Axpert doesn't tell me how much the PVs are producing

[Nuno]

Hi folks!

I recently noticed that my Axpert MPPT inverter only reports the charge current/power, and not exactly how much the PVs are producing. So, even in full sun, if the batteries are full, it only reports around 100W of power coming in.

This is fine, except for the fact that I wanted to connect a Raspberry PI to it to gather data and have statistics on the production around the year.

Any suggestions on how to go around this?

[Youda]

Hmm... how about to put some AC load on it?

I mean, if the batteries are full and there is no load on the AC side of the inverter, then no power can be generated.

[Nuno]

I can put some load, but I wanted to get the stats of how much power can be captured regardless of the load. How do all the solar monitors work? I had the idea  that the stats you usually see online are the max power output of the panels, regardless of consumption. It never occurred to me that might not be possible if power is not being used (facepalm)

[Youda]

Well, most of the popular "solar stats" that you can see online are comming from ongrid inverters. For ongrid inverter the grid itself is a huge load. Whatever these ongrid inverters will push into the grid, someone on the other side of the wires will consume.

 

For offgrid PV systems, you need some kind of "ideal load", for example an empty battery. If the batteries are near full and there are no other loads then the generated PV power will drop:

This is my system as of today, where you can see large drops in generated power, since I had no use for that power:

 

Same here: Batteries were charged in the morning, but then I had no use for the excess power. So the generation went down during the day:

 

This is an older sample, where I was able to use all the electricity. Drops in the power generation were caused by the clouds only:

 

 

Edited March 4, 2019 by Youda

[Nuno]

of course, you are right! makes sense.. should have thought better about this before buying the pi

in that last graph, you have increase output in the afternoon with "pv2" is its a group of panels oriented differently? I was wondering if it would be more useful to have 3 of my panels facing south and the other 3 towards SW to boost the production a bit at a later part of the day. But would it be better than just having more production earlier? Have you conducted such tests?

[Youda]

Correct - the PV2 string is facing West in my case.

For the maximum production, the best is to make all the panels to face the sun directly at noon (in the Europe it's South, for SA it's North).
Ongrid installations are typically oriented just like that (if not utilizing trackers).

For an offgrid install with a smaller battery bank, it's okay to spread panels in a way that some of them will catch the morning sun, most of them noon and the rest will catch some sunrays in the late afternoon. But I personally, prefer to focus on the morning and the noon:

- morning sun is quite strong, and will help you to quickly charge the batteries after a long night
- noon sun is the strongest, it will top-up the batteries and will power the loads through the most of the day
With the above setup, it's quite often that you will have a full battery when then night starts. So, it does not make a real sense to focus on the West too much. On the other hand, in the morning the batteries are always hungry, so it's good to give them a nice "breakfast" That's why I'm convinced that East is more important then West, for the offgrid setup.

 

 

 

 

[Coulomb]

6 hours ago, Nuno said:

I wanted to get the stats of how much power can be captured regardless of the load.

The only way to find that out is to pull as much power as possible, and see how much that is. There might be clever ways of estimating it, but I don't know of any system that does this. There may well be some, e.g. those Zappi EVSEs for charging electric vehicles from "available" solar energy only.

Axperts don't measure the power going into the MPPT. But they're pretty efficient, I think 98% or more, so the difference between input power and output power, at least at full load, is slight.

[Youda]

Nuno,

if you are familiar with programming PI, then you can code a short "load automation" that will pull as much power as possible, while not draining the batteries at the same time. Something like this, for example:

- Have the two relays that will turn-on a heater element. 2kW each.
- Add the SSR relay with 0-10V control, put third 2kW heating on it.
- Read-out the stats from Axpert via RS232

All of the above can be controlled by a single PI, where code logic is like this:

- read battery charging current from Axpert, if it's more than 0.5A, then:

• close the SSR relay a little (via PWM to 0-10V conversion)
• if the SSR is fully closed (10V), then open the SSR (0V) and fully close the first relay
• if the SSR is fully closed AND first relay is closed then open the SSR (0V) and close second relay

- read battery charging current from Axpert, if it's less than 0.5A, then:

• do the same like the above, but reversed
• read AC Watts from axpert ->  now you know what's the maximum power available just now

With the code above, you have an "intelligent load" that's varying from 0 to 6 kW in a very smooth steps. And it's a nice PI project too

PS: Yes, you need to add IF + ELSE + ELSEIF statements to the above, in order to work it properly. But the logic is obvious - with just couple of relays and a weak SSR you can scale this to tens of kilowatts of smooth load.

Edited March 5, 2019 by Youda