ezbee:

 

I see that LiFEPo4 batteries are starting to become available.
Though no famous brand backing ?
https://www.mitre10.co.nz/shop/vectral-lifepo4-rechargeable-solar-batteries-pack-of-2/p/329289

 

AA are only 600mAH , but LiFEPo4 don't have a tendency to combust.
Technology has reputation of being robust and a very high cycle life.

 

At 3.2V for each cell thats equivalent of 1.6V x 2. 
So put in 1/2 the batteries in a device, and make a dummy to fill the empty spots ?

 

Potential Problems 
No major bands yet ? 
No specs on maximum discharge, self discharge and other parameters.
You need a different charger than NIMH, and other Lithium Cells.
Capacity for these so-so. 600mAH for AA, what you might expect for a AAA.
If a major brand gets involved they may use better LiFePo4 technology.
LiFePo4 don't give a nice slow voltage drop, rather, steady voltage with sudden drop at the end.
Assuring minimum voltage of 2.5V, might be tricky.
Unless equipment refuses to operate of give warning to ensure you don;t over discharge.

 

 

 

you can use one of these as the dummy cell, just connect both terminals with a wire - AAA to AA adaptor.

also need to make sure nobody replaces the battery with 2 of those LiFePO4 cells.

as far as energy goes, 3.2V x 600mAh = 1920 mAH not too far from a decent NiMH cell.

not sure why you think the discharge pattern is a 'potential problem'... it generally means the device is functional for much longer. unless you're thinking of getting caught off guard when it drops off. if anything, a protected cell will have an even sharper drop off when voltage protection is triggered. but that's a good thing for the cell - that's how you ensure it doesn't go below minimum voltage.

next time i'm at M10, i might pick up one of these just to test. i know jaycar also has them. might be good to compare.