Sizing an MPPT correctly

I just noticed something unusual today. If I were to size a 250/100 MPPT using a solar panel of 37.2Voc and 9.07Isc. I would do this 250/37.2 to know how many I can connect in series and 100/9.07 to know how many strings I can connect in parallel.
But today while checking Blue solar MPPTs datasheet, I noticed they specified the Max. PV array current to be 70. As opposed to 100. 100 is the output current of the MPPT.
We have a site that the MPPTs were sized using this method. And they've been operating for a while. Any help understanding how MPPTs should be sized exactly?

There is a magic number, like 30 or 40 percent voc over absorb voltage..... ive never honored it even though it is good design. Most of our runs to a charge controller fall between 80v and 160v anyhow and charge 48v batteries.

I use the function purely to get power home at higher electrical pressure in a fight against resistance and high current in pv source circuits for building practical size wire and conduit arrangements in the home run and pv overcurrent protection, etc.

You will likely not get the proportions you are looking for. It is just a result of the electronic design. Rarely if ever, would you need as much array current as you do charge current and they are related through power, power should be reflected when you do the math backwards. A pwm or other control will do this but we are talking step down controls and having 100a pv feeding a 100a control at higher than battery voltage would melt stuff.

Nother edit) your coldest temps recorded with a safety margin is required to approach max volts. This is a critical design parameter for these devices. The do not exceed voltage has to be known and the temp co-efficient in order to know how many (modules) in the series strings.

It is amazing to use -55 degF and see what would have popped a controller once in 20 or 30 years. When they are yours and even better yours and inexpensive, you can get courageous.

I just got back from wiring 2 flexmax 100-300 controls...... at 1000$ apiece, im not letting it be in my portfolio or have to bear the cost. Those controls cost as much as half my entire off grid system and i cannot afford the gold i spin for a living, especially not as a paper weight or door stop.

An FM 100 would make a great small boat anchor!

[quote= and i cannot afford the gold i spin for a living, especially not as a paper weight or door stop.

An FM 100 would make a great small boat anchor!


Now that is funny. It made my Friday night. I often feel when I try to explain load shifting or inserting the smart human to manage an off grid system many of today's clients with 5000watt arrays and auto generators just look at me strange... I will put in a new array for myself this year probably in the 2700 watt range for my 24 volt system. A pretty penny but probably time.
Cheers... David

I hoped other installers would get a kick out of that. I use leftovers to great effect and a cheap, but good inverter for now. Im used to droppinga 6-8k$ inverter system,

but for 225$ the powerbright 2300w 24v inverter has done an admirable job. 12w standby and it runs everything except a large portable compressor at -10 deg...... shrunken parts and thick oil, i guess runs fine any other time of year. The thing has been on for years, never turn it off!

These guys are 9600w, 1100ah/48v, big array structure sched 40 3" steel with the bottom edge 4.5' off the ground for snow drift/shed avoidance.

Not bad for a cabin in the woods and way cheaper than 250k plus the lifetime payment plan for electrons on tap.

Edit; thats not my drill motor.... !!!