PV not reaching 58volts after terminated into inverter

Welcome to the forum Klippies,

I guess you are somewhere around Capetown South Africa?

Is this your inverter (combined with MPPT charge controller)?

https://www.inverter-warehouse.co.za/products/axpert-inverter-mks-5k-48v-4000w-mppt

And these are your 325 Watt panels?

https://cdn.shopify.com/s/files/1/0763/7835/files/Canadian_Solar_CS6U-330.pdf?14437442069782673902

And what size/kind of battery bank do you have (i.e., 8x 6 volt @ 220 AH flooded cell batteries, or what)?

In general, the first thing you need to look at your battery bank first... If you take the batteries to too low of state of charge, need to get them fully recharged as quick as you can. If not, you can ruin your battery bank in weeks or days.

So, what is the voltage at your battery bank terminals right now? And what is the state (charging, discharging, resting)?

You need to get these batteries back over ~90% state of charge (roughly >58-59 volts on the battery bus while charging flooded cell deep cycle lead acid batteries--And hold that voltage for 2-8 hours--closer to 8 hours total if batteries are well discharged). And you probably should do an equalization charge after bank is charged (typically around 60 VDC and measure specific gravity every ~1 hour, when SG stops rising that is your "new" 100% state of charge).

Next, your solar array, how was that wired? All panels in series, or 2 series x 2 parallel? More or less, it appears that your MPPT solar input for the inverter-solar charger would take around Vmp-array-std of ~100 VDC maximum.

Your panels (guessing) are Vmp~34 volts per panel. 2 series would be (2x34=) 68 volt Vmp-array-std. And 4 series (4x34=) 134 Vmp-array-std.

You measuring 56 volts in the combiner box if there was no connection to the inverter-charger would seem to indicate that you installed them all in parallel (one 16 amp fuse per + string connection for 4 fuses?).

And measuring 41 volts -- would also seem to indicate a 4p connected array (if trying to run the inverter-charger).

I have not gone through everything and I am making lots of guesses... But, the general requirements for an MPPT charge controller on a 48 volt Battery Bank would nominally be a Vmp-array-std (std=standard test conditions, or "marketing" specifications)--Would be Vmp-array-std~72 to ~100 VDC (could be higher than 100 VDC if you don't have cold winters).

And the issue is that your 4x solar panels (again, guessing specifications), are not really a good fit for your system... 2s x 2p with Vmp-array-std of 68 volts--In a cool climate, is a bit lower than I would like to see. And 4s connected panels would be too high of Vmp-array-std (cold panels, their Vmp/Voc goes higher--And in subfreezing weather, your solar array could "over voltage" your MPPT controller input).

So, nominally, 3 panels in series is probably appropriate for your system.

With your panels reading ~41 volts--That has me worried about your battery bank voltage... In most cases, the minimum voltage on your panel inputs cannot be lower than your battery bus voltage---So if your batteries are at ~41 volts, they are dead at the moment... And you need to get them charging as quickly as you can (see above).

You also need some tools... If you have flooded cell lead acid batteries, you need a hydrometer to measure the specific gravity of each cell (the "gold standard" for state of charge measurements and monitoring the health of your batteries).

And, I would highly suggest you get a DC Current Clamp DMM... These meters you just clamp around one wire and your can immediately measure the DC current (or AC current for your inverter output) going through the cable. You can see if your solar panels are producing current, and if the battery is getting charged. You are not in the US, but here are some quick links to give you an idea of what to look for (note there are AC only current clamp meters and AC+DC current clamp meters--For our needs you need the DC capable meter--Look closely, because the specifications for the Current Clamp can be mixed with the DMM specifications (i.e., AC current clamp & AC+DC volt meter--Which you do not want).

https://www.amazon.com/gp/product/B07546L9RT (cheap meter, good enough for our needs)
https://www.amazon.com/gp/product/B019CY4FB4 (a nice mid-cost meter)

And some battery links to review:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

And a long FAQ thread with lots of random/useful information and links:

https://forum.solar-electric.com/discussion/4426/working-thread-for-solar-beginner-post-faq/p1

-Bill