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Advice re Axpert 5kVA (unit with large heatsink) and connection to solar panels....


VaughanT
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Dear Braintrust - some time ago I installed the above-mentioned unit for my folks to help get them through load-shedding and power outages. For the most part the unit has been solid and has a bank of 105AH, deep-cycle Enertec batteries as the storage in the system. 

Ultimately I'd like to consider adding solar panels to the specific inverter along with upgrading the present battery bank to a technology that would allow it to be regularly cycled. 

From what I understand - this particular inverter has some limitations in terms of;

1) Maximum PV voltage (limiting the number of panels as could be applied)

2) Battery voltage settings as would be applicable to the higher voltage lithium cells. 

The PV market is rapidly evolving with emphasis being on panels that obviously operate on higher voltages which will further limit the choices of what panels could be used.

I would be grateful to receive your responses on how best I  could manage the situation at hand.

Vaughan

 

 

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12 hours ago, VaughanT said:

For the most part the unit has been solid and has a bank of 105AH, deep-cycle Enertec batteries as the storage in the system. 

If that's a single string of 105 Ah modules, that's a little light for a 4 kW system. But if it's two strings of 105 Ah, that's fine.

12 hours ago, VaughanT said:

From what I understand - this particular inverter has some limitations in terms of;

1) Maximum PV voltage (limiting the number of panels as could be applied)

It's almost certainly the model with 145 V max PV open circuit voltage. PV charge power derates above 130 V, so try to keep the working voltage (Vmp) well below 130 V. Usually that means limiting the panels to 2S, though 3S for 60-cell panels (now rare). You can parallel many strings (my system has 14 strings of 200 W panels, two inverters) to make up the power. Those old models are probably 60 A max (at the battery side, so 3000 W of charging power). So keep the total panel power below 3375 W (25% over nominal), preferably under 3600 W (20% over nominal).

12 hours ago, VaughanT said:

2) Battery voltage settings as would be applicable to the higher voltage lithium cells. 

Check my recent posts for the recommended Pylontech voltage settings; use the settings with 52.5 V max. The manufacturer recommended voltage (53.2 V) is too close to its shutdown voltage (54.0). You can also use those figures (perhaps adjust the maximum charge current) for other battery brands, as long as they are 15S. If 16S, multiply the voltage settings by a factor of 16/15.

12 hours ago, VaughanT said:

The PV market is rapidly evolving with emphasis being on panels that obviously operate on higher voltages which will further limit the choices of what panels could be used.

I believe that you can still get 72-cell and perhaps even 60-cell panels. But even 80-cell panels can be used if wired 2S. Silicon solar cells tend to have the same voltage characteristics: around 0.6 V open circuit, and 0.5 V at Maximum Power Point (MPP).

The larger panels mostly have larger cells, which have similar voltage ratings, but higher current. That means you need fewer paralleled strings with the larger panels.

 

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