jofemodo

What's more is every time the inrush short to the inverter bus caps occur, it shortens the life of the cap. So its a good thing to perform pre-charging always. 

Hi @Tinbum!
It worked! You saved my ass! Thanks for your advice!
I pre-charged the inverter caps using an old electric heater (couldn't find any legacy bulbs around here!) and everything started to work like a charm. The system is now running smoothly awaiting for the sunrise to take the first sunlight-breath!
Now i understand a little bit better how all this work and my little PV system is alive. I just ordered a good 30ohm/200Watt resistor to have a more reliable precharge system. Of course, a second battery is a good idea anyway, but first, i will enjoy a few days of my sweet victory! Step by step!
Really thanks!

Hi guys!
Here a newbie in all this, nice to talk with you after 2 days of intense reading. This forum is really the best i've found. Congrats!
I'm building my first "small-but-serious" PV system:
+ 8 x 400V panels
+ 1 x Axpert King 5KV Rack
+ 1 x Pylontech US3000C  (i hope to add more ASAP)
I just finished the installation and double-cheked everything: continuity, polarity, protections, grounding, etc.
The problem:
Having the battery connected to the inverter, no matter if AC input/output or DC input is connected (i've tried all combinations), when i switch-on the battery, all leds light once for 1-2 seconds, and the inverter seems to soft-start (it always does, no matter if inverter switch is on or off.). The display backlight shines and the fan start to work, but after 1-2 seconds, it switches off and the battery goes to protection mode with red led solid light and all the rest off. It tries to restart automatically after a few seconds,  minutes and so on, but the result is always the same.
If i disconnect the battery from the inverter and switch on, then, after the initial all-light, the RUN green led starts to blink. It blinks 0.2s and stay off for 3-4 seconds, what manual says is normal idle operation (no charge/discharge). When i measure the battery voltage in this state, i get 50.5V, what seems quite normal to me.
The battery has been stored for 13-14 months but i did a recharge 6 months ago, using the same inverter connected to the grid. Everything worked fine when i did the recharge. Perhaps i should had recharged more frequently in this time, but i think it's not so bad.
I've reached the point where i'm stuck, so any fresh idea or thought would be really appreciated.
What do you think could be the problem? Is it battery or inverter related? What can i do? 
 
Thanks a lot!

The batteries have a soft start and don't like surges on start up. The battery will see it as a short and shut down. I don't know your inverter so am not sure how it started last time. Is it possible it was powered up from the grid before you started the battery? Some times hitting the battery start button in quick succession may work, as when starting the second time, the capacitors in the inverter already have some charge in them from the first attempt. You could try pre-charging the capacitors first- do a search for methods, eg light bulb.
New batteries only come charged to 50% but the manual does say to recharge to min 90% every 6 months, so that's not your problem.

You need another battery really. The start up current of the inverter is too much for one battery.
 
(Batteries like to be stored at 50% SOC)

A single battery wouldn't boot my Sunny island. The inverter takes a vast current on start up. Look at pre charging the capacitors in the inverter.