I have one solar panel with rated power of 440 W±3% at maximum power point (Vmp) and 14.41 A±3% of short-circuit current (Isc) and 38.98 V±3% of open-circuit voltage (Voc).
Is SmartSolar 75/15 charger a right choice? From the maximum power (24V battery will be used) point of view and maximum open-circuit voltage this charger should be ok. I just worry about maximum allowed current of the charger which is 15 A.
If I take into account 14.41 A and add 3% I got 14.81 A. However, since Isc is a function of irradiance - at irradiances higher than 1000 W/m2 Isc could be even higher in my opinon. However, i think that much more than 15 A is not very likely.
Current at maximum-power point equals to 13.46 A by panel’s specs.Current at maximum-power point equals to 13.46 A by solar panel’s specs.
I put all needed info in the calculator and I got a suggestion - SmartSolar 100/30. Any idea how the calculator got such powerful charger’s model? I hoped that at least model 100/20 would be more than enough?
Chrigu, you are right. I wasn’t paying enough attention, I selected 12V
On 24V I don’t get any suggestion: No result found. Link: https://mppt.victronenergy.com#7VY8xCoAwEAS_YpHqWCF32QQlT7D0BSIpFFGx8P1GO5lui1kmW2ZySldX3wzHWS4043bcZV_WKSdmaGBCBJRUgPSYvxwESz1aiwhd3_0bRSvvm1SvqAm9PA
I was trying to increase the Vmp voltage from 32.69 V with 0.1 V steps and at 34.9 V calculator suggested model 100/20. It looks like that calculator “see” too small difference between battery’s voltage and Vmp of the solar module in my example. It looks like that calculator “needs” enough difference when battery is full also. I’m planning to use LiFePO4 battery which should not have a voltage higher than 29.2 V. In the datasheet is clearly noted that difference between battery’s voltage and Vmp must be higher than 5 V for the start of charging and then diff can be only 1 V. From my point of view it is completly fine if charger doesn’t start to charge the battery till its voltage drops to around 27 V. At this voltage battery would have around 80-90% SOC. Taking into account this limitation It looks that model 100/20 would work also on 24 V system? Did I forget something?
The MPPT itself needs to have 5V of difference in order to start working, that is correct. If you are very close to that border, then it might be possible that on cloudy days or now in the winter you might not be able to surpass that.
Panels are rated at STC, standard test conditions, which means 1kW/m2 irradiation, optimal angle to the sun and cell temperature of 25°C. You very rarely get all of these conditions at the same time in reality. Even 1kW/m2 irradiation alone is wishful thinking for most of the year.
In theory your PV voltage should be high enough even with a 24V battery connected, but there might be long stretches of cloudy weather or similar that will lead to no charge at all.
I think the 100/20 is due to the MPPTs output current, since 20Ax24V is 480W and a 75/15 would only be able to output 360W max, so you would be limited to that, but im actually not entirely sure why, since if you use multiple of your panels in series, both the hot and cold current are always the same, and always the max of the particular MPPT, which seems strange. Are you sure that the temperature coefficients are correct?
yes, coefficients are pretty much okay. Temp. coefficient of Voc is -0.25%/°C and temp. coefficienf of Isc is +0.046%/°C, however during calculating +0.05%/°C was used, which doesn’t bring a very huge difference.
Datasheet of the solar panel (model: stp440s-c54/nshtb+):
Completly understood and agree with you. I’m just now checking how Voc changes with irradiance (at fixed temperature) and it looks like that affect is not very big. Bigger affect on Voc has a change of the panel’s temperature. However, I agree with you it could be very tricky - if panel’s Vmp drops to 30 V, start of the charging would begin when voltage of the battery would drop to 25 V. At that voltage battery would be almost empty
To proceed with 12 V system would be much more reliable as it looks.
