This sounds all too familiar.
For the first time today my new K3 started showing "Battery Depleted" when the 2 new Pentax batteries were fully charged, AND when the two backups I carry were inserted.
After some fiddling I realised it's camera, not the batteries, and as stated by others above: turnjng the camera/grip off and waiting a few seconds seems to fix it.
I have a theory as to the cause, and it relates to another issue I have experienced with two K5 bodies and now this K3.
It usually appears after a short time of using older heavier lenses with AF and always those with electronic couplings:
SMC Pentax-F * 300mm 1:4.5 ED(IF)
SMC Pentax-FA star 200mm f2.8 ED IF
Tokina AT-X PRO 80-200mm 1:2.8
I believe that some lenses start to develop a small amount of rotational 'play' in their mount. What could be expected if the locking pin protruding from the camera into the lens mount socket over time made the socket enlarge. The mounts on these lenses are typically aluminium after all, and the amount of rotational play in some of my lenses is quite noticeable. However the locking socket does not appear distended or damaged, so evidently the amount of distortion is miniscule.
Somehow this rotational play seems to become worse the longer and more I use some of these larger lenses, and along with it another problem begins to surface, firstly:
The f - - problem.
Even though the lens aperture is in A mode, and the camera generally set to Av mode, I frequently notice the camera will display f - - when I wiggle the lens slightly. This is caused by the electronics between the lens and the camera becoming uncoupled... disconnected. Exactly the same as if you had switched the lens out of A and into one of the manual Apertures, or if you had pressed the release and begun to or actually removed the lens.
Now here's the clincher:
On several occasions I have been holding down the cameras release button in continuous fire mode, rattling off shots as fast as the camera can fire, and the lens has rotated slightly, the electronics uncouple, and :
-in the case of the K5:
The camera completely locks up.
The LCD is still displayed, and the shutter appears to remain stuck open exposing the sensor, but the camera is effectively dead.
The only fix I have found is to remove both batteries (grip and body) and wait a minute or two. Sometimes just popping them out and straight back in will cause the camera to immediately close it's shutter and recover.
Any images in the buffer will be gone.. kapoot, vanished.
Shutter actuation counts will be regressed somewhat, sometimes by the number of buffered shots lost, sometimes back to the last time the camera was turned on. (yeah, read that again)
If the camera shoots in f - - mode and doesn't crash/lock up, which is its typical behaviour if you aren't buffering a bunch of frames at the time, then it is shooting with the aperture wide open like Av mode when the lens set to one of its f/stops instead of A mode (expected behaviour for manual aperture in Av mode). However the exif data *may* still register the user specified aperture. The image will almost always be over-exposed.
-in the case of the K3:
The camera will often destroy the image, sometimes making a collage of several images, or sometimes simply producing a black frame, but it does not seem to hesitate or stop at all. Other times AF will fail, internal readouts turn off, and the shutter cannot be released until the lens/camera is wiggled slightly to make the lens/body connection short again.
The exif data will (afaik) always reflect the user specified Av mode setting for Aperture, but will be shooting wide open, and over-exposed.
However the camera does not lock up, and i often I won't even notice that it's happened till I find a zero byte image later.
In the K3 and K5 the AF will usually fail for as long as f - - is in effect.
I strongly suspect that the lens<->body electronics becoming decoupled, and the resulting f - - problems, and the way the K5 would simply lock up whereas the K3 seems to handle it gracefully are related to the "battery depleted" of the K3
I believe that the K5 just chokes and dies, whereas the K3 tries to deal with it but often can't, so the electronics goes into a power-problem shutdown state.
Note however, that the above has never happened on any of these 3 cameras when using (note that they are smaller & lighter):
SMC Pentax DA 10-17mm f3.5-4.5
SMC Pentax-F Zoom 1:3.5-4.5 28-80mm
And I have excluded these lenses, which have never manifested the problem, but probably becasue they are either M-series with no electronics, or A-series with no AF... AND because they are not as heavy and long and have never developed any noticeable rotational play in the mount/body connection:
SMC Pentax-M 1:2.8 28mm
SMC Pentax-A 1:2.8 28mm
SMC Pentax-M 1:1.2 50mm
SMC Pentax-A 1:1.2 50mm
SMC Pentax-M 1:1.4 50mm
SMC Pentax-A 1:1.4 50mm
SMC Pentax-A 1:1.7 50mm
Zeiss 85mm F/1.4 ZK Planar T*
... or are just light, and not seen a lot of use, much less being slung about on a monopod all day:
SMC Pentax-F 1:2.8 100mm macro