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Time to parallel my Axpert


Tony Swash

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Hi all,

I am taking the next step and installing a second Axpert MKS 5kw inverter to be in parallel with my existing one. Both have cards but not sure of the cables I need, or for that matter the ins and outs of setting them up.

I have 20x 265 watt panels in 5 strings of 4 giving approx 135vdc input with two 7.4kw mypower lifepo batteries. Must I split the panels and the batts into two or what is the config?

Any assistance will be greatly appreciated. 

Rgds

Tony

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On 2019/11/24 at 4:00 PM, Tony Swash said:

Must I split the panels and the batts into two or what is the config?

Split the panels to allow both units to contribute to charging and supplying the load.  Do not split the batteries. The inverters should share  the battery bank. 

On 2019/11/24 at 4:00 PM, Tony Swash said:

Both have cards but not sure of the cables I need, or for that matter the ins and outs of setting them up.

Firstly, I guess you are aware that the units should be identical and running the same firmware. 

The cables should be in the box, you will need 2 x current sharing cables as well as 2 times parallel cables. You should get one each with the inverter.  

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9 hours ago, Tony Swash said:

5 strings of 4 giving approx 135vdc input

4S is way too much for a 145 V max SCC. At 135 V, the Solar Charge Controller is already derating itself to 67% of capacity.

If your panels are 72-cell: use 2S.

If your panels are 60-cell or 66-cell (often as 132 half-cells), then use 3S.

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I have just connected the panels three in series with an input voltage of 105 vdc. The charger brings this down to 78vdc.  78/105 = 0.74

If I connect the panels four in series then the input is 131.5vdc. The charger brings this down to 100vdc. 131.5 = 0.76 

Doesnt seem to be much difference.  Isnt the purpose of the MPPT to change the voltage accordingly to provide maximum current. I often see 60 amps form my array which at 135vdc is impossible. However at 100vdc it is mathematically correct

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1 hour ago, Tony Swash said:

So what is the optimum input voltage?

There isn't an optimal input voltage as such. You just have to stay inside the various limits. The open circuit voltage, even when cold, should not exceed 145 V, as a hard "never exceed" limit. Panels put out some 7% more voltage when at 0°C than at 25°C.

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31 minutes ago, Tony Swash said:

I have just connected the panels three in series with an input voltage of 105 vdc. The charger brings this down to 78vdc.  78/105 = 0.74

That ratio is just a characteristic of your panels. It tells you nothing about maximum power.

31 minutes ago, Tony Swash said:

If I connect the panels four in series then the input is 131.5vdc. The charger brings this down to 100vdc. 131.5 = 0.76 

But on a cold morning, it might be too close to 145 V for the MPPT to be able to drag the panels down into the working zone, which is 60-130 V.

31 minutes ago, Tony Swash said:

I often see 60 amps form my array which at 135vdc is impossible. However at 100vdc it is mathematically correct

Don't forget that the PV current reported by the inverter-charger is on the battery-side, i.e. at the output not the input. So 60 A is about 3200 W @ 53.3 V. Your panels have a nominal power rating at 25°C and 1000 W/m² insolation of 20 x 265 = 5300 W. You'd expect to see a maximum of about 75% of that most of the time, or around 4000 W. So depending on where this 60 A figure came from, you are not getting the most out of your panels.

Unfortunately, 20 panels doesn't divide into three evenly, nor does 10 (splitting them between two inverters). Adding one more panel would allow you to have 4 strings on one inverter, and 3 on the other.

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A pv cell makes around 0.65V (to over 0.75V if it is very cold) when open circuit, and pulls down to around 0.5V when working a the maximum power point. You will always see it pull down by about 15%, in fact if you don't see this, there would be reason for concern (and support people like myself actually USE that fact to figure out what is wrong in some cases).

With three modules in series, and a voltage of around 105V, I would have to assume you have 60-cell modules, so that's 180 cells in series. 105V means 0.58V per cell. That seems more or less correct (and it probably isn't very cold where you are). The 78V seems a tad low though. I would have expected high 80s.

As you also observed, at 4-series the ratios remain about the same.

I recently discovered that there is a kind of PV module that has a voltage clamp on it (the particular case I dealt with had Trina "Smart" panels). It clamps the open circuit voltage at a panel level so as to reduce the open circuit voltage. This allows squeezing in an extra module in cold climates without exceeding open circuit limits, and makes little functional difference to the MPPT, but it is something to be aware of when you're looking at voltages and the amount by which they "sag" when the MPPT loads them down. These "smart" modules will appear to sag less.

Edited by plonkster
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4 minutes ago, Tony Swash said:

I am adding another inverter, so should I go with 24 panels 12 on each inverter and 4 strings of three then? Or 3 strings of four?

A 60-cell module can make up to 45V per module in really cold weather. If you put 4 in series, you're WAY over the 145V max your MPPT allows. So 4 is too much, which pretty much (by a process of elimination) means you have to go for 3-series.

The rest  cannot answer, or rather, I prefer to leave it to people who know this inverter/charger/mppt combo better.

Edited by plonkster
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