After some hemming and hawing, concerned it wouldn't be powerful enough or that battery life would be terrible, I picked up a Ninja Blast portable smoothie maker a couple of weeks ago from John Lewis (£45). It seems great. It charges to full in about 2 hours with a USB-C cable, and one gets about 8 smoothies out of that. If the battery gives up halfway through the last smoothie you can plug it in and finish the job. (Not sure if it will run continuously while plugged in, but plugging it in for 30 seconds and then leaving it plugged in, one can get at least one more smoothie out of it.)
I've only tried it with frozen blueberries and fresh bananas, but it has no problem with those as long as there is enough liquid. With a food processor at home, I sometimes use very little liquid and the result is like ice cream. This couldn't cope with that, you need more liquid - with the minimum amount of liquid you get something between straightforwardly drinkable and wanting to use a spoon.
It's quite compact, roughly the size of a wine bottle, so can live in a Cali easily. It's very easy to buy frozen blueberries on the road so it's a way of getting a healthy frozen-ish dessert on the road. Planning now to try vegetable based smoothies too.
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