2 or 3 years ago I bought something like that in Aldi, not waterproof and only 10,000mAh but I got it for only $9,90, I thought it was a pricing error and quickly grabbed the last 2.
I didn't buy it for the solar surface but the capacity/price ratio.
I tried it once directly in the sun and an hour later I decided never to do that again because it got far too hot for my liking and what I consider safe for standard Li-Ion batteries.
That little array has about 1W, the battery has 20Ah x 3.7V capacity = 74Wh.
Roughly 10-12 full sunny days to charge at an expected 6-8Wh a day. By then the battery capacity would have lost some Ah due to prolonged heat exposure unless you charged it in colder parts of the world.
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