The efficient way to convert battery voltage to a +5 volt level is to use a small switcher power circuit. There are several different circuit configurations that work to step up the battery to +5V or step it down to +5V. Some designs are configured to be able to supply the +5V from the input voltage both lower than the +5V and higher than the +5V.

I would suggest that you select your battery configuration so that the input voltage to the switcher is higher than the +5 volt output level so the switcher always operates step down.

Another very effective choice for you to consider is to change all your digital circuits to use 3.3 volts and then use one of the newest LDO regulators. Then you should be able to easily supply from a battery voltage as low as 4.0 volts or so. Then maybe you can get everything to work rather well with 4 NiMH cells.

Michael Karas

PS. Keep in mind that the batteries voltage goes down with discharge and so the design you select has to account for still enough voltage to drive whatever regualtor or converter you select at near the end of life of the batteries. MJK