I came across this product Parallax Li-ion Power Pack / Charger – 2 Cell so my device can be run off of 2 rechargeable Li-Ion batteries or from AC power. On the last page of the document they show how to hook up the power pack to another device to be able to handle the switching back-and-forth from battery power to AC power (Figure 7). They show a capacitor hooked up to supply power during the switch over, however they simply say to make sure you select one large enough without saying how large, or how to figure it out. Is there anyway to calculate this from the information supplied or a good rough estimate of how large a capacitor I should use? Or am I simply going to have to do trial and error? Any suggestions or comments are greatly appreciated and thanks in advance.

 

The OP is referring to this (Fig 7)

The capacitor size will depend on the load. It's that simple; you have to define the load to calculate an answer.

 

How about treating it the same way the capacitors in AC power supplies are treated? Calculate the "ripple voltage", or decrease in voltage on the capacitor from AC peak to AC peak, or in your case from AC to battery switchover time.

http://en.wikipedia.org/wiki/Ripple_(electrical)

As wayneh said above, you do need to know the load requirements.

 

The OP is referring to this (Fig 7)

The capacitor size will depend on the load. It's that simple; you have to define the load to calculate an answer.

Is it just the load requirement? Wouldn't the switch over time be as important?

 

Yes of course, it's the product of (current) x (time) that tells you how much charge you need stored.

The bad news is, a capacitor stores very little compared to a battery of the same size. If you have much load at all, the whole idea is impractical. With a small load and a fast switch, maybe you have a shot.