Solar powered Mintyboost?

[olbetsy]

Hey all, I've been toying around with the idea of converting a Mintyboost to run off of solar power so I can create a solar USB power source, or something similar.

I have some solar panels that can generate either 800mA @ 4.25V (850mA Isc and 5V Voc) or 1600mA @ 2.13V (1900mA Isc and 2.5V Voc) depending on how I wire them up.

They actually work, in direct and unobstructed sunlight, as a direct USB source for a lot of devices when hooked up in series (5V "mode"), even high powered devices as they can put out 500mA @ roughly 4.67V (a little below spec). But, if arranged in parallel, they'd work fairly nicely for a Mintyboost input, simulating a set of NimH batteries rather well.

Now, since the sun isn't always direct and bright, I'm thinking the best solution is to use them to charge a set of batteries and then use those to power the Mintyboost output.

I'm trying to figure out what the best way to charge up those batteries via the panels is. I've found descriptions online, but they are very simple and don't provide any protection from overcharging, nor any intelligent charging capability.

So I'm thinking maybe the best solution would be to get an off the shelf USB AA charger and power that through a Mintyboost (or similar circuit). Though from the Mintyboost design, it seems it is not really intended to give an output of up to 500mA, right? If so, could this be changed through selecting a different converter and tweaking the circuit somewhat?

I also found this circuit here that seems to offer the charging capability I'm after. However, I'm not an EE by any means and I'm not sure I could build that. It would be nice though as I could then simply flip a switch and then use the charged batteries to run a Mintyboost circuit.

I'm just trying to learn a little about electronics with this little project and I'm wondering if anyone here might have some advice.

Thanks,

OB

[magician13134]

You might want to talk to JPL, he's been working along the same lines, here:
http://www.ladyada.net/forums/viewtopic.php?t=5621

[olbetsy]

Thanks! That might work, but I am somewhat worried about charging the batteries with such a simple circuit. I would think it would definitely shorten the lifespan of the batteries unless closely monitored.

What I was just thinking of is that I have a little lithium battery that charges via USB, and also has USB output. It seems to work with low current, but draws the full 500mA when it's dead. If I hooked up the solar cells to the Minty circuit, then it would provide the 5V output that the lithium would want to see, and then I could just use it's USB output instead.

Kinda convoluted, but I think it would work. And since the lithium guy has all it's protective circuitry, I wouldn't have to worry about being in over my head trying to design some battery charging circuit.

My only worry is that the Minty doesn't seem to love supplying high current. How hard would it be to swap the MAX756 for a LT1302 or something that would be happier supplying the higher currents? I'm imagining that wouldn't be a plug in play swap.

--

Edit: Actually, perusing the MAX756 data sheet, it seems it is OK with supplying 500mA, it just needs more like 4V to do so. Since my panels can do that, it looks like I should be OK.

Solar Minty, here I come!