Best Battery Bank?

[Sage Oldmann]

Close, but no cigar.
A damaged panel can be safely backfed by just 1 identical panel (or identical string) But any more than 1 backfeed source, and the damaged panel can overheat and start a fire. Panels generally have a sticker on the backside with a spec for a Series Fuse. That spec is always less then 2x Isc but a bit more than Isc. My 9 amp panels call for a 15A fuse. 18A would be too large and not protect.

incorrect/poor wording highlighted in red. If you had 4 strings of 8 amp each, 30a fuses would NOT protect a defective panel/string

Thanks for your easy to understand explanation. If I calculate this correctly my 4, 100 watt panels in parallel, which call for a 15 amp fuse max each, would be well protected by a 10 amp fuse on each panel. Do I have that right?

[inetdog]

Thanks for your easy to understand explanation. If I calculate this correctly my 4, 100 watt panels in parallel, which call for a 15 amp fuse max each, would be well protected by a 10 amp fuse on each panel. Do I have that right?

Yes, I think you've got it!

[lkruper]

Yes, I think you've got it!

I was getting lost but I think I figured it out.

If each panel can put out maximum 10 amps and there are 4 panels in parallel, if one panel dies in a particular way so that it consumes power and creates heat, the fuses should be greater than the 10 legitimate normal operating max amperage and less than 2 X that amperage (ie 20) so that if there is a failure it blows the fuse? So in this contrived example a fuse of between 11 to 19 amps would work?

[inetdog]

I was getting lost but I think I figured it out.

If each panel can put out maximum 10 amps and there are 4 panels in parallel, if one panel dies in a particular way so that it consumes power and creates heat, the fuses should be greater than the 10 legitimate normal operating max amperage and less than 2 X that amperage (ie 20) so that if there is a failure it blows the fuse? So in this contrived example a fuse of between 11 to 19 amps would work?

You too have got it! Almost. See below:

As a rule of thumb if you want to be sure of a prompt (minutes!!) blowing of the fuse with any fuse within nominal tolerance values you should size so that the overload will be at least 135%.
So if you want to be sure that 20A would blow the fuse you should not use any larger than a 15A fuse.
Be sure to allow for temperature variation and cloud/snow effects on the low end too, so no smaller than 12A if the panel is rated for 10A Isc at STC to avoid nuisance blowing. If the fuse is in a high temp environment or the connections to it are high resistance, the fusing current will be slightly lower. Ambient temp affects breakers even more severely.

[lkruper]

You too have got it! Almost. See below:

As a rule of thumb if you want to be sure of a prompt (minutes!!) blowing of the fuse with any fuse within nominal tolerance values you should size so that the overload will be at least 135%.
So if you want to be sure that 20A would blow the fuse you should not use any larger than a 15A fuse.
Be sure to allow for temperature variation and cloud/snow effects on the low end too, so no smaller than 12A if the panel is rated for 10A Isc at STC to avoid nuisance blowing. If the fuse is in a high temp environment or the connections to it are high resistance, the fusing current will be slightly lower. Ambient temp affects breakers even more severely.

20 / 1.35 = 14.81

Thanks

[inetdog]

20 / 1.35 = 14.81

But who's counting.

[Sage Oldmann]

You too have got it! Almost. See below:

As a rule of thumb if you want to be sure of a prompt (minutes!!) blowing of the fuse with any fuse within nominal tolerance values you should size so that the overload will be at least 135%.
So if you want to be sure that 20A would blow the fuse you should not use any larger than a 15A fuse.
Be sure to allow for temperature variation and cloud/snow effects on the low end too, so no smaller than 12A if the panel is rated for 10A Isc at STC to avoid nuisance blowing. If the fuse is in a high temp environment or the connections to it are high resistance, the fusing current will be slightly lower. Ambient temp affects breakers even more severely.

The panels are rate 5.75 ISC. So should I use a 10 amp fuse?

[lkruper]

The panels are rate 5.75 ISC. So should I use a 10 amp fuse?

In my understanding, but I am still learning, so lets let the experts weigh in:

Your panel is 5.75. The fuse must be greater than 5.75 and less than 2 X 5.75. Furthermore it should protect the panel by no more than 135%. So take 11.5 (2 X 5.75) / 1.35 = 8.5 amps fuse. Therefore if the panel fails and the other three panels push their 3 X 5.75 into it, the fuse will blow. If temperature or other conditions legitimately raise the current above isc, it should be no less than about 20% (6.9), and not blow the fuse.

Another calculation would be 5.75 X 1.35 = 7.7 amps. So 8 looks like a good value to protect the panel.

However, VERIFY this from an expert!

[inetdog]

I will not call myself an expert, but I did supply those recommendations, so yes, 8 would be good. Slightly smaller will probably last but 6 is risking nuisance blowing.
Slightly larger than 8.5 would still be OK too, but definitely no larger than 10.
Stamp of approval is in the mail.