MPPT charger wishlist

[FurtherThanTesla]

Hello, I'm an electrical engineer and I got involved in a project requiring a solar battery charger. I surprised to find how expensive MPPT chargers are, especially the american versions "Morning Star, BlueSky, etc.". Why would I spend $100 or more on an MPPT that gives me a 30% efficiency boost, when (in the face of lowering solar panel costs) it's cheaper to just spend 30% more on solar cells? I decided to put in some effort to designing my own with the aim of turning it into a marketable product for people like myself, and other DIY-ers interested in small-scale solar projects. I'm not looking to sell or advertise anything to anyone on this forum. Just some "market research" from one Maker to another. Give me your MPPT charger wishlist!

My prototype is essentially the same specs as the Chinese MPPTs, but at a lower cost, and (possibly) integrated wifi connectivity for viewing data, and configuration. My questions are...

do other people find the cost of MPPTs too high when compared to using PWM with a slightly larger PV?
Is wifi connectivity something you would actually use to view charge data?
would you prefer a hardware interface like RS-485, or RS-232? Or just a plain LCD?
How many people are using 120-480W MPPTs for RVs, boats, etc. or industrial uses?
Is a waterproof MPPT particularly desirable?
What operating life do you expect to get from a budget solar-setup?
How many would be willing to pay $10 extra for double the service life?

The estimated specs of my current design are as follows:
Charges 12-24v lead-acid battery at 20A (240W for 12v system, 480W for 24v)
3-stage charging (bulk, absorption, float)
95% efficiency
Integrated panel diode (No diode needed on panel)
Short-circuit protection (Current limiting)
Status LED to indicate MPPT, absorbtion, float and error
Minimum operating life 6yrs
Est. price $40-$60

Possible features:
LCD display, or integrated Wi-Fi (display charge data on a screen, or on the internet?) at no extra cost
Waterproof (If so, then no LCD) at $10 extra
RS-485 (MODBUS) interface
Better quality Caps (12yr est. operating life) at ~$10 extra cost
Logic level low-battery indicator output (wire to a relay or something controlling battery load)

I have a lot of respect for the knowledgeable people on this forum, and would really appreciate any feedback you may have to offer. Thanks!

 

[xenodius]

I'm in the planning stage of a ground-up carbon fiber/solar faired velomobile, I'll be using semi-flex monocrystalline panels primarily, and possibly polycrystalline panels for the extra curvy areas. I will definitely be using a charge controller, and I'm interested to hear about your process of building one yourself as I will most likely have to do the same. First couple I looked at were arduino based, which is great-- but, those are again designed around really low voltages.

I'd like one that charges up to 90v, but I haven't seen any small commercial options in that range that are not nearly 4 figures. If I ever want to balance my pack via my Adaptto display off of solar instead of plugging in, then I'll have to have a way to lower that to less than ~66 volts, but I'm not sure if that's as important to me. Depends on how much I can really get out of a "400w" set of panels, if I can get 200w for just a few hours each day then I might be set for my regular commute. I suspect i can get about that much power from them for longer than 3 hours... anything over 300wh/day would be truly fantastic.

Curious what layout you go with, as it sounds like you have a lot more electronics experience than I.

 

[FurtherThanTesla]

I'm in the planning stage of a ground-up carbon fiber/solar faired velomobile, I'll be using semi-flex monocrystalline panels primarily, and possibly polycrystalline panels for the extra curvy areas. I will definitely be using a charge controller, and I'm interested to hear about your process of building one yourself as I will most likely have to do the same. First couple I looked at were arduino based, which is great-- but, those are again designed around really low voltages.

I'd like one that charges up to 90v, but I haven't seen any small commercial options in that range that are not nearly 4 figures. If I ever want to balance my pack via my Adaptto display off of solar instead of plugging in, then I'll have to have a way to lower that to less than ~66 volts, but I'm not sure if that's as important to me. Depends on how much I can really get out of a "400w" set of panels, if I can get 200w for just a few hours each day then I might be set for my regular commute. I suspect i can get about that much power from them for longer than 3 hours... anything over 300wh/day would be truly fantastic.

