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Old Mar 05, 2011, 10:32 PM
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A 4 cell AAA External Battery Pack for the #11 HD Key Cam

I decided to get some better use from the emergency charger for this camera that was donated to me (thanks lostheli). My goal was to have a portable power pack that could allow my #11 to record a continuous video until my 4GB card fills up. Since the stock 250 mAH lipo can record at least 40 minutes of video with a total file size of about 2.12 GB, doubling that capacity to 500 mAH should easily fill up a 4GB card. So, having a number of 800 mAH AAA Sanyo Eneloop NiMH rechargeable cells laying around doing nothing, I decided to use 4 of them as the external power source.

I found this neat fully enclosed, switched, 4 cell AAA battery box at my local Radio Shack store. Since it is switched, I thought it would be nice to also have an LED power indicator, so I also picked up a pack of these LEDs and these load resistors. So, with about $6 invested in the parts, I proceeded to hack them together with the special USB charge/power plug from the stock emergency charger. The pics below will document the process in case someone else wants to do something similar.

Recording Test Results and Specs
  • camera: #11 HD key cam, CMOS version 2, 120 mAH internal temporary lipo, 7060 kbps ave. total bit rate (sound plus video), 4GB flash card
  • pack size: 63mm x 49 mm x 15mm
  • pack weight (with 4 AAA 800 mAH Sanyo Eneloop NiMH cells): 72 gms
  • pack capacity: 800 mAH
  • pack voltage fully charged, resting: 5.69 V
  • pack voltage after recording stopped on low power, resting: 4.22 V
  • Charge put back into pack after test: 859 mAH (107 % of rated capacity)
  • recording time with 120 mAH lipo cell alone: 25 min 57 sec.
  • recording time with external pack and lipo battery (both fully charged): 123 min. 23 sec.
Comments: The external battery pack will power the camera while keeping the internal battery fully charged until the pack voltage drops below the expected nominal 5V input at the USB connector. At that point, the supply power via the regulated USB connection has dwindling added value, with the camera's internal battery then doing most of the work to finish the recording.

In my test with a 4GB flash card, I recorded until the camera stopped with a full card (73 min. 4 sec. of video) and measured the voltage on the internal and external pack. The camera battery was still fully charged, and the external pack had a resting voltage of 5.05 V, approaching it's maximum added power ability. I wiped the card, and continued recording without charging until the recording stopped due to low power. The last 45 sec. of video had the intermittant buzzing sound indicating low power, and the internal lipo was discharged.

The 120 mAH camera battery alone could record for 25 min. 57 sec., and the external pack added about 97 min. 26 sec. of additional recording time, much more than needed to fill my 4GB flash card on it's own, with the internal battery in reserve, easily meeting my goal for this external pack. When I replace the temporary 120 mAH lipo in the camera with a new stock 240 mAH cell, I should get at least 20 min. more recording time, or about 2 hr. 23 min. total.

Further testing by Isoprop has confirmed that with a flash card larger than 4GB, the recording will stop and save the file when the file size reaches a full 4GB in size, then continue recording several seconds later. This is due to the 4GB max. file size limit of the FAT 32 file system. So if your camera records with a total average bit rate of about 7060 kbps as mine does, with flash cards larger than 4GB your recording will be saved in individual 4GB max. clips with about 75 minutes playing time each. If your camera is one that records at about 10,000 kbps total bit rate, the playing time of each clip will be proportionally reduce to about 53 minutes each.

So what kind of recording time might be expected from a pack made with 2000 mAH AA cells instead of the smaller 800 mAH AAA cells? Based on the fact the 800 AAA pack delivered just over it's rated capacity and extended the recording time of the camera by 97.4 min., it could be assumed the 2000 mAH cells could deliver a proportionally extended recording time, or about 2000 / 800 x 97.4 = 243.5 extra min., plus the internal 240 mAH lipo battery time of about 45 min., for a total recording time of about 288.4 min., or about 4.8 hours! For the bit rate my camera has, this would produce a total of about 14.9 GB of recording (three 4GB clips and one 2.9GB clip), requiring a 16GB flash card to hold it all!

<EDIT 2/4/2013>
Questions have been asked about this pack delivering about 5.6V when freshly charged, well over the USB spec of 5V (+/- 5%). I have powered the camera with this pack with no adverse results that I could see, but my use of that power supply is very limited. So while I had no problems, it could be stressing the circuitry and lead to problems later.

I subsequently made a new extension cable to power the camera that is both longer and incorporates a diode (on the downstream side of the pack plug) to drop the delivered voltage to the camera by about .7V. I have not tested to see how much recording time I lose with this arrangement since the pack lasts long enough for my purposes. I still use the other extension cable for charging the batteries while still in the pack since the diode would prevent charging.

If I were doing another pack, I would consider using one of these nifty small USB power supply components in the extension cable to the camera instead of a diode. This would not only clamp the delivered battery voltage at 5V from a freshly charged pack, but might also give even longer recording time than the batteries normally would on their own by maintaining the 5V power supply to the camera as the battery voltage drops below that point. The only potential problem I can see with this added component is it might continue to power the camera while discharging the cells below their healthy discharge voltage (about .9V/cell) and cause the cells to fail sooner than normal.
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Last edited by Tom Frank; Feb 04, 2013 at 10:08 AM. Reason: added file 4GB file size info