Solar Charger for a satphone battery

G'day all. Here's a curly one for all the techies out there:
I have a Motorola 9500 satphone with a spare battery. While I'm in a vehicle, no problems, I plug it into the lighter outlet. However, when I'm out in remote country surveying or collecting samples for quite a few days carrying the satphone in my backpack I have a problem.

The 3.6v Lithium Ion batteries are designed to discharge at 5% per day. For safety reasons, and in case I find something interesting and need to make detailed reports on scene, AND, I'm out in the weeds well away from the vehicle for a week or more, I would like the ability to "top up" the batteries. Here's the rub - The solar panel supplied by Motorola is too heavy to carry and far too expensive to buy (almost as much as a new phone!)

So what I need is a small light (fit in a pack pocket or under the top flap)and relatively inexpensive solar panel set up ( I have the gear and skills to construct one to specs or from a kit) that will charge the 3.6V LI ION batteries. I've been told that a "special setup" is needed to charge these particular type of batteries. Is this the case? Are these batteries the same as the usual mobile phone batteries?

Can anyone technically/electrically/solar minded help me out with this problem? Cheers, Mario