Curious what layout you go with, as it sounds like you have a lot more electronics experience than I.

Correct me if i'm wrong, but it sounds like you are using a 400W set of panels to charge a 90v battery pack for an EV? The main reason there are no 90V MPPTs is because most of them (including my design) use a "buck" topology, where the output voltage must be less than the input voltage. It is possible to make a "boost" version, which was a considered design of mine, but it ends up being a little more expensive for the same power. If I were you, I would get an MPPT that outputs 24v or 48v (depending on how many panels in series), feed it into a beefy capacitor, then use a DC-DC to boost that 24 up to your charging voltage, and feed it to a charge controller/BMS. Plain old DC-DCs can be had pretty cheap.

The layout I am using is not anything new. As mentioned, it's a typical "buck" converter run off a 32-bit ARM uC. In fact, i'm not really using any new design at all but I simply found the MPPTs I have seen could be designed a lot better, and could be given better parts and more longevity for the same cost. I worked around the solar industry for a year, and even though the build quality was good, I was generally disappointed with the outdated design practices used on industrial-scale MPPTs and inverters. I stopped working for the company that bought them, thinking I should be designing them. Now I'm unemployed and spending all my savings on electronics parts and equipment, hoping it gets me somewhere!

 

[FurtherThanTesla]

Have you looked around at any of the existing DIY MPPT projects?

http://www.google.com/search?q=diy+mppt

I have seen a good few DIY MPPTs. Most are more simplistic and less powerful than what my breadboard prototype is now. I would be interested to see what tracking algorithm others are using. I get great power output from mine when controlled manually, but my P&O MPPT tracking algorithm is misbehaving, and needs more work.

 

[xenodius]

Correct me if i'm wrong, but it sounds like you are using a 400W set of panels to charge a 90v battery pack for an EV? The main reason there are no 90V MPPTs is because most of them (including my design) use a "buck" topology, where the output voltage must be less than the input voltage. It is possible to make a "boost" version, which was a considered design of mine, but it ends up being a little more expensive for the same power. If I were you, I would get an MPPT that outputs 24v or 48v (depending on how many panels in series), feed it into a beefy capacitor, then use a DC-DC to boost that 24 up to your charging voltage, and feed it to a charge controller/BMS. Plain old DC-DCs can be had pretty cheap.

The layout I am using is not anything new. As mentioned, it's a typical "buck" converter run off a 32-bit ARM uC. In fact, i'm not really using any new design at all but I simply found the MPPTs I have seen could be designed a lot better, and could be given better parts and more longevity for the same cost. I worked around the solar industry for a year, and even though the build quality was good, I was generally disappointed with the outdated design practices used on industrial-scale MPPTs and inverters. I stopped working for the company that bought them, thinking I should be designing them. Now I'm unemployed and spending all my savings on electronics parts and equipment, hoping it gets me somewhere!

That is actually a really obvious approach... Ideally, I'd have two MPPT's-- one for balance charging at ~50v, another for 90V bulk charging. Most of the converters I've looked at are only ~85% efficient, though. There are so many 12v DC-DC converters and junk products out there-- do you recommend a brand? I know Meanwell makes some, but they're not particularly efficient either. Everything else I've found are just efficient but small and expensive SMT components, nothing with the right power or voltage output...

Perhaps if you want something done right, it should be done yourself... =) Though I wonder if I could mod a DC-DC Meanwell for higher voltage!

 

[FurtherThanTesla]

I cannot vouch for the reliability of any particular converter. I see what you are saying about the efficiency though. Your application is rather unique, and I think this is the point where I would start building my own device to fit the need. I have not seen anyone else using low voltage solar panels to charge a high voltage battery bank, but it makes sense in your setup. Mean well supplies are pretty good in my experience, but I doubt they could be modded for much higher voltage. I did tweak a 54v supply to give me 57.6v, but that was it. I'd figure you could replace caps with higher voltage ones, but the main switching element will be a hard limit.

I believe the reason most of the converters you mentioned suffer from poor efficiency is that they use a basic buck topology. Since you are converting to a much higher voltage, and need serious efficiency, I would say to look for a full-bridge topology, or make one yourself.

