Energy Matters Forums

[Ray]

Thanks Gordon,

Yes now that i look at the raw figures it puts me right on the line and makes sense as to the possible cause. I am in the process of making a bigger array with better match of 250 Watt panels so this is great to know as it changes my whole plan of wiring.

For now back to the current setup i will rewire it in the next day or so and see how it performs

Ray

Btw would really like to know how Kev's outcome was ?

[SandyP]

Maximum efficiency with MPPT charge controllers happens when your rated PV input is ~10V above your maximum charging voltage. It decreases significantly if you go too high, near the CC's limit (assuming a relatively low battery voltage), and drops off even more if you exceed the max input voltage and suffer de-rating.

Interesting info Gordon,
Any reason why my installer would have arranged our panels so that the Outback CC sees 88 volts input from our panels that each have a Voc of 45.2V?
The wire run from the panels to the CC is only ~12 metres.

[Gordon-Loomberah]

Hi Sandy, it is a trade-off between MPPT efficiency and cable diameter, ie cost. What you lose in the MPPT efficiency is the price for less expensive cables.

Your 88V input will just be due to warm panels, the voltage decreases as the cell temp increases, and rated voltage is for cells at 25C, which only happens when the sun is shining on them in air that is significantly cooler than that.

Running 2 X 72 cell panels in series is the norm for 24 and 48V (Pb-acid) systems, but it is more efficient at 48V. It does cut the losses by 3/4 for a given cable diameter and current over running single panels in parallel.

[kemills1]

I've given up worrying about the alarms and derating graphs. The controller seems to be working okay at the moment. From 1400W of panels, I'm seeing well over 1500W on cooler days. The Vmp seems to be around 110V and the array current can reach about 14.5A. At the batteries (24V) I get up to the 60A max of the controller.
Voc has reached 142V which is going to go higher as the winter approaches. I've made a voltage clamp for the array which is limiting the voltage to around 138V so there shouldn't be any problems.
We'll see what happens.
Kev.

[Ray]

I have finally rewired my system also changed the panels.

I am running a 48 volt system as i was before

Before i was running at about 123-131 volts from the array and would get the "High Array Voltage Current Limit" warning almost every day.

Now i have connected different panels (upgrade to matching panels) my input from the array is about 92-108 volts.

I have not seen the problem since.

Ray

[kemills1]

Well, my voltage clamp didn't seem to work. It was getting very hot sometimes which was unexpected. I connected a volt meter across the array and set it to record max and min voltages. What I observed when it was getting hot was quite alarming. Max voltage was recorded at +240v and Min voltage was recorded at -312v! I've since changed the array configuration so I get a voc of about 75v. I don't harvest as much power but I don't get any more alarms either and I don't need the voltage clamp...
Next time I buy a charge controller, I'll pay more attention to the specs.
Kev.

[Ray]

Wow those voltages are scary lucky you didn't kill the charger.

Bit surprised that you are not getting as much power, mine has more power than before but could just be as a result of different panels tho.

Your running a 24 volt setup so your input voltage compared to the system is still high. ???

Ray

[offgridQLD]

Midnite Solar models do 150, 200 and 250V max.

Actually the Midnites will do a thing called hyper VOC where they can handle MAX voltage + battery voltage so on a 48v system a classic 150 can deal with up to 200v. It wont charge the battery while in hyper VOC but it wont let the smoke out. I guess this could be handy on a real cold morning with a array that's pushing the limits.

That said if your up that high in Voltage its worth considering running less panels in series if you can as the Midnites run much cooler around 80ish volts.

Kurt

[kemills1]

Well, things are going down hill. The other day, I had to move a panel and had to disconnect the array from the controller. In the process I managed to short the controller wires together which resulted in a spark. After reconnecting the panels I noticed the array voltage and array power on the wed page were reading zero. According to the manual and the literature the controller is protected against array short circuits. If this hadn't been stated, I would have taken precautions. I contacted Morningstar again and they told me the damage wasn't covered and I' have to buy a new one. Now I'm finding it doesn't go into night mode, the controller is back feeding into the panels and the logging is messed up in that it can't determine when a new day starts or ends.
It seems their specs and claims have to be taken with a pinch of salt....
Kev.

[kemills1]

I decided to take a look at the Tristar today. I found an array of 8 FET's which isolate the array from the controller, probably to stop back feeding into the array at night. Five out of the eight were blown (short circuit) which would cause the symptoms I've observed. Looking for replacement devices (IRF6646) it became apparent that the devices are rated to a max of 80V Vdss. How Morningstar can claim the CC is safe to a PV voltage of 150V I don't know.