Right, this is a non-IKEA tip! Actually, the store (kijkshop) is something really Dutch. I figure, similar products are available around the globe.
I purchased this product many years ago, with an entirely different scope in mind. However, today, it comes handy when backing up the batteries in my van.
And here it is:
Once I installed the second, i.e. house, battery, the solar thing will charge it up... at its own pace.