I'm building a watch and I would like to use a rechargable 3v lithium battery (http://www.digikey.com/product-detail/en/ML-1220%2FF1AN/P295-ND/447504) to power it. I don't really want to put a whole lot of money into the watch so I'll be charging it using a little solar cell I ripped out of a calculator. It provides about 2.9v no load and 2.8v with a 50uA load (directly under my desk lamp). I realize this isn't the ideal charging voltage and that I'll only get about 40% max capacity on the battery, but it should be fine for my purpose. So the question is, what do I need between the solar cell and the battery to charge it? Do I need a diode to prevent reverse current flow when there's low light? This will of course drop the voltage further.
a diode and a step up, because one lithium cell is 3,6 V. Also you need a bit of logic to prevent lipo from damage. There are chip that already do everything for you, take a look here: http://ladyada.net/make/solarlipo/
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.
take a look here: http://www.rchelicopterfun.com/rc-lipo-batteries.html
Again, this isn't a regular 3.7v lipo, and I'm not exactly charging it using conventional methods. I just checked digikey and they have pretty cheap step up converters( http://semicon.njr.co.jp/eng/PDF/NJU7261series_E.pdf ). This one seems to do what I need, 3v output. I have a question though. In the equivelent schematic it shows a voltage divider to get an output voltage reference for the comparator. I don't see any info on the datasheet about reverse current draw though. Does anyone know the typical reverse current draw for one of these, if input voltage is too low?