controller calc

Re: controller calc

ws9876 wrote: »

say you have 2 panels 24v 250 watts each wired at at 48 volts.Average amps.
You want to run a 12 v inverter off of a 12v 100 amp battery bank for a year until you get more gear.Later it would be wired into a 48 v inverter and battery bank.
Can you run a 30 amp controller mppt for awhile ??? even if it wont run at max charge???

Yes you can.
You're basically looking at 500 Watts of array into an MPPT controller.
Output to a 12 Volt system would be (500 * 0.77 / 12) 32 Amps peak, which you may never see depending on conditions. Any good MPPT controller will simply 'clip' that extra 2 Amps.

BTW on 48 Volts you could have two problems: one would be if those "24 Volt" panels had a Vmp around 30 instead of 35 in which case the Voltage could be too low to properly charge a 48 Volt system; the other would be the mere 8 Amps of current you'd get from that 500 Watt array being probably too low for any practical purpose.

Re: controller calc

ws9876 wrote: »

So you think the KID controller would be ok???
btw later there would be more panels and the voltage in series would be maybe 70-100v ..
but about the other....yes your charge current could be low with 3-4 panels but watts is watts so I dont think of it as too little to be useful.
1 amp to a 12v battery is a joke but 1 amp to a 48v is much more really,no??

More like 1 Amp to a battery larger than 10 Amp hours at any Voltage is a joke.

The Kid should handle this fine.

Re: controller calc

unseenone wrote: »

I am running a similar configuration. Reality of panel outputs don't seem to come near expectations. I'm not familiar with the KID, but maybe it is similar? If you used, say a Morningstar mppt 45 controller according to the manual (I would suggest considering the 60) you could run;

Nominal Maximum Input Power*

PANEL
VOLTS

---

MPPT30

---

MPPT45

---

MPPT60
12 Volt

---

400 Watts

---

600 Watts

---

800 Watts
24 Volt

---

800 Watts

---

1200 Watts

---

1600 Watts
48 Volt

---

1600 Watts

---

2400 Watts

---

3200 Watts

It isn't the controller that makes the difference so much as the panels in the real world versus "standard test conditions" (STC) i.e. laboratory testing that gives them their rating. When you 'flash test' panels under controlled circumstances they put out their 'ideal' values which are not the same as what you typically see under normal operating conditions.

So for a 30 Amp controller the 'lab math' is 30 Amps * Voltage, and they can select any Voltage within the operating range for the system. You thus get a 12 Volt range of 360 to 444 Watts for input.

The real world calculation for approximation is to use the nominal system Voltage (which is as low as you want to go as it represents about 50% SOC and is the point where maximum current is needed - loads are disregarded because they are too variable) and the 'typical' 77% average operating efficiency. Thus you get 30 Amps * 12 Volts / 0.77 or 467 Watts maximum on the controller.

That is just a starting point. You have to take into consideration your particular operating circumstances, such as high heat or cold temps or anything else that varies insolation, as well as the controller's own recommendations for maximums. Some will easily take much more Watts than others, 'clipping' any surplus power. Some have very strict limits on input vs. output even though they are MPPT types. An example of this difference would be the MidNite Kid as being very flexible and a Blue Sky MPPT as being not flexible for input.

Sorry it's not black-and-white and that this point of design is not emphasized enough. The rules-of-thumb and basic calculations only get you to a starting point. That will work most of the time but the particulars of the installation should be checked to see if it is right for your application.