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Josep morancho avatar image
Josep morancho asked

large serial string or small parallel strings to charge 1oKw lithoum batery bank

I am setting up an off-grid system. I will have 8 panels of 545w (50V Voc and 14A Isc) and 10kw pylontech battery bank (2 x 5kw) with 80Ah recommendd charge/discharge current. Distance between panels and charge controller is 20 meters.

My doubt is, if it is better to build 4 parallel strings of 2 panels in serial (Voc 100V and Isc 56A) or 2 parallel string of 4 panels in serial (Voc 200V and Isc 28A) to reach the recommended 80Ah of charge current.

My trend would be to reduce as much as possible de current to minimize the power loses in the cable (as large serial strings as possible) but perhaps with the low 28A nput current, the SmartSolar charger will not be able to provide the recommended 80Ah charge current and it is better to increase the smartsolar input current with the 4 parallel string configuration. I know that in both cases I will need diferent SmartSolar devices: 150/85 or 250/85.

What would it be better?

Who determine the current charge (solar charger or batery bms) and how to limit?

thanks


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2 Answers
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Mike Dorsett answered ·

Firstly, charge current would be in A, Ah is battery capacity...

Using higher voltage strings on the PV side just reduces the copper loss in the PV charging, it does NOT affect the output current from an MPPT controller: This is determined by the input power and the battery voltage. I'm running 4 strings of 3 x 400W panels in series into a MPPT150/70 charger with 6.6kWh of 24V lithium battery.

This has been working fine - the MPPT can only convert ~50% of the max solar input, but it also does well on cloudy days. The input SC Current is within the MPPT's rating of 50A.

I have not seen anywhere that Lithium's require a minimum charge rate - only a maximum, which can be as low as 0.5C (A good battery can charge at up to 3C to 80%). C is the Ah rating of the battery, for 10kWh at 50v nominal, this would be 200Ah. An 0.5C charge current would be up to 100A. Your 4.36kW of panels should produce ~87A at 50V battery voltage - with either arrangement. If the string cables are long, then you would be better off with the higher voltage to reduce the loss in the PV cables.

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Josep morancho answered ·

Mike, why do you say that mppt can only convert around 50% of solar power input if smartsolar specs say that max efficiency is 98%?

On the other hand, my doubt was if the internal mppt dc/dc transformer can transform high voltage and low current input (solar input: 200V and 28A) in a low voltage and high current (battery charge output: 60V and 58A) without problems.

Additionally, i am trying to get maximum current output to reach the maximum battery charge current recommended as i understand that then the batteries will charge faster. Isn't it? Who determine the battery current charge?

Thanks


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