Battery Vehicle Society

Just a quick note about a project I'm working on; it's a solar charger for a 24V lead acid battery. It's a small (and I mean small - 25mmx40mm) PCB which runs from a 20V PV array - the sort you'd use to charge a 12V battery with a shunt regulator.
To do the maximum power point thing, it has a AVR ATtiny5 microprocessor on it - this thing has just 6 legs but that's more than enough to measure array amps and volts and provide a control output to set the array voltage.
So far the "float charge" control works beautifully (controls output to 28V max) and the "bulk charge" controller, where it does its maximum power point tracking thing, also works beautifully, controlling the array output voltage to anything you want (provided it's lower than the array open circuit voltage of course...).
We got the maximum power point tracking software going today - brilliant fun, you can see the array volts stepping up and down as it searches for the maximum power point (I use the perturb & observe or hill climbing algorithm) but there's a fair bit of optimisation to be done in that area still.
So I've just sent off for the 2nd generation PCBs (with errors corrected and redundant elements removed) - they'll be back in 10 days.
I've done the circuit for my brother's gate automation business, but it looks as though they will be commercially available from a firm in Ashton - we'll see.

What's it for? It means you can get 10 to 30% more power out of your solar panel compared to a linear shunt or series regulator. That can save you £30 on a £130 panel. That puts a firm upper limit on the price of the thing (though it does replace the normal series/shunt regulator)

Boards are back, no. 1 built up & experiments ongoing: I'm looking at the effects of search step size, step rate etc. a various insolation levels. It looks as though a simple adaptive search step size is appropriate, with larger voltage search steps at lower light levels (i.e. lower array currents) - it's all to do with quantisation errors in the 8 bit ADCs. We want to test the "incremental conductance" method for optimisation as well (I don't have much faith....it's presently running with a "hill climber" or "perturb & observe" system). I'll show some pics when I can - but it's looking pretty good.
These particular little boards are suited to a 30W panel; they are VERY dinky & it's pretty cool watching the array voltage stepping up & down as it searches for the maximum power point - sort of thing (like pen plotters) that you can happily watch (wasting your life) for ages....
In bright sun it finds the maximum power point in about a second - in low light it can take quite a lot longer as its behaviour is quite a bit more "chaotic"

Oversampling the ADCs let me increase the resolution & has improved low light performance no end. The incremental conductance method was a non- starter on this board with 8bit ADCs. I now have a complete tool chain from "C" source to programming the PCBs at home so am progressing again. Looking good - I'll post a couple of pictures when it's all hunky dory.
I'm sort of thinking about going partially off grid, - get a couple of lead acids & solar cells (just a few hundred watts) & have a 12V bus around the house - it will easily cope with lighting & telly's/computers, but probably not hoover/kettle/dishwasher etc. 12V has a sort of "standard" connector (the cigar lighter socket) & lots of suitable kit - it looks quite technically appealing... and would mean that my home can pretty much ignore power cuts!

Sounds interesting. You would have to keep the wire runs for a 12v bus as short as possible as you would experience a large voltage drop over distance making it very inefficient. Perhaps you should consider a second 240 volt ring and use an inverter to take the 12volt power and change it to 240 volt. If you get a large enough inverter then you could continue to use the large appliances in your house. I did this some years back, and I was lucky enough to get the batteries out of a UPS - totalling about 10kwhs of storage. I'll try to find the forum I put it all on and link it here.