As to how it can be a battery is still not very known, because a normal water battery has two different types of metals used to make a galvanic reaction and that gives you voltage. A water captret uses two of the same metal but it still gives you voltage. One reason why it gives voltage might be that the aluminium is paramagnetic and the water is diamagnetic, but I really don't know.
In this video I hook 10 water captrets up to get about 800mV. I really want to make a water captret that runs a LED light so that I can really test the life of the water captret. I'm sure it has a good life because they seem to work better when given a short or a load.