I invested in a few back-up batteries but none of them made sense. Then I went for a Solio but it annoyed me that I either had to leave it somewhere or worry about which was it was facing depending on where the Sun was. In the end, I grabbed myself a solar backpack from Infinit. Works great for phone, DS and my GPS, especially when I'm out on my bike. Works in daylight too and even better in the sunlight.
Got it here:
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