Powering the Pi with 6AAs?

[jackwilsdon]

I have created a circuit using a 5V regulator (LM7805), however it only lasts one hour on 2500mAh NiCad batteries. Could this be due to the condition of the batteries (they are losing charge)? I had a screen, usb keyboard and a WiPi plugged in at the time. With normal AAs, could I expect it to last longer?

If not, can anyone recommend a power supply that would be viable for a small robot (not too heavy, not too big)?

[PiGraham]

If the Pi draws an average of 2.5W you need >2.5WH (watt hours) to run for an hour.
2,600mAH NiCads, in top condition, could provide 2.6AH x 1.2V = 3.12Wh per cell.

Six such cells potentially give you size time the energy.

HOWEVER, if you waste energy in conversion, and if your circuit can't provide usable power when the voltage drops, then you won't get close to what the basic numbers suggest.

Don't use a linear regulator. They waste a lot of energy. Use a switchmode DC-DC converter.
Put batteries in series. This means that as the voltage drops the input to the converter is still well above the operating level.

A linear regulator like the 7805 will waste more energy if the input voltage is higher. A switchmode will draw less current when the input voltage is higher, keeping the power use more or less constant.

Rechargeables heave a limited life. If these are old NiCads they won't hold anywhere the rated energy.

[Burngate]

... however it only lasts one hour on 2500mAh NiCad batteries.

So either 1) your curcuitry - Pi, keyboard, WiPi - are using 2500mA
Or 2) the batteries are lying

The Pi takes in the region of 700mA, the keyboard maybe 100mA, and the WiPi who knows? though probably less than 500mA
So we conclude your batteries aren't providing their full 2500mAh

Reasons for this:
1) they're old and weak
2) they've still got life in them, just not enough volts to drive the Pi after the 7805 has taken its minimum - it could need a volt of headroom to give out its 5v, meaning the batteries have to provide >6v for you to get 5v to the Pi
3) when you first started this, the batteries did their best. But having been discharged so low that the volts reached 6v, one or more of them went flat, then the rest drove it / them reverse-biased, a condition almost guaranteed to destroy nicads. After that they're old and weak

Use a switch-mode supply - boost or buck.
Boost has the advantage that all the cells will be in parallel, so share the current . that means individually they're not as stressed. Also, they can never get reverse-biased.

[jackwilsdon]

I forgot to mention that I measured the power being used by the Pi, and it averages around 0.4A (400mA) to 0.5A (500mA). I think the batteries must be faulty. How long would normal AAs last? The average AA is 500-1000mAh, so if I said my AAs were 1000mAh, would it last around 2 hours?