Efficiency of 2 AA batteries with step-up to 9V vs 9V battery

Question

I have a device (LEGO PowerFunctions IR receiver to be precise), which operates at 9V, and usually, it's powered with 6AAA batteries. Needless to say, 6AAA batteries fit in quite bulky battery box, so sometimes people just use 9V batteries for it, which, on the other hand - yields much lower performance in terms of capacity (and they're more expensive than AAAs or AAs).

So - how about using 2 AA batteries (NiMH rechargeable, so with voltage of 1.2) with step-up converter to put it up to 9V? Assume the ones capable of 2600mAh, and the efficiency of step-up at 80% level, let's do some calculations:

2.4V / 9V * 2600mAh * 80% = 554.66mAh

So it would seem it's better than the 9V NiMH batteries on the market can provide. What would be the other considerations that I miss?

Answer 1

You might try lower voltages to see where it actually stops operating. It's possible they've a step down converter on the inside.

Answer 2

For a given voltage, a lower current draw will typically result in better long term performance. Wording it a bit differently, for the same current draw a larger battery will perform better long term.

Not only is this referring to the larger battery lasting longer because of it's larger capacity, but because a smaller current/size will result in a better recombination process.

Assuming your demand is 0.9 W...9 V at 100 mA, your current demand from each AAA battery is the 100 mA. Using two AA batteries and stepping up 3x will result in a current draw of 300 mA + losses, so maybe 330 mA.

The currents I used were a reference, if your demands are larger still then the recombination inefficiency will be even greater.

The 9 V battery may be under performing because it cannot supply the demanded current.

Instead of a step-up converter, I would recommend a step down converter. Using these 12 V batteries, for example. They won't last you as long as the 6 AAA, but that is the price for lower weight and smaller size. On performance through, they would be just as good, and the current draw from them will be 9/12 of the load current, or 75 mA from our previous example, thus less drain from recombination. Stacking two for a 24 V - 9 V converter and you'll half that current draw.