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Workaround for Cheap MPPT Controller?

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MagicBlueSmoke:
Hi,

I have an urgent need to get a better solar power system going. I have a chance to get a 280W panel for dirt cheap, and I'll need a better controller than the Harbor Freight one I have.

While searching for one, I've run accross the MPT-7210A controller. It seems to be a real mppt device, the catch being that it's a boost converter based controller. Which makes it suitable for those almost unheard of situations where the storage banks are higher voltage than the panels used for charging them.

This poses a problem for me: The panel I have this chance to get produces 35.2Vpmax, 44.8Voc, 7.95Apmax, 8.33Asc. I currently have a single 35AH AGM-SLA battery from Harbor Freight. Getting a higher voltage battery bank isn't an option for me in the immediate future. With the higher power panel, I could at least run my laptop during daylight while still keeping my battery charged for lighting at night.

Than I had the "bright" idea (solar pun intended) of sticking a cheap buck DC-DC converter in between the panel and the MPT-7210A, reducing the voltage to around 12V to let the boost controller do its mppt thing to charge the battery. Would this be feasible? Would the buck converter let the variability of the panel's output just pass through to be handled efficiently by the boost converter?

(And if my idea is absurd... Please be gentle!   :-DD )

NiHaoMike:
I think the use case is for 48V systems. That's a common voltage used by those who have expanded their systems beyond what a 12V system can practically do.

For your case, maybe build your own buck MPPT controller? Start with an existing buck converter design and add a signal to limit the duty cycle. (Consult the block diagram of the switching controller used to find out how.) Then find a way to use a microcontroller to perform MPPT.

Or go all the way to a digital power design with a dsPIC or similar directly controlling the buck converter. I'm actually working on one for a 65W, 18V panel I recently managed to get for cheap.

MagicBlueSmoke:
Thanks for the suggestions. I'd love to take on a project like that. But my resources & situation don't allow for it any time soon.

I did have a facepalm moment though: Maybe I could just put a buck converter between the cheap mppt controller and my battery? The controller's output should be fairly stable, enough for a buck converter to handle?

I understand there's the split-pi converter topology, which is a boost-buck converter. So maybe I'm not completely bonkers? I know this wouldn't be a proper split-pi device, but do I have a chance of getting it to work without blowing up what little equipment I have without losing too much power? I'm just looking for something to get me by until I can put together the resources to do a proper job of it. I really do hate resorting to cobbling things together like this, but I'm in total bootstrap mode right now out of sheer necessity. I hope I'm not being a total pest.

NiHaoMike:
If you try to cascade the two converters as described, it would probably just end up being unstable.

A buck converter with the duty cycle limited to a certain value (can generally be done by clamping the compensation pin voltage with a potentiometer to ground or similar) would likely work fairly well (would need manual adjustment with the help of a multimeter) and be easily upgraded to a proper MPPT setup later on. No need for the boost part.

Someone:

--- Quote from: MagicBlueSmoke on September 11, 2016, 12:01:20 am ---Than I had the "bright" idea (solar pun intended) of sticking a cheap buck DC-DC converter in between the panel and the MPT-7210A, reducing the voltage to around 12V to let the boost controller do its mppt thing to charge the battery. Would this be feasible?
--- End quote ---
Battery charging/management relies on measuring the battery voltage (among other parameters) so you would need to do quite some work to have the output of the MPT-7210A (or any other battery charging/management device) "see" the battery load it is expecting. If you want a system with separate MPPT and battery management you'll need modules designed for this.

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