I need a fresh look at this. I've spent countless hours and written several e-mails back and forth with Linear Tech about this and so far I haven't come up with a working solution. So maybe someone here can ...
I need to put two, single cell LiPo (or Li-Ion) batteries in series for the higher voltage. Each one will need to have its own charging circuit as well. Where I am stuck is, how do you tie that together in series? I can't simply tie one battery's negative pole to the next one's positive pole because when the charging circuit kicks in, current will flow all over the place between them. It just won't work. And on top of that, when the batteries are depleted and a DC Input is provided to recharge, that too needs to supply voltage to the system load, so I need to switch between the batteries and DC input.
I'm going to use the LTC3633 to generate the voltages and current needed for the system load. What I need now is to figure out how do I tie the batteries together so that their charging circuits don't screw with each other, and a way to switch between the batteries and DC input (when provided.) Again, Linear has some muxing ICs that can do the switching and also monitor the batteries to prevent an over-discharge, but they are all between DC, BATT1 and BATT2 ... and I need it between DC and both batteries in series, not a parallel setup.