"All that is required is a small supply of a sugary drink... Bio-batteries are fully biodegradable and have, on a single charge, a potential life-span three to four times longer than conventional lithium batteries." When someone sees a quote like this, that too on a reliable news aggregator like Google, it's hard to just move on and click 'next'. According to a report in CRN.com.au, a paper on a Coca Cola-powered mobile phone was recently published by Chinese designer Daizi Zheng, the aforementioned quoter. Basically, as a part of an Eco project, the designer used the sweet soda to power a mobile phone. The claims are impressive and say that the battery can last 4 times longer, and is fully biodegradable.
My take on this is simple: I don't think Coke wants to be known as a battery fluid, as i do remember my 8 th standard Physics clearly- battery fluid is strongly acidic. There have been reports that Coke has been used as an industrial cleaner, and for that matter any of the colas can be used for that due to its phosphoric acid content. But going further into the battery topic, the efficiency of this battery is questionable, and also safety of the use of an alternative electrolyte like Coke. There is a chance of it reacting wrongly and even exploding. To nullify this in commercial production, the R&D costs go up.
Recharging means buying a new Coke can, so it's no doubt cheap in theory, but mass production of the battery itself will be expensive. There are other sources of BioFuel like edible foods such as edible oil seeds, Corn and Jatropha.
But Bio batteries are not all theory. None other than Sony has been toying around, literally, with the concept of Bio batteries. Geek.com very recently covered a report on toys that use few drops of sugar filled liquid to power its small toys, batteries provided by Sony. "Such batteries run on just a few drops of sugar solution making them an ideal and safe replacement for batteries in children’s toys."
On further web browsing I stumbled upon an article on how to make your own Bio battery, thus it's not really a heavy commercial process, DIY dudes can surely check this one out.