If you're OK with the diode voltage drop already, then those two diodes alone are enough to do what you need.
If voltage drop across the diodes is a concern, you could also go the mosFET approach. At least that's what I remember the folk over at picoUPS do.
Why not use some recharge-ables and leave them in the circuit all the time. At least then you won't have the system failing because someone forgot to change the batteries. Then you power supply just has to be regulated to the proper voltage for the batteries. NiCads peak out at approx 1.25 V/cell and run through a good part of their discharge cycle approx 1.2 V/cell so you coud regulate to 5V and supply that 5V after the regulator, straight to the arduino and the batteries.
Hmmmm, the MCU could track battery replacement date and cumulative usage...
QuoteHmmmm, the MCU could track battery replacement date and cumulative usage...and voltage? If they drop below a set value the Arduino can warn you before the backup is too low to be useful.Pete
Well, it'd be an open-circuit voltage from the battery. Not sure how useful that'd be.
QuoteWell, it'd be an open-circuit voltage from the battery. Not sure how useful that'd be.With alkaline batteries, if you stick with one brand, model, and size I suspect you will get reasonable results. I have#. But, to get a "percent life remaining" you will have to develop a "battery profile".# I recommend Rayovac. If you're interested, I'll get the model and size I've been using.
Let's see ... there it is ... Rayovac 815 (AA). The datasheet is here...
Sounds like you've really done the homework!
I've tended toward Duracell or Energizer, but not for any particular reason.
Rayovac tends to be a little cheaper around here, but not as common.
I do have some Rayovac NiMH AA cells which are quite old, they just keep going and going, oh wait that's the other guy's line XD