Tried also continuous Hi shooting mode for several chains of shots until the camera slows down, no problem of any. LiveView was also enabled so as to drain more battery juice purposely.
Now my K-x is even lighter than using 4 disposable AA Lithium of the L91), but the LiFePO4 batteries should be placed in the central two battery slots to enable better weight balance of which the difference can be noticed if they are placed differently elsewhere (for the different combinations of real and dummy hollow batteries) .
Okay, before I can really tell more about the battery life in actual use. I have done some more voltage measurements and comparison for the make-up voltage for fresh and used L91 against those of the LiFePO4 batteries as below. I used the AA battery holder/adaptor D-BH109 for my K-r to do the job and the measurements have been made at near the same time with the same voltmeter:-
Fresh L91, ~6.8V:-
Used L91, ~6.1V:-
Fresh LiFePO4, ~6.8V:-
Used LiFePO4, ~6.2V:-
So, it can be safely concluded that the voltage characteristics of 4 x L91s and 4 x LiFePO4s are very close and similar. The capacity of the LiFePO4s should be lower, though. I shall tell more about its exact real-life battery life later on by the picture counter, as the first set has not yet been exhausted but only that the voltage has been found to be decreased/dropped significantly already, as seen in the above.
Btw, I think my this battery (also an entry level camera hacking) project has been succeeded!~ :-D
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LiFePO4 AA Batteries - Part 2: Unboxing
New Lithium Rechargable Battery Type in AA Format - LiFePO4