Hi everyone, I’ve got a project I’m wanting to build, and its quite complex, but I think I can handle it with just a little guidance.
The scope of the project is to build a 3 legged MPPT system. The physical layout of my property, and other limiting physical factors, such as buildings, and trees, require that I have my PV panels laid out in 3 separate arrays, one facing E, one S, and the last SSW. Each array consists of 2 x 15W panels. 22Voc.
My current charge controller simply combines the arrays in parallel through a diode on each leg input. This combined input then feeds a PWM charge controller which is connected to a pair of 100Ah “deep-ish” cycle batteries.(automotive style liquid lead acid trolling motor), LVCO(loads) and night light controller.
So I’ve been reading and collecting parts lists, and I think what I have settled on is a synchronous buck arrangement. Well, 3 of them. I’ve found several schematics for these, most of which use IFRZ44 FETs, and a driver chip. Easy enough. While I realize that these parts are a bit of overkill for the current size system, if I can make this controller work, panel and storage upgrades will be next. (also let it be known that this project is as much about having an idea and trying to make it work as it is about actually improving my PV system)
Here is where the design decisions become sticky, and I’d like some input. The panels are currently paralleled in each leg, meaning that each input wire coming in from outside is 22Voc and the panels are + to + and – to – out in the field. Theory was that I could get 2xpanel current coming in on each of the 3 legs. Due to long wire losses, and the impedance mismatch between the panels and the battery, at best I’m getting 1A @14.7v from each leg during charging. Half of the current I would get ideally.
So the basis for all this was that I was thinking that if I put the panels in series in each leg, thus getting 44Voc, but trying to push less amps through long wires, I could use a MPPT circuit and improve the system efficiency.
So, using a pro micro, and figuring I’ll need 4 measurements from each of the arrays, plus a control signal to each of the MPPT’s, I’ve run outta pins. Enter the I2c ADC. I’ve seen 8bit ones, 10bit ones and 12 and 16’s. But really, how much resolution do I need? I’m thinking that 10bits should be enough(cause the pro micro uses 10bit for it’s internal ADC’s). And they have 4 and 8 channel ones. I’m thinking this sounds great, I2c is easy, and I can have up to 4 of the 4chanel adc’s I found all on the same com lines. If I use 3 of them, one for each array, That gives me 12 inputs(and in my mind will make coding easier cause when I address the ADC I can just group my readings together) and then I can read the battery state directly through the on-board pro micro ADC
what about timing? Can I read that many inputs through the ADC’s via I2c fast enough to be useful feedback to control the 3 MPPT’s?
Would an analog MUX be faster? I’d have to use a couple pins on the pro micro as control lines to the MUX, but if I have all of my readings coming in through the MUX, then I have pins to spare. That also raises the question of noise, or reading instability, by going through the MUX.
Are the 3 MPPT circuits going to battle at their outputs? Do they need to be combined through diodes so that when one is a little higher V output, this doesn’t fool the others into trying to reduce their outputs?
Going further into the project, I’d like to collect the data, and post it online so I can see it from my phone from anywhere. I could also see adding the night light control function and LVCO, but those will be easy if I can get all the rest to work.
So, whaddya think? Can a 5V 16Mhz Pro Micro handle all this?