Fuses and circuit breakers

hillside

Haven't had much luck finding fast blow 40-50A fuses so I've been using Jex brand circuit breakers. These are for 2 separate strings of 2-450W panels each going to it's own controller and both charging 4 new Rolls 235AH batts.
I'm not sure if these are helping or hindering performance. One 40A kept tripping so I replaced it with a 50A which seems to be fine. But between a few clouds and hot weather (80-88* most days it seems like it's producing less power overall.
Should I experiment with taking one or both out completely?
Appreciate your thoughts.
These
https://www.ebay.com/itm/251598706525

BB.

What is your battery bank voltage? (12/24/?? volt)

Where are the fuses? On the battery main bus, from the charge controller, etc...

What is the expected current through the breakers? 40 amps from charge controller?

Guessing 24 volt battery system (AC inverter bus voltage?).

900 Watts * 1/24 volts charging (MPPT controller) = 37.5 amps "max expected"
1,800 Watt AC inverter / 21 Volts battery cutoff voltage = 87.7 Amps full power worst case (excluding typical 2x inverter surge rating)

Typical fuse/breaker (UL Listed) generally will not trip at 80% or less of current rating. And "should trip" at 100%+ of current rating... So between 80% to 100% of rating, breaker/fuses "may trip" or may not, especially for thermal tripping devices at "hours" of max rated current.

Also, highly suggested to use 80% as wiring/branch circuit derating for solar circuits ("hours" of max rated charging current from solar/MPPT controller into battery bank...

So--Wiring/breakers/fuses from charge controller to battery bank should be derates by 80% (or 1/0.8 = 1.25 times current)...

If really getting 37.5 Amps:
37.5 amps * 1.25 derating = 46.9 ~ 50 amp minimum rated branch circuit wire/breaker rating (for best system reliability).

Typically, best nominal current is really 77% (or a bit less) of solar panel output Wattage and operational battery bank voltage... Just to plug in some numbers:

900 Watts * 0.77 = 693 Watts panel Derating (temperature/Vmp voltage)
693 Watts / 24 volts charging a nominally discharged battery = 28.875 Amps typical expected max current

So, I would not expect to pop a 40 Amp breaker at 29 Amps in a typical solar power system with 900 Watts of panels. Of course, if the 1,800 Watt inverter is in this breaker circuit--Then normally I would suggest a heavier branch circuit rating:

1,800 Watts inverter load * 1/0.8 inverter eff * 1/21 volts auto shutdown = 107 amps max load current (not taking 2x typical surge current rating).

That leaves an open question... The breakers you are using are typically for car audio systems--Which are not known for "accurate" current trip ratings.

Not sure I would work at getting "fast trip" breakers or fuses... At least on the solar side, breakers and fuses are not really there to protect the electronics--The over current protective devices are there to protect the wiring from overheating/starting a fire. Electronics typically shutdown (such as MPPT controllers) much faster than any fuse/breaker. And are damaged by over current/over voltage faster than any breaker/fuse can trip.

You can also get a very nice AC/DC current clamp DMM and measure your current flow. Just a couple of examples (make sure is AC+DC meter, not AC only clamp meter):

https://www.amazon.com/UNI-T-Multimeter-Voltmeter-Continuity-Capacitor/dp/B0BBMKLL5H ($60 meter)
https://www.amazon.com/gp/product/B019CY4FB4 ($122 mid-priced meter)

-Bill