So I thought. The battery is actually 3v, it says so in the datasheet. As for the voltage doubler, I'd rather not buy a chip to do that. I found something called a dickson charge pump doubler, consisting of 2 caps and 2 diodes. I don't know what I would clock it with though. Would this kind of doubler work at 50 uA? Alternatively I could just put a switch between the solar cell and the battery, and make sure only to charge it if there's a lot of light.