PowerSwitch

I got myself a pair of these for £25 http://www.chinavasion.com/product_info ... -supply-2/ (details on bottom of page)

They are for charging a lithium power pack
http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0166927906
(It has over current and over charging protection built in.)

Using one panel works OK in bright light.

What I want to do is use both panels wired in series to increase the performance in low light. I am thinking this will increase the voltage to approx 6-8v, I can then regulate this down to approx 5.5v and thus my power pack will charge in lower light.

I found this http://www.engadget.com/2006/06/13/how- ... arm-clock/ And have got a lm7805 to wire in.

I also found this on using a 7805 with solar http://www.instructables.com/id/How-to- ... -mounting/

You obviously want to be a _bit careful_ with charging lithium batteries..

So I thought I would get a second opinion before I go ahead.

Does this seem OK?

Thanks

Sam

I don't think connecting them in series will give you much (if any extra charging capabilities. Most decent panels are designed to charge on cloudy days by putting out considerably more voltage than is needed (12V panels put out around 21V). I think you'd be better off with them in parallel.

PV is a current source, not a voltage source -- therefore wiring in series might boost the current but not necessarily the voltage. A battery actually has quite a low resistance until it begins to reach full capacity, so you're not going to charge them very efficiently just by coupling them up.

What you need is a 5.5V charge controller ('CC'). That provides a variable resistance to keep the system voltage high, although you'll lose a few milliamps of current as the trade-off. In fact if you can get the manufacturer's data sheet to find the 'maximum power point' value ('MPP' -- http://en.wikipedia.org/wiki/Maximum_po ... nt_tracker), and run the system at that voltage, you'll probably make-up for the system losses with the higher panel output efficiency.

The only difficult is that I'm not aware of a CC consumer unit for sale that works at 5.5V -- so you'll either have to make your own (can't we all?), or cannibalise the CC from a solar light that operates around 5.5/6V

Also, forget the lithium batteries. With such small panels you're not going to efficiently charge them according to spec. and this will artifically shorten their life -- it'll work, but in the long-run it's just a waste of good lithium batteries. Get a cheap 6V sealed lead acid, or get a discarded 6V moped/motorbike battery, and use that instead -- although the sealed lead acid is better then an auto-battery because they're designed to take regular discharges of 30% to 40% of their capacity, whereas auto-batteries will begin to warp their plates if you regularly discharge more then 10% to 15%.

It is common practise to wire nonimal 12V PV panels in series to get 24, 36 or 48V outputs, Mobbsey. Agree with the MPPT controller advice, though.

It is best practice to restrict the discharge of lead acid batteries to 50% or 60% depth of discharge (DOD), that is leaving 60% of the charge still in the battery. It will extend the life of the battery from less than a year at 10% DOD to over five years at 60% DOD.

kenneal wrote:It is common practise to wire nonimal 12V PV panels in series to get 24, 36 or 48V outputs

The major problem with wiring in series is shading. If the first panel is in full sun, but the next has a line of shade across it, it will create a resistance to current flow from the first panel. Over-shadowing is a big issue with PV for this reason -- you must always ensure that each panel in the "string" (to use the jargon term for a series of panels wired to produce the required output) is not shadowed in a way that might cut the output from the whole string.

Although panels are rated according to voltage, based around their optimum 'maximum power point', being current devices they will still float to a voltage based upon the current they produce and the system's circuit resistance. My 12V thin films panels will quite happily sail up about 21V off-load (an infinite resistance) but stick 'em on a half-ohm dump load and you'll only get 6 volts. That's why you need to have a charge controller to ensure the battery is charged at the most efficient voltage for the panel's MPP.

HomePower have a handout on this at:
http://www.homepower.com/view/?file=HP1 ... PageBasics
... and a load more consumer-oriented stuff at:
http://www.homepower.com/basics/solar/