Idea to make higher-capacity AA batteries

Sure, AA batteries have a round cross-section, but have you ever really looked at the battery compartment in your assorted portable devices? I have, and there's often a lot of space in there.

My idea is to use the space typically found in a battery compartment to boost the capacity of a AA battery. They won't fit in everything, but for those things they do fit in, the extra capacity will undoubtedly be handy.

In the picture above there are three shapes, labelled as cross sections for, respectively, a normal AA battery, my suggestion for the most compatible extra-capacity battery and my suggestion for a maximum extra capacity battery. I have used the + naming style you might find with paper sizes slightly larger than normal metric sizes (eg; A4, A4+, A4++).

AA+ is designed based on the premise that many battery compartments only require a curve on one side of the battery. The AA++ design is based on compartments with no guides beyond where the circular cross-section of a normal battery would meet the inside of a square.

A selection of typical battery compartment layouts is illustrated below.

You can see how I think the two new shapes would fit.

I've had this idea bouncing around in my head for a while, but when Moixa Energy came out with the "USB Cell", I thought it was worth putting together an explaination of my idea in case the room could be used for the charging circuitry, even if current battery cell technology still favours a circular cross-section.