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Battery Life

A project log for Affordable Reflectance Transformation Imaging Dome

A simple and inexpensive way to image and analyze subtle surface details on objects.

leszek-pawlowiczLeszek Pawlowicz 10/21/2016 at 23:310 Comments

The small 12" dome whose build I documented here was mainly designed to be a portable system, for use outdoors in the field as well as for visits for museums, archives, collections, and so on. Since there's not likely to be a power outlet in the middle of nowhere, and power outlets may be difficult to access in other locations, the system can be operated off of a battery pack. In the components list, I specify a 6-AA battery pack for alkalines, and an 8-AA pack for NiMH rechargeables. But I've never actually tested how long the system can run just on battery power, using these two options. So, I thought I'd give it a try.

Conditions: The LED current is set at 350 mA, which results in a light intensity more than bright enough for short exposure times with such a small dome. The system has 48 LEDs, less than the maximum of 64 supported by the controller. The LEDs are on for 1 second, which is more than enough for any exposure conditions, even with the aperture stopped down for better depth of field. The camera is not attached, but the current draw for shutter control is likely to be insignificant. The endurance program is set up to run 100 sets of LED series at a time, so I may not get the exact count before the system runs out of power. The alkaline batteries are standard type, rated capacity of 2900 mAh; total battery pack mAh is 17,400 mAh. The NiMH rechargeables are charge-retaining (like Eneloops), rated capacity 2200 mAh; total battery pack mAh is 17,600 mAh.

Results:

Battery type# of full LED seriesBattery pack voltage at shutdown
6 AA alkaline 5205.1V
8 AA NiMH rechargeables3606.5V

This was a surprise to me. Standard alkaline batteries have a higher output resistance than NiMH, and given that the system draws a fair amount of current, I expected that the NiMH batteries would have lasted longer. But the standard alkalines lasted almost 50% longer. It's possible that at the highest current levels of 1A, the results might shift somewhat in favor of NiMH, but I'm not going to try that. The system will work with 8 AA alkalines, so I may drop the 6 AA battery pack as a recommended power option, and just stick with the 8 AA battery pack option.

NiMH are still recommended, though, since in the long run you'll save a lot more money using them than disposable AA alkalines. 360 artifacts photographed with 48 LEDs is almost certainly more than anyone could reasonably do in a single day; you'll run out of camera batteries and SD card space before the system quits on you. With 64 LEDs, that works out to 270 artifacts, still a ton.

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