I have procured a Garmin etrex 20X GPS. The Garmin Dakota is extremely similar aside from the etrex's lack of a touchscreen. The battery save and backlight dimming features can both be deactivated in software, so there is no reason to construct an automated finger for testing. Why Batteroo didn't simply read the manual and do the same will remain a persistent mystery.
Here is the initial data from testing the unit directly out of the box. The etrex 20x uses two AA alkaline, lithium, or NiMH rechargeable batteries and has a mode setting for each such that the low battery warning functions appropriately.
Battery cutout voltages:
Alkaline Mode: 1.05V per cell
Lithium Mode: 1.29V per cell
NiMH Rechargeable Mode: 1.05V per cell
Notes:
- These are single data point readings. I have not had time to do more.
- These measurements were taken indoors with no GPS lock. I am assuming that this is an equivalent condition under which Batteroo conducted this test.
- Screen brightness set to 100%
- All power management functions disabled
- This device, without a doubt, already has a boost converter installed. Here is a test run in alkaline mode, full screen brightness, no power conservation features
V I (mA) mW
================
3.2 147 470
3.0 150 450
2.8 160 448
2.6 172 447
2.4 188 451
2.2 207 455
2.1 218 458
This is clear evidence that adding a "Batteriser" type of external boost converter could only result in doubling up on converters and reduce the overall efficiency of the device. This device is already engineered to maximize the depletion of a alkaline battery by operating in constant power mode.
From looking through the GPS, I believe that there are numerous ways to alter the results of a rundown test. I will be working on those next. Simply by turning off the backlight, one can reduce the power draw by over 50%. This can have a very dramatic effect on battery life as evidenced by the attached graph extracted from Duracell's MN1500 data sheet for AA alkaline batteries. Changing the operating mode of the GPS such that its 450mW typical power use turns down to 220 mW, will change the expected operation time of the device from approximately 3 hours to over 8 hours.
In short, reducing the rate at which power is extracted from a battery is how to extract the absolute most energy from the battery. Increasing the power draw, by doubling up boost converters is not a plausible method by which to extract more energy from a battery. It is exactly the *wrong* direction one has to go in order maximize battery performance.
More to come....