Thanks for the reply. I also found an article by searching google for Circuit Voltage Booster. I knew about dc-dc converters, but I haven't had any luck working with them. The step-ups I've seen generated a lot of HF noise, couldn't supply a lot of current, got less efficient and greater ripple as more current was drawn, and required inductors & booster ICs. I ruled them out for this project because of the problems I had.
Could rapidly switching batteries between parallel and series connections (?100Hz~2MHz) provide two distinct voltages? I'm building a handheld device, about the size of a digital multimeter, and it needs two voltages. They are 2.4V and between 5.5-8V. My original solution was to use two rechargeable NiMH AA's in series for 2.4V and a rechargeable NiMH 9V (funny thing - those 9V NiMH's are actually 7.3V = 6*1.2V). It'd be really cool if I could build a 6xAAA battery pack that supplies 2.4V@2700mAh (3|2*1.2V, 3*900mAh) and 7.2V@900mAh (6*1.2V, 1*900mAh) Does anyone know if it's plausible? The power draw from all 6 batteries should be nearly equal (the switching circuit would drain one battery marginally faster).