 

[xenodius]

Thank you for providing excellent keywords to guide my search. I don't have any experience with circuit design beyond basic ebike wiring, but it'd be very cool to build my own device... although, I may simply find some efficient, adjustable 48v supplies and use a pair of them in series-- We'll see. If doing it myself means I shave off ten pounds, or get an extra 5% efficiency, I might do it! I just have to get as much out of a given surface area as possible. I'll look into running commercial MPPT units in series, that might be the ticket...

 

[FurtherThanTesla]

Woah, hold on there. If you wire the outputs of two MPPTs in series, they will fry! Reason being is a typical buck converter's neg output is shorted with its neg input. It's just a fundamental part of the design. Tying the neg output of one with the positive of the other is the same as shorting it. That would only work if the converter uses an "isolated" topology, which none of them probably do because they are not designed to be wired in series. Only very high power MPPTs are isolated, and that's there as more of a safety feature.

It really sounds like you are either going to have to use a DC-DC, and deal with the efficiency hit, or build your own MPPT from scratch with a full-bridge topology.

I would not recommend the latter option. I can say from experience that power electronics is a tricky business. The parts are expensive, the test equipment is expensive, and one wiring mistake can fry it all! I once tried to build my own motor driver for my electric bike to get some extra power. I spent 20hrs cutting traces and soldering components. I had it up and running for 90sec on my test motor before a wire slipped off the motor and sent an EMF spike back into the driver which subsequently fried my microcontroller, driver chips, 2 mosfets and my oscilloscope. As for the circuit board I spent two days making, it just caught fire. It's worth the educational experience, but it's not worth a 5% efficiency gain! Good luck though!

 

[xenodius]

That's unfortunate. I've already emailed a couple suppliers if it was possible, hope it gives them a laugh. I'll probably look for some particularly efficient DC-DC's that support wiring in series. But, I do have access to a lot of test equipment and oscilloscopes... Anyway. Thanks again for all the input!

 

[jaunty]

How about a digitally controlled MPPT?
I have a Chinese version which has 12V and 24V volt settings. Some versatility would be awesome.

 

[Alan B]

Isolated converters are required for series connections.

 

[HydroDan]

this sounds cool. i have wanted to do a DIY charge controllers

i am surprised you are complaining about $100 charge controllers!!! the good ones we use, like Apollo MPPT charge controller, are around $800+!

we often use the cheaper xantrex c40 charge controllers, which are around $100. but i dont think they are really "MPPT" but simpler PWM charge controllers.

what i would be interested in is these things:

• can charge 48v packs
• high input voltage, like at least 100v, ideally even higher like 500v
• adjustable full charge/float voltage settings so i can use other battery types like LifePo4 (the xantrex c40 even has this)
• actual MPPT, not just PWM

 

[dmwahl]

Why would I spend $100 or more on an MPPT that gives me a 30% efficiency boost, when (in the face of lowering solar panel costs) it's cheaper to just spend 30% more on solar cells?

Because not everyone has infinite space.

Is wifi connectivity something you would actually use to view charge data?

Yes, definitely.

would you prefer a hardware interface like RS-485, or RS-232? Or just a plain LCD?

If I had to choose 1, I'd go with a hardware interface and use the wifi in place of an LCD. I happen to carry a small, wifi connected LCD in my pocket most of the time anyway.

Is a waterproof MPPT particularly desirable?

Water resistant is probably a good idea, but I don't think you need full waterproofing. They usually go near the battery packs anyway, which is generally a protected location.

What operating life do you expect to get from a budget solar-setup?
How many would be willing to pay $10 extra for double the service life?

Even the cheap chinese stuff should last a while if used within it's limits in a stationary setup. For mobile use then I'd certainly pay the extra money, but be prepared to demonstrate how you know it will extend the service life. Simply stating it's "more rugged" or whatever won't cut it.

Also, some kind of interface to stop charging would be nice. Unlike lead, with lithium you don't want to keep charging after they're full. A normally high input that can be pulled down by an opto-isolator to cut off charge would be ideal, and is similar to what already exists on other chargers.