We recently put our Kestrel 4000 to a test when a customer logging data of the weather conditions outside, to his Android via bluetooth of while in the comfort of his home.   He was surprised, as it lasted about 24 hours and then would auto shut off.

What we do know is this....  
If running the Bluetooth at 2 sec sample rates, the approximate current required is 25 mA. (no backlight on)
Fresh batteries provide at least 1200mAhr of capacity. If we factor the efficiency of the regulator at the increased current and the auto-off of 35%, we calculate about 17 to 20 hours of continuous operation. This sounds like what the customer is observing – 24 hours. 

The Test: 

Determine running time and Bluetooth cutout capacity for the low battery condition.  Here were the findings:

With a fresh set of batteries in the K4000, a setup on a PC program to poll the K4000 every 2 seconds like the customer had done.
Once a minute, instead of polling the environmental data, it was polled the K4000 for its status and logged the battery capacity.

The K4000 lasted 22.5 hours before the battery indication shut the Bluetooth radio off. The last registered capacity was 36%. It was therefore assumed 35% was the cutoff. After cutoff occurred, (It got up to 2 hours later) they cycled the K4000 to see what capacity it would display. It was 52%.

The confusion stems from the fact that under the Bluetooth power, the battery capacity at cutoff is 35%. Without the Bluetooth radio, the same batteries indicate 50%. With the 50% indication, the customer can expect another 250 hours of operation out of the K4000 without Bluetooth. Unfortunately, the battery cutoff was necessary to ensure the Bluetooth module has enough power to operate properly. 

This is very much in sync with most Bluetooth Classic connectivity as it relates to battery usage.  For example, if one were to use a Bluetooth headset continuously for 22.5 hours with their iPhone and the battery drained, that would be understandable (within reason).

This was a great customer challenge.  The engineers at NK are awesome, they can always find the answer!

Hope you found this helpful!