There are a few other things to worry about, and I hope that when you add them all up you conclude that this isn't a good area for a novice to design in.
U/L labs approval...if you plug something into the wall, your insurance carrier should insist on it having U/L approval. There are a lot of really interesting specs that U/L has conjured up over many years of evaluating the safety of various designs, and they will apply ALL of them to you. Give the process from 6 weeks (a rush job for which you pay big $$) to 6 months. U/L cares about what kind of batteries you're using, whether they're rechargeable or not, and if they are, the actual schematics and code running any microcontrollers inside the recharge unit. For Li+ batts you need a pretty capable recharge unit to stay out of danger (overcharging being one of the dangers). Many people turn to wall-warts because those can be independently U/L evaluated and approved, and then it may look as though your entire product has been U/L approved even though it actually wasn't. (Don't believe me? Check out the back of almost any recent rechargeable device and notice there's no U/L on it, only on the wall-wart. Yes, U/L has noticed this end-run and are not happy about it.) Also, current and voltage are not the only things you must get right. Computers contain some digital devices that draw drastically different amounts of current depending on what they're doing, microprocessors being among the worse offenders. You must make sure that not only does your supply provide the peak current needed, it must also respond fast enough to current changes and the resulting dI/dT induced voltage swings must be restrained by appropriate decoupling. If you get this part wrong, you probably won't notice it until you've shipped enough machines that somebody out there ran some software that drove the CPU through a big enough current swing to droop your supply and cause a malfunction. It takes major engineering talent to identify and resolve this if it happens. The best advice you got so far is to dissect a laptop and do what they do. Even better, hire yourself a consultant who knows this area, and she can keep you out of trouble. -BobC
