Is this a valid way of looking and calculating power budgets?
I have 24 "AA" Cell NiMH cells that I want to use to power an autonomous line seeking robot. The cells are rated at 2100 mAh.
I have several constraints. I need 500 mA at 24VDC to run the motors, and I'd like to reserve 250 mA at 12VDC for future wireless stuff.
So I ask myself how much 5V current can I then get from the battery pack if I want a 2 hour run time.
I'm planning on using switching regulators, so I come up with something like this:
2100 mAH / 2 Hours = 1000 mA current per hour 24VDC * 1000 mA = 24 W power budget24VDC @ 500 mA / 85% efficiency of regulator = 14.12 W
12VDC @ 250 mA / 92% efficiency of regulator = 3.26 W ----- 24 W Budget - 17.38 W 6.62 W6.62 W * 85% efficiency of regulator / 5 VDC = 1125 mA
or if I look at it this way
5VDC @ 1000m mA / 85% efficiency of regulator = 5.88 W + 17.38 W (from above) --------- 23.26 Wor 97% of the available 24W. Does that amount to a 3% safety margin?
Putting aside that I may not get a full two hours from the battery if I draw the current that fast (not sure what the internal resistance is) and I'm trusting the efficiency numbers from the spec sheets alot; can I safely believe that I have 1A at 5VDC for the rest of the system (sensors, controller and such)?
Is this a valid way to approach power budgeting of a battery pack? I'm very interested in factors I might have missed in thinking this through.
Thanks,
Kevin