Backyard Aquaponics
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Whats happens if you cover 2/3's of the panel, does the voltage drop 2/3's

This is a good explanation:

http://ezinearticles.com/?What-Are-Solar-Bypass-Diodes?&id=4657969

I am planning on getting one, but it's 6 weeks away, and I'm desperate to know how much power I'm going to have.

http://www.ebay.com.au/itm/MPPT-15A-Sol ... 854wt_1028

@Privatteer The panel and battery are to be used to power my electric trolling motor on my epic murray river adventure.

I'll be underway for all the time the panel is making power, and should still have around 2 hours flat out (7 kph) up my sleeve for emergencies (to get to the next town/pub after a night time snake bite) and for a bit of night time lighting, and to keep GPS, iPhone, and laptop powered.

I'm just really interested to see how much my trolling motor pulls so I can see what speed I'll be able to run it at.

To test it, I figured I'd put the solar panel to the battery, and run the motor in a tub of water for a few hours, and watch the charge state to see what speed I can run at and keep a fully charged battery.

From the small amount of playing around I've done, when shading more and more of the cells, it seems the voltage stays at almost full and then goes to zero with almost nothing in between.

I take it you were thinking along the lines of a roller blind as a charge controller.

What is this epic adeventure about? Im courious.....

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That makes sense. That might explain why my little 6 in 1 solar toy set sees the motor run when you shade a third of the panel (the top) but not when you shade a third of one of the sides.

If my panel can be converted to 2 x 12 volts, does that mean I'll be able to buy this inexpensive charge controller?

This is 1/5 the price of all the others, and I still dont know if they will do what I want.

From the information provided, I cant tell if all the MPPT ones can have 24v+ input and 12v output. The blerb often reads like it's either 12 volt OR 24. Is there a phrase I should be searching for to find models that will cope with anything in, and whatever I want out?

Here is an example of one that happens to mention that it wont do what I want. (although perhaps it will if I can arrange the 2 x 12 volt option on my panel)

I still cant find one that actually says it CAN do what I want it to.

http://www.siliconchip.com.au/cms/A_112335/article.html

From what i can tell, if you can get you panel to output 12 volts then the cheaper one will do the job. Is getting another battery an option?

Um, nothing will happen. The panel will just sit there not providing any power to the battery.
Same with a lot of PWM/MPPT chargers if the input is less than battery voltage they don't boost it meaning there is no charge generated.

For the scenario Bull is looking at the dc regulator I linked to might be his best bet. Wide input voltage so even if the panel is slightly shaded some charge should be generated. Besides its waterproof as well.
Otherwise a MPPT charger, like this one, but they are a bit more expensive.
I must admit since I last looked there is a lot more options for higher voltage panels.

Bull, at 14.8v (ie panel split direct connect, you could do your test. I assume you have a multimeter so watch the battery voltage and only let it sit at 14.5-14.8v for a maximum of about 2hours. A lot of charge controllers do this anyway in the bulk charge period.
Edit: I wasn't sure if you were going to be sticking this in a boat and then leaving it for a month charging.

The B02 charger won't be any good at 12v unless you buy 2 and run in parallel. You have halved the voltage, so you can get up to about 16.8Amps from it.

Oh yeah. I forgot I'll need one that can deal with more amps.

Not really. Weight is a major factor. I'd need to buy two smaller ones and leave the one I have at home, and this has to be done on what most people wouldn't even call a budget, but then again I got a free iPhone on the strength of blogging about my future trip, so perhaps I should talk to a battery manufacturer.

Is there any advantage to being 12 or 24 volt?

I figure there will be losses at each conversion so I'm guessing I'd be better off running a 12 volt system given that's what the motor wants.

Given that your on a river, shaded at points, probably horizontal panel, keeping the 24v panel might be better.
There is no point having 5amps from the panel available if its only at 11v for example since the battery voltage will be hopefully higher and therefore no power from the panel will be used.

Better to have 2.5amps at 22v and convert it to your charge voltage/current battery voltage.

if you are converting it why does 11v or 22v make a difference?