Help getting design started....

[sincity337]

Hi Folks,

I am very new to the world of PIC's, and would like some advice on which direction to head in a project I'd like to implement using PIC's. Some backround on myself first and then the design. This is long so get comfortable...:-)
I have a basic grasp of electronics (just passed ISCET certification, studying for FCC) and have been interested in using PIC's for a long time. I bought a bunch of books and have a programmer and demo boards, but never got past blinking an led.
Just recently, I finally convinced work in footing the bill for the PIC class offered by RCG Research. It was a great class for someone like me. Someone who has never really done anything with PIC's, but understands the general usefulness of them. It also helped that I had some electronics training.
Anyway, after having gone through the class, I feel that I can now jump in with both feet and begin experimenting. Which brings me to where I think I need help in deciding which way to go, as I have an idea where the PIC and all it's power can help me out.
Here is what I have currently: I have a battery that is comprised of 15 cells connected together. It is charged via a constant current battery charger that feeds 4 amps (voltage varies) to the battery until I decide if all of the cells reach 2.05 volts. Each cell has to be checked individually as they NEVER come up to full charge at the same time and it is critical that they do not exceed the 2.05 volts for any length of time, as it tends to short the cell.
Since cells do not come up to 2.05 volts evenly, AND the battery as a whole is being fed 4 amps, when a cell does come up to charge, it is "shorted" with a resistive wire, while the other cells continue to charge. That "short" cannot stay on for too long, as the cell will now slowly lose it's charge. As you can see it becomes a rather tedious task of shorting, and unshorting each cell until they all come to their full charge. Lot's of fun let me tell you.
What I would like to do it feed the charge current to a bus that can supply each cell individually through a relay, MOSFET, or some high current capable "switch" that can be controlled via a PIC or series of PIC's. Obviously the PIC would need to read each cells voltage, then make a decision as to either open the "switch" or leave it on based on that cells indivdual voltage reading. I would also like to display a cells voltage if selected (through a button press or maybe a rotory switch?). As well, during a "discharge" cycle, the PIC should be able to turn that switch on or off depending on the cell's voltage.
I know it is possible to do this with PIC(s), but what kind of approach should I take? Do I use a high pin-count PIC that can do it all, or do I use multiple PIC's talking to a "Master" PIC? I don't want to take a super long time developing this, cells are VERY expensive and this would greatly extend their useful life. I can buy a really fancy battery management system for about $80,000 US, but I would like to do this as inexpensively as possible.
There are a lot of really smart people here and I would like your educated opinion in which way to lean as far as using lot's of low pin-count PIC's or a few/one high pin-count PIC. By all means, if you have questions ask away!
Any advice would be greatly appreciated.

Thanks in advance.....

James

[dmairspotter]

but it would seem like you could implement this on one PIC chip, like the 16F876A

PORTA analog inputs

PORTB 8 digital out
PORTC 8 digital out

either use a mux chip to select the cells to be read by the on chip A/D or use an external A/D chip (or two) to read in all the cell voltages. Use ports B&C to control the cell switches.

[rossfree]

Hi James,

Curious... this multi-cell battery arrangement... are all of these cells in series? If so, can you add wires in-between each of the cells... and pulse charge them individually depending on their current voltage reading?

I'm no genious... but it seems if you had access to each cell, you could first read it's voltage, then smack it with a charge pulse and then move to the next cell. If the cell was fully charged, you'd skip over it. You could store each of the cells voltage for recall and viewing at any time on a display. You would not need to short the cell with a resistive charge. This assumes you have the multi-cell battery off-line for charging... such as a cordless drill battery.

The pic is a good choice. If you are new to PICS might I suggest that you get playing with some code a bit... scrounge the internet for some circuits that might be related to what you are doing and start playing with it.

I assume you have PicBasic Pro.

I purchased Microcode Studio Plus with it's Bootloader. The 16f877 is a nice fat chip with plenty of breathing room and peripherals. It's more than you might need but inexpensive enough and you can stick the bootloader in it. I can't say enough how much the bootloader has helped me along. It's great for a beginner. You write a bit of code, press a button (to load your program into the chip) and see what you've done... instant feedback!

I would love to hear more about your project.

Good Luck,

Ross

[email protected]

[sincity337]

Ross,

Thanks for the info. I think you are suggesting some of the same things that I was leaning towards. I have been doing lots of searching on the net and found some "universal" chemistry battery charger chips from Maxim, can't think of the model number off the top of my head right now. Apparently it will do what I want, I just have to set up the voltage and current reference and it applies and releases the load as needed. I guess I could use the PIC as a "supervisor" if you will, to enable each cells charger and even read and display it's voltage reading.
I am currently waiting for my supply guys to order the evaluation kits for the Maxim IC's. If I get them to work, and work with a PIC I will definitely let you know how it goes.
Thank you so much for the input. It is really important for us novices to get input and helpful hints from people that have more experience.

james

[sincity337]

Just for the curious:

I have requested information from Vicor for their power supplies that may work for my battery cells. I need 2v @ 4A constant current to charge the cells, and they have a system called the MegaPAC that holds their power supplies mounted as cards. Each card is controllable via a logic gate to turn it off and on. I think I might be able to easily (HA!) program a PIC to monitor voltages for each cell and switch the power supplies off when the cell voltage reaches 2.0v.
Of course I really need to sit down and decide what other features or controls I would like to include in my program.
Anyway, thought you guys would at least like an update on what I was doing with this project.

James

[Acetronics2]

Hi, Sincity

There's a simple solution, not needing PIC's !!!

Have a trip to Modeller's forums ( www.rcgroups.com i.e. ) and search for LiPo Balancers ...


THE link : http://www.rcgroups.com/forums/showthread.php?t=270580

Dan Baldwin's scheme is, I think, the most efficient here ...


The goal is not to have a cell voltage over 4.2 volts ...

I think it sounds something good for you !!!

Alain