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?

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.

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).

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.

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?)

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 :)

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.

http://ecomodder.com/forum/attachmen...1&d=1239833307

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.

Here is some arduino code that compiles that should convey the functionality of the above circuit (post 7):

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:
http://ecomodder.com/forum/attachmen...1&d=1239838467

The timing diagram:
http://ecomodder.com/forum/attachmen...1&d=1239838673

you have to control the low sides too. and the 555 needs resistors and capacitors etc.

the fets come in quad packages so an avr and two chips would be three chips total for the microcontroller route (mosfets included).

This is good stuff, guys. Thanks. Digesting & reading your code, dcb...

Missed your description of the panel, nevermind!

The correct way is to use a boost converter. However, not any common boost converter can be used as in a solar application, as it must attempt to regulate the *input* voltage as well as the output voltage. The reason is that solar panels are more or less current sources up to near their open circuit voltage. (Read about the photoelectric effect for more information, but the basic explanation is that the electrons knocked free by light have a certain velocity and therefore a certain amount of voltage difference they can overcome.) The boost converter should try to pull down the input voltage to the desired value as long as the output voltage is not too high. Use a potentiometer to set it to maximize efficiency. If the panel voltage is clamped too low, efficiency suffers as a lot of the electron energy is wasted. If it is let to go too high, the low energy electrons will not be able to make their way to your circuit. The optimum point is something like 13-18v for a 12v panel.

In what sense is a boost converter the "correct" way? (Efficiency? Simplicity?)

If I was wanting to do a 12V solar charger for 72V I would probably build the one dcb has listed. It is a simple circuit so that means there are fewer things that can go wrong.

Simpler is always better if it does the same thing as the more complicated circuit. But for my setup I will probably stick with the one I posted since it does continuous balancing of the batteries and swapping out the 555 in the circuit for an arduino could give me a lot more capabilities like measuring volts/amps and having a display show total KW used or something. I can never leave well enough alone it seems like :)

Both simpler and more efficient. It is simpler than trying to switch the panel between the batteries and more efficient as well as it will allow the panel to operate at its most efficient output voltage. The efficiency gained by hitting the sweet spot is greater than the losses in the boost converter.

Mike, any thoughts on "more current" meaning more electrolite stirring in the battery? Part of Darins idea is that it would be better to occasionally stir up the electrolite by applying more charging current individually, (i.e. by rotating which battery is being charged). Not sure how much current you need for that though.

It is actually better to charge at low current than at high current. As for keeping the electrolyte mixed, it's a valid point for a stationary system but in the case of a car, just driving would produce enough vibration to keep it mixed.

I think I'm with mike on this one, jack up the output voltage (or add three more panels) and/or build a solar powered desulfator, and optionally optimize the current draw from the panels (gets more complicated quickly though)

Probably the easiest would be to dissect it and rewire the panels for 48 volts though.

One way to make a solar powered desulfator is to use a boost-type circuit without the output capacitor. Use a comparator circuit to activate a one-shot timer every time the input voltage rises above a certain threshold (about 17v for your panel, but tweak for best results). Size the input capacitor large enough to ensure the input voltage only drops a little bit during the on period, use an inductor rated for 20A or so (can salvage one from a PC power supply), and adjust the period such that the peak current is something like 15A. Also use MOSFETs and diodes rated for high voltage (200v minimum) as there will probably be voltage spikes when in operation.