Hi guys

What is the proper way to protect battery banks with fuses? My current installation (4 x 4 strings 12V) does not have any fuses on it. There is a breaker between the inverter and the battery bank but no fuses. Should each individual battery have a fuse (between the 4 batteries in a string or should I only install a fuse per string? I assume it is sufficient to only put a fuse on either the positive or negative of the battery, not both?

How do I size the fuses? What should their rating be? Can anyone recommend fuses?

Cobus

Would also like to know. I have 8 x 150Ah 12V Trojans in two strings of 4.

I suppose you can calculate the amount of power draw that should pop the fuse, so if my inverter pulls 10kW (which it can do for short times, but I have nothing that big), that should pull 200A at 50V so a 200A fuse will be adequate?

Or should you put a 100A fuse on each string?

A 100A fuse pops at 100A correct? or is there a factor?

16 minutes ago, DeepBass9 said:

200A at 50V

What I have been taught, you use the lowest voltage the inverter will go down to, and use that voltage.

On UPS we used to take the max the UPS can power, taking into account the peak for wot, 10 sec, with the lowest cutoff voltage, for that is where the drama starts with high amps, to not blow the fuse.

52 minutes ago, DeepBass9 said:

A 100A fuse pops at 100A correct? or is there a factor?

There is a 25% temperature derating over 25^oC.

So what does that mean practically? For battery fuses you have 100,150,200,300 A fuses, so you can't cut the cake too finely.

 

27 minutes ago, DeepBass9 said:

So what does that mean practically? For battery fuses you have 100,150,200,300 A fuses, so you can't cut the cake too finely.

Rubicon have 40,50,63,80,100,125,160,200,250,315,350,400 and 500A fuses so there is a reasonable selection.

Work out you max draw (in my case 83A) and derate by 25% (103A in my case). I went with 100A fuses since what is going to happen is the may blow sooner on a hot day. Last summer we went to 49^oC and indoors it was 34^oC. I did not try and run near capacity so I do not know whether the fuses would have blown early but I did not have any nuisance blowing. Initially they could only supply 63A fuses and I blew one charging up the capacitors on the inverter even before we started.

Chris

If I remember correctly you only have one string of 4 batteries. Do you have only one fuse installed on the string? Where is it installed?

In my case I have 4 strings of batteries, should I divide the amp rating by 4 and install one fuse on each string?

No need to install fuses on each battery?

What exactly is the fuse protecting against? If an internal short on a battery occurs, will the fuse on that string then blow due to the increased amps pulled by the string? Is this a realistic protection?

Thanks
Cobus

I have one fuse in series with my one string of batteries. It's rated ad 160A because that covers the peak use on the inverter. The point of the fuse is to blow if something really bad happens. Since the battery is good for well over 500A in a short-circuit condition, the fuses will protect me.

If I had more than one string I'd put in a fuse per string, but I would still oversize it a bit, as the point of the fuse is not to blow as soon as I go slightly over the max current... it's job is to blow when things go wrong in a REALLY bad way.

Their job is to protect the cable, and neighbouring batteries to some extent.

The inverter has its own built-in fuse on the input, which is there to protect the inverter itself.

I have been looking at these, to put one on each postive pole and be done with it.

https://www.powerforum-store.co.za/collections/accessories/products/fuse-block-terminal-fuse-single

EDIT: Obviously with the right size fuse.
EDIT 2: I have 4 batts - maybe just one. 

Batteries connected in series essentially make one big battery so I have a fused Mersen disconnect between my battery bank and inverter.

If I was to have multiple strings I would fuse each string dividing the amp rating as you suggested. I like the look of the terminal fuses @Energy got.

That Aussie dude who I see has a fuse on each string. Not that I would copy everything he does. 

Incorporating fuses are a "hard to sell". If you are meticulous and the system operates the way it should you never need them. Its when you standing in the shell of what's left of your home that fuses gain popularity.

I see Plonky's post has just come through so we concur fuse per string.

I looked at the PowerForum Store (would like to support the forum store) and found the fuse below. My inverter is normally limited to max 20A charge and discharge (while grid is available) but it is a 3kW inverter so when grid is not available it would draw the full load from the bank. It can also be overloaded quite a bit. So if I use 4 x 30 A fuses (one per string) it should allow for max load of 4.2kW taking into account 25% derating.

