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dagfroderekdal avatar image
dagfroderekdal asked

SmartSolar MPPT 100/20 minimum PV voltage for 48 V battery

I have a Lifepo4 battery pack with 51.2 nominal voltage which I want to charge with a SmartSolar MPPT 100/20. I have read about the +5 V minimum PV voltage in order for the solar panels to charge. However I am still a little uncertain as what the recommended minimum PV voltage is in order for having a well-functioning system.
Questions:
1. Is the +5 V minimum PV voltage requirement the same for 48 V batteries as for 12 and 24 volt batteries? So the minimum PV voltage for charging a 48 V battery with 51.2 V nominal is 56.2 V.
2. If using 3 panels in series, each having a voltage of 19.8 V at maximum output giving 59.4 V, will that be sufficiently high voltage or is it recommended to have an even higher voltage?

Thanks!

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2 Answers
snoobler avatar image
snoobler answered ·

1. Yes. And this is Voc, i.e., it doesn't have to deliver current at that voltage, it just has to present that voltage.


2. Yes, but it will be close. LFP gets floated at about 54.4V.


Is 19.8V Voc or Vmp, and have you actually confirmed they meet spec?

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dagfroderekdal avatar image dagfroderekdal commented ·
19.8 V is the Vmp (I assume this means the Voltage at maximum output). I do not intend to charge the batteries to 100 %, but only around 90 %. To increase the lifespan of the batteries. I haven't bought the panels yet, just read the data sheet.


Does the Vmp from the panels have to be 54.4 V + 5V to reach float? Or is 5 V just a convient way of saying that in the Vmp has to be +5V above the nominal voltage to be sure to reach the float voltage?

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bbmz avatar image
bbmz answered ·

1: I expect this to be true for all battery voltages.

2: From my point of view this is a very small headroom. I would suggest to grab a fourth panel. There is a high probability that your MPPT will get errors in MPP-tracking especially in combination with high temperatures or shading.


Keep also in mind, that the voltage of an PV-panel is not constant. It’s changing with temperature and sun intensity. Warmer panels will deliver less voltage.

If the panels have diodes included - to bypass in case of (partial) shading - this will also reduce voltage.

Please align all panels (if connected in series) in the same direction and keep in mind, that (if your panels haven‘t integrated bypass-diodes) even partial shadows on one panel will stop the production of the complete array.

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dagfroderekdal avatar image dagfroderekdal commented ·
But with 4 panels, won't they will be dangerously close to the Vmax for the charge controller (100 V) in cold weather? In Norway cold weather is also a factor :)
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kevgermany avatar image kevgermany ♦♦ dagfroderekdal commented ·

Yes. You should do the calcs first. There should be a label on the panels giving the value per degree centigrade and the starting temp, normally 25C. You're looking at a drop of more than 50C. Don't forget to multiply the increase by the number of panels.

Also be aware that the charge starts at actual battery voltage, not nominal. So if your system is at 54V, you need 59V from the panels to start charging.


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