SuperStan

Hi Gav,
See attached. I think this is what you are after.
1614805_shoto_sda10_48100_manual.pdf

It could be the inrush current, if you don't have any pre-charge arrangement. Any inverter larger than about 1 kW should really be started with a resistor in series, of a few ohms. That limits the in-rush current to a tolerable ≈20 A. Once the capacitors in the inverter have reached some 90% of the battery voltage, the resistor can be shorted, at which point it is safe to start taking load from the battery, or to charge it. The resistor has to be a special type capable of very high peak power, preferably about 25x its continuous rating. Usually, it will be rated for 50-200 W continuous (so 1250-5000 W of peak power). My system uses a 3.3 Ω resistor, rated at (from memory) 100 W. A small computer monitors the voltage at the inverter terminals, and turns on a contactor to short the resistor when the voltage is close enough. The contactors themselves have a large inrush current, so they are started in a staggered fashion. So my system starts up from cold with a rapid clack-clack-clack... sound (there are about 7 contactors total). These are all EV200s, capable of switching 200 A DC at lowish voltages (such as the nominally 48 V battery, or 100+ V of solar power).
You mention that you eventually got the inverter started. It's possible that each attempt left the inverter batteries with a little more voltage than before, until finally the capacitors were close enough to the battery voltage that the inrush current no longer tripped the battery's over-current protection.
My understanding is that many battery designs, including that of the Pylontechs, include MOSFETs for battery protection, and that these naturally or by design are able to limit the current out of the battery for a short period of time. The short time is because these MOSFETs would be dissipating the same energy in a short pulse of power that the pre-charge resistor would be, i.e. several kilowatts. They can withstand such power levels for a short period of time (less than a second), without needing impractically large heat-sinks. Perhaps the Shoto battery doesn't use MOSFETs, or doesn't employ them in such a fashion that they naturally provide a pre-charge or soft-start function.

There is no such thing as an Axpert King MKSIII; perhaps you meant three models, the King, the MKS III, and the VM III?
The King is an unusual model, in that it is the only one with dual conversion from AC-in to DC, and DC to AC-out. Thus, it can offer zero transfer time (between the loads being powered by the battery and by AC-in). It's also one of the now relatively few models that has a 145 V absolute max Solar Charge Controller (SCC). It can be paralleled.
The Axpert MKS III, also known as the PIP-MGX, is a version of the Axpert MKS II with a removable display. This model has a high voltage (450 or 500 V max) SCC. It can be paralleled.
The Axpert VM III is a "value line" machine, which can't be paralleled and has a few other minor limitations. It also has a high voltage SCC. So it's like a cheaper version of the MKS III / PIP-MGX.
All the above models come in 3 kW and 5 kW versions. A 7.2 kW and 8 kW version of the Axpert MKS III is called the Axpert MAX.
There are also the Axpert MKS models (without a II or III as part of the name). These have no removable display, have a low voltage SCC, and can be paralleled. An Axpert VM II is like an Axpert VM III without the removable display; it's a far less common model.

Hi Jay,
How is this unit doing? Are you still impressed with Growatt? I'm looking to assemble a lithium UPS for an older lady which is reliable, but not too expensive. This unit looks like a good option! Any advice for an alternative inverter/charger will be appreciated!

We built this for a client. Was very much plug and play. I have a few pics. This was for off grid. But the inveter is a hybrid. I'm very impressed with growatt.
Jay