From the description it seems like I cannot connect it directly to the battery terminal (why not?). So do I use the device below and bolt the flat piece to the battery terminal and then fasten the cable to the inverter on top of the fuse?

10 minutes ago, plonkster said:

If I had more than one string I'd put in a fuse per string, but I would still oversize it a bit, as the point of the fuse is not to blow as soon as I go slightly over the max current... it's job is to blow when things go wrong in a REALLY bad way.

Ok, so if I go for 4 x 50A fuses that would allow operation up to 6.9kW, 4 x 75A will be 10.3kW.

I obviously don't want nuisance fuse pops but on the other hand oversizing it too much will reduce the protection.

I also think I read somewhere that on parallel strings (as in my case) the fuses prevent a faulty string from pulling current from the other strings e.g. in case of a short circuit so it protects the other battery strings and also protects against fire. Is that correct? (My knowledge on anything electrical is dangerous.)

Cobus

Cobus, just a thought, as I forget this all the time, when you buy the fuses, get a spare or two just in case there is an issue for whatever cause.

Thanks TTT, will definitely do that. Don't want a string to be out of operation for a couple of days because I don't have a spare fuse.

I like the maths for the 30A fuses.

20 minutes ago, cvzyl said:

in case of a short circuit so it protects the other battery strings and also protects against fire. Is that correct?

Yes :-)

20 minutes ago, cvzyl said:

in case of a short circuit so it protects the other battery strings and also protects against fire. Is that correct?

Yes :-)

A bit late to the party...

Cobus, according to the infini manual the max discharge current is 92A. If you take derating and temperature into consideration I would suggest a 125A fuse or breaker between the bank and the inverter.

Then, if you want to fuse each string I would divide that by the number of strings. 92 + 25% => 115.   115 / 4 => 28.75  I would use a fuse as close as possible to that per string, e.g. 30A, 32A or 35A, depending what is available.

 

48 minutes ago, superdiy said:

divide that by the number of strings.

That is the bit I am a little unsure of. Let's set the stage for a scenario I want to sketch: say you have 4 strings and you want it to max out at 100A, so you fuse each string at 25A.

Suppose now one of the fuses blows, for whatever reason. While the remaining 3 strings might be perfectly capable of supplying the 100A that is needed, you now only have 75A available, divided slightly unequally across the strings. The next time you draw 72A (for example), due to the slight unequal division, another fuse blows. Now you only have 50A available so the remaining two topple like dominos shortly after.

Hence I think that fusing to exactly the total you want might be a problem. It might be better to slightly over-fuse each battery string and then add another fuse on the inverter side which you then size for 100A. Eg, fuse every battery at 35A, and the inverter at 100A.

The fuse is there to protect the cable, and the battery to some extent. So fuse it for the rating of the cable and/or the battery.

I'm just thinking out loud here though.

Yes, I totally agree with you.

So then the formula will be something like: ((max inverter current + 25%) / (stringcount - 1)) e.g. ((92A +25%) / (4 - 1)) => 38.33A ~> 40A per string 

That compensates for one blown fuse, but if more than one fuse fails, you are back to square one.

Best solution still is to NOT HAVE PARALLEL STRINGS. 

Thanks superdiy. I know that a single string would be better but I have a "special" setup. 

I'll see where I can find 40A fuses. Think the Powerforum store only has 30 and 50A.

C

8 minutes ago, cvzyl said:

Thanks superdiy. I know that a single string would be better but I have a "special" setup. 

I'll see where I can find 40A fuses. Think the Powerforum store only has 30 and 50A.

C

As per @plonkster's explanation, then rather go for 50A fuses.

Hi

I use these between my battery and inventor.
http://www.solarsolved.co.za/index.php/products/miscellaneous/accesories/fuse-switch-disconnector-detail

Its a fuse and also a switch.
it takes the standard fuses so you can choose whatever amps you reqire

I use a 100A fuse on my 250A bank since the Axpert only draws 93A. You get some nice blade fuses at the audio installation places for about R50. The fuse holder cost about R200. Rockford Forcegate makes one with a built in Ampere meter as well.