Greetings, 

I have an Axpert 3kva that has a limitation of 600w I have 2 X 250wp feeding the inverter currently. I would like to a more solar without exceedin the restriction. Is it possible to charge another set of batteries with different solar panels and feed the inverter via the solar input port. This will obviously start at night when the main panels aren't supplying power. 

2 X 100ah 12V AGM batteries are installed. 

9 hours ago, Azimuth said:

I have an Axpert 3kva that has a limitation of 600w

I presume yours is an Axpert MKS 3K-24. (It saves a lot of time if you give the model number).

600 W is the battery voltage times the maximum solar charge current, so there is no allowance for "overclocking" at all. I would think you could add another 250 Wp panel (presuming that the panels are in parallel).

I would not feed a battery into the PV input. There will be no limit to the current the battery could provide. PV panels are current limiting.

If you need more PV charging, add a separate MPPT charge controller. The output of the external charge contoller connects directly to the battery. Make the external controller's settings a little lower (in voltage) than that of the inverter. So they work together for most of the charge, but the inverter takes it from roughly 90% to 100% SOC on its own. I have a similar setup with an old 5 kVA 4 kW model, which came with only a 3000 W SCC (Solar Charge Controller). So I have 3200 Wp of PV connected directly to the inverter, and another 2400 W of panels on an external solar charge controller, rated at 1500 W. You could do much the same, scaled down of course. I note that the external charge controller I'm using (Blue Sky Energy) is very good at handling excess PV power, much better than the Axpert itself. So I'd only connect about 750 Wp, no more, to your Axpert's PV input.

yeah (disclaimer - I am a newby) but you can handle 3 panels in parallel if they are under 25A and your cabling can handle it?

27 minutes ago, Dex_ said:

you can handle 3 panels in parallel if they are under 25A and your cabling can handle it?

The limit won't be quite 25 A; 25 A is on the battery side. But with 1S, the PV voltage and battery voltage won't be vastly different.

Let's say your Vmppt is 35 V, and you're charging your battery near the end of charge, but before the current ramps down. So say 28 V and 25 V on the battery side, or 700 W. Ignoring losses (they should be about 2%), that's 700 / 35 = 20 A on the PV side. The MPPT is like a "DC transformer".

Suppose your panels can do 8.5 A (that would make them 300 W, but play along); that would be a total of 25.5 A under ideal conditions. But your MPPT will only deliver 25 A on the battery side, so it will only draw 20 A on the PV side. So the rest of the available current is wasted. But in non-ideal conditions, you'll need the extra capacity just to make the 20 A, so most of the time it won't be wasted at all. You'll just have your peak production clipped at some 600 - 700 W, depending on the battery voltage.