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Old 04-15-2009, 05:23 PM   #1 (permalink)
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Solar panel EV charging (ForkenSwift): can anyone help set up a 555/relay circuit?

I want to put a solar panel on the ForkenSwift to trickle charge its 48v battery pack. The car often sits unused for days at a time, and when it is used, it's for short trips only. So using a panel isn't an outrageous idea in this case; it could measurably reduce grid energy consumption.

I don't want to modify the panel itself (12v nominal, 85w, rated for 21v peak, open circuit).

I also don't want to modify the pack (eg. to split it into four 12v pairs, wired in parallel for 12v charging).

Instead, what I was thinking was "rotating" output from the panel to pairs of 6v batteries using an automated relay circuit that switches sequentially through the 4 pairs of batteries every N minutes (where N is a small value, say 2, to minimize the chance of unbalancing the pack as light conditions change).

I've read a small amount about the "555" timer integrated circuit, and it seems (to my non-expert electronics mind) like it might be a suitable foundation for this idea, in combination with some standard 12v relays.

Any of you electronics wizzes willing to guide me in setting up such a circuit?

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Old 04-15-2009, 05:35 PM   #2 (permalink)
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I might suggest a dc-dc converter of some sort. Conceptually you take the solar panel output and drive an oscillator with it, then run that through a transformer to step it up to the voltage you want (> 48 volts), then rectify that and charge the batteries with it.

So basically a 555 and a ~30volt dc wall wart should have everything you need (24v would probably work fine too).

There is probably a chip that does it too, that takes a little digging though. I'll see if I can sketch up what Im talking about.
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Old 04-15-2009, 05:39 PM   #3 (permalink)
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Thanks for the idea.

One reason I thought of using the "merry-go-round" relay approach is that it's a more efficient use of the panel. I assumed there would be greater losses going through a DC-DC setup.

EDIT: also, I like the idea of hitting 12v pairs with higher current every N minutes, rather than 1/4 (or less) of the current at a constant 48+ volts nominal across the entire pack. The higher current will mix the electrolyte better, and may be more effective against preventing sulfation (an EE who is an EV owner mentioned that to me).
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Old 04-15-2009, 06:00 PM   #4 (permalink)
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You could use several power fets to do that (similiar to synchronous rectifying). But if you go down the 555 path, you start adding somewhat dated counter logic chips where a pic/avr (I'm a broken record, I know) would be both low power and simpler to design a circuit around. I would probably run the avr off a couple (3) nimhs just to keep it simple and avoid draining anything else, and turn the avr clock down to 1mhz (or less) on the internal clock for nice long run time. I'll draw a picture.
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Old 04-15-2009, 06:05 PM   #5 (permalink)
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I'm not married to the 555 route. I just read about them once (blinking LED project), saw the word "timer"... And if the only tool you've read about is a hammer...

I have basic experience with the 'duino, and everything needed to program its chip to put it in an iDuino. Would that work in place of pic/avr? (Are they effectively the same thing?)
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Old 04-15-2009, 06:20 PM   #6 (permalink)
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They are basically the same thing, and it probably could be programmed in arduino IDE, but an ISP programmer would be good for setting the cpu frequency lower so it uses a lot less power (a consideration here). I know I have done an arduino at 1mhz before, just have to remember how I got there
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Old 04-15-2009, 07:09 PM   #7 (permalink)
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here's a pic, the cpu just needs the battery hookup and 8 digital pins once it is set to run on the internal oscillator. Not sure what mosfets go where, but it's a start.

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Old 04-15-2009, 07:20 PM   #8 (permalink)
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Porsche 914 EV Conversion: Battery Equalizer Schematics Rev 1.0

could use that circuit. Just hook the charger to a middle battery and let the circuit balance them out over time. If nothing else just double the capacitors up or go to much larger ones so you get more current flow between the batteries.

That is one of the things I am considering doing for my car so I can use solar to charge the 12V battery and the 72V pack at the same time.
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Old 04-15-2009, 07:34 PM   #9 (permalink)
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Here is some arduino code that compiles that should convey the functionality of the above circuit (post 7):
PHP Code:
//haven't done my mosfet homework so making these defines

//pin value for turning on a p channel mosfet
#define pcon HIGH

//pin value for turning off a p channel mosfet
#define pcoff LOW

//pin value for turning on a n channel mosfet
#define ncon HIGH

//pin value for turning off a n channel mosfet
#define ncoff LOW

//number of batteries
#define nbat 4

//milliseconds to wait between switching batteries
#define pause 60000

//define the control pins
#define a 2
#define aa 3
#define b 4
#define bb 5
#define c 6
#define cc 7
#define d 8
#define dd 9


//put the pins in arrays for smaller program
int  npins [] = {a,b,c,d};
int  ppins [] = {aa,bb,cc,dd};


void setup(){
  for(
int x 0nbatx++){
    
pinMode(npins[x],OUTPUT);
    
pinMode(ppins[x],OUTPUT);
  }
}

int bat=0;
void loop(){
  
//turn off all pins
  
for(int x 0nbatx++){
    
digitalWrite(npins[x],ncoff);
    
digitalWrite(ppins[x],pcoff);
  }

  
//turn on the battery to charge
  
digitalWrite(npins[bat],ncon);
  
digitalWrite(ppins[bat],pcon);
  
  
delay(pause);

  
bat=(bat+1) % nbat//point to the next battery modulo
    

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Last edited by dcb; 04-15-2009 at 07:45 PM..
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Old 04-15-2009, 08:39 PM   #10 (permalink)
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Darin, I think you're asking for a small logic circuit that would drive something like relays (with low ohmic resistance in the ON state) and driven by some sort of clock that could multiplex one input into four outputs. If so, maybe this would work. It's a 555 clocking a JK Flip Flop. The sequencing comes from wiring a 4 NAND gates between the 555, FF and some drivers. The drivers could drive relays, MOSFETs, IGFETs, etc:


The timing diagram:

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