Solar Freaks

Dear Forum,

for a long backpackung trip (CDT trail), I try to construct a simple and light solar charger for the DSLR-camera batteries (2 cell Lipoly with a protection circuit, 7.4V 1Ah, charging voltage 8.4V) . I opened the battery charger and the battery and found that the charger is just a constant voltage supply of 8.4V, and the battery includes a small protection circuit.

As I don't have access a workshop in the moment, I tried to use alread finished parts for my setup.
I bought a cheap and light (but inefficient) 15V 1.5Watt solar panel which I planned to keep on the backpack all time.
http://www.amazon.com/PowerFilm-Peel-St ... 596&sr=8-2


and a small step-down converter board, based on the LM2841 http://www.national.com/ds/LM/LM2841.pdf from Pololu electronics http://www.pololu.com/catalog/product/2104/specs

I had to modify the board slightly by putting a schotty diode at the output (but before the feedback loop of the LM2841). Now everything is working from a power supply, and I would have a 2 ounce charger including the solar cell.

However, using the solar cell it doesn't work: As soon as the battery is slightly discharged, the charging current increases, the input current to the step-down converter increases and therefore the solar cell voltage strongly drops.

Sometimes, it is still charging, but the solar cell is far away from its MPPT point and therefore the efficiency is low; sometimes, it doesn't charge at all and oscillates, likely as the step-down converter has problem operating if the input voltage-output voltage difference gets to small.

So what I would need, is some charging current limitation as soon as the input voltage drops.
One solution would be the LT3652
http://www.linear.com/product/LT3652

but I don't know how it operates with the very small amount of power I'm dealing with (input power from the solar cell; ~80ma 15V for good sunshine, and 20ma 15V, in less perfect conditions)
As I have no workshop here, I could only buy the Eval board for $150, which I would only spend if I would know that it would perfectly work.
Do you have any idea if there is a simple modification of my setup which could work, or if there is any "finished" product which I could buy (and modify) for my purpose.
Or does anybody know if the LT3652 would work and maybe even has a board he could sell me?

Thank you
Thom