Still here - how is YOUR summer?

Just a quick note:  am still here - and have not abandoned this project.    But have been kind of distracted with maintenance needed on our boat after it sat unattended for a full year.  Have also been working a bit on refinements to the Alternator Regulator project.

I do have the new v0.0.3 PCBs on hand, and parts for it all ready to solder up.  But with all that we have 'planned' for the summer there is a good chance I will not have much time to work on this for another month, or even more, but I am still interested in completing this MPPT platform.   So hand in there.

And if by chance there is anyone wanting to pitch in, I do have a couple of extra blank PCBs!

-al-

Time to checkout the Magic Smoke!

I have finished assembling a solar MPPT controller - here are a couple photos:

I really should not have the camera THAT close.
Those Caps look like towers!

Have also been working to clean up the Arduino IDE and the port of the ATmega64M1 CPU.  I think I have most things fixed and will post corrections as I continue the debugging effort.  A bit on that: the porting is to the IDE version 1.0.5 / 1.0.6.   Arduino just released their 'updated' IDE v 1.6.0, and it will be much better - mostly because it uses an updated version of AVRDUDE and the GCC compiler - providing native support for the ATmegaxxM1 series of CPUs.   However, there has been some large changes in the support files - specifically around the hardware serial port, and it will take me some time to work though that all.  So for now, I am continuing my efforts under the 1.0.x line of the IDE.

Just last night I moved to start some live testing using the Solar Panels on Viking Star.  It is perhaps a good time to do this as the weather has changed, and our overcast winter sky's limit the panels output.  So far results tell me there is still a bit of work to do.  Am not happy with the +12v boost power source, and have worked though a fuse or two already...

But will carry on, and see what all I can get working.

Turning the PCB - and Arduino internal ADC accuracy

I have found enough errata to want to do a turn of the PCB.  v0.0.2 was send to OSHPark Thursday, as with a Mouser order.  Should have them in in a couple of week.  I will work on porting Tom Nolan's simple MPPT software (https://web.archive.org/web/20140324221651/http://www.timnolan.com/index.php?page=arduino-ppt-solar-charger)  to the SmartMPPT controller in preparation.  v0.0.2 has several changes, including:

• Corrected SMT footprints
• Cost reduced power supply, USB designs.
• Removal of SERVICE port (functions could be accessed via ICSP port)
• Improved voltage sampling buffering ckt - removal of offset errors in op-amps

Overall cleanup of board, and improvement in tight Hi/Low FET layout.

One area I struggled with a little was the voltage sensing design - based around using the Arduino's built in A/D.   In short - how much $ vs. desired accuracy?   One might noticed I backed off from the 0.1% resistors in the voltage divider, and now just use 1% devices.  This saved about $1 in BOM cost.  And here was my thinking:   The ATmega32M1 is able to selected one of two sources for its A/D converter reference:  External (Vaa, or +5 in this design), or an internal 2.56 source.   And there is the basic issue.  The internal source is known to be rather stable over time and temperature, but not all that accurate (2.5-5% or so).  Using Vaa tied to +5, even with the filtering, can present a significant error its self (And is dependent on the accuracy fo the +5 supply to boot).

All told, using high precision components in the op-amp ckt seemed kind of like putting silk on a pig's ear.  Additional cost could have been added by adding an external voltage reference (i.e. LM4040) - but I think even then we would not know the BATTERIES voltage, only what we see at the output of the MPPT controller.

So, I have  backed off - and accept a few % error in absolute accuracy for voltage readings, but still look for stability.  As far as the MPPT logic is concerned, it is trying to maximize a value - and it really does not care too much how accurate that value is, just that it is repeatable.


For accurate battery voltage information, to say decide charging states and accommodate voltage drops over the battery cables, will look to the CAN BMS device attached via a simple CAT-5 cable.
http://smartbms.blogspot.com/

Status of prototype build.

We are now in the Portland OR area and could fetch mail (and boxes) from our mail drop.   Most all the components have arrived, and the PCB should be here soon.   But there is one delay:  the inductors from CoilCraft are on back-order until the end of January... 

Oh Well, will give me time to play with other parts of the bringup.

v0.0.1a of hardware design released

Today I edited this BLOG to put in the resource links above, and also posted the final release hardware design for the initial build v0.0.1a:

See 'Schematic' link above for larger image .pdf file

And here are some 3D renderings:

Top view

 

Bottom view

The module is approx 3x4" (actually, 80mm x 100mm), utilized 100v FETs and caps (I am suggesting keeping solar panel voltage below 85v), and can support up to 25A battery current.  These give it the following range of charging support per module:

• 12v battery:         345W capacity
• 24v battery:         690W
• 32/36v battery: 1,000W
• 48v battery:      1,380W

 To each the above capacities one or more solar panel can be used.  Example, for a 12v system one could use a single large 280W panel per controller.  For a 48v system, 4 such panels could be deployed in a combination of serial/parallel connection.

The costed BOM come to $73.30 + price of PCBs, which could be found for under $10 using overseas supplier.  I expect parts to arrive some time in December and will start assembling one unit then, and when I return to Viking Star will be able to start playing with trial runs on our solar panels as well as start developing the more advanced firmware.


For those interested there is a LTSPICE model of the buck converter using in this design under the CAD resource tab above - kind of fun to play with.