorry for the delay.
It is correct that you protect the cables but also the battery strings if you have more than one.
So you must have the fuse on the batterys and that for every string.

I personal put a second fuse also in the middle from every string. that to avoid a tool shorting the battery terminals ( yes it happens ).
Also in every earthquake zone it is importand to first fuse by mutiple strings.

I have seen a lot off burnt fuses after strong eartquakes.

greetings from Greece

DeltaFox · January 2010

Re: Fusing of battery bank

I just got a new inverter that is 1000 watts and it said to use #2 wire and I have 4 batterys going to the inverter . I put a 150 amp fuse inbetween these.
The chart said that it would be enough. I was useing a car type of a fuse holder and a waffer fuse.

westbranch · January 2010

Re: Fusing of battery bank

what is your question? you are good on both counts by the tables, slightly oversized.

Fusing of battery bank

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