I purchased a AA-battery adapter from eBay
see:
https://www.pentaxforums.com/forums/pentax-k-r-forum/121010-where-get-battery...ml#post1468094
I tried it and was disappointed to get the message"Battery Depleted" when I tried to turn the camera on. I have a multi-meter I tested the open-circuit voltage across the terminals of the battery adapter with the batteries (NiMH) inside and got 5.18V.
Considering that the batteries are nominally rated at 1.2V, this works out to 1.295V average per battery. I then put a 12 Ohm load across the terminals and measured it and it was 4.95V. I also measured the Li-ion battery the camera came with and it came to 8.22V open-circuit and 7.99V witha 12 Ohm load (across the top and third contact). The difference in voltage between the Li-ion and NiMH is very big (about 3V)
I then swapped one of the NiMH batteries for another of a different brand and the open-circuit voltage was now 5.12V and with 12 Ohm load was 4.94V. This time the camera switched on without the message and was fully useable.
The only issue was that the battery level indicated on the display was half full in an orange colour and then a short time later to just the outline of the battery in red. I tried to set the "AA Battery type" to NiMH instead of "auto" but it made no difference. The camera appears to work OK but I haven't tried taking many shots. LiveView works fine and I know this is very demanding on the batteries.
I have just moved up from a Canon compact and I had a similar issue. This compact took just 2x AA batteries and was always giving me warning messages of low battery yet would last for the normal complement of shots (200-300+).
What are the experiences of other people using the battery adapter with AA rechargeable NiMH??
Does the battery level indicator work properly or is it always appearing to be at the very end of its charge level?
Or are there particular brands of batteries that work better with cameras?
I used "DLG" batteries which are rated at 2300mAh and then swapped to "Titanium" brand which are rated at 2400mAh
I upgraded the camera firmware to the 1.01 version before trying the battery adapter.