Just thought I'd post an update about this as I'm aware of the fact that people had had problems with this in the past. I used a Garmin 800 and the Garmin External Battery Pack installed on my TriBars for my ride in the Mersey Road 24hr TT last weekend, you can see it below my right wrist. With this setup I recorded the data set for the 24 hr race without any problems, here is what I did.
Firstly the firmware on the GPS unit was the latest version at the time of writing which is v2.50 and the GPS software version was v2.86. As I was riding a marshalled race I did not load a route, I was using the Garmin 800 as a recording device rather than a navigation device. During the race I recorded all the usual parameters (position, speed, elevation, temperature, heart rate etc. and cadence and power data were also recorded, the data being supplied by an SRM unit.
I set the Garmin 800 to record at one second intervals and did not use smart recording as I wanted the data to be as clean, uniform and detailed as possible. I elected to use the Garmin 800 instead of the SRM PowerControl 7 head unit because the stopwatch functions suited the event better and I could constantly display data such as average speed over the last 10 miles as well as overall average speed which I was using to manage my effort and monitor my progress towards my race target.
I elected not to manually mark any laps at all during the race but set the Garmin 800 to automatically record a lap every 10 miles and to display that information, this allowed me to monitor my progress. During the night, to conserve battery power when a 10 mile lap was completed the backlight lit up for just 15 seconds so that I could briefly see the screen and know how things were going without constantly burning juice by keep activating the backlight or having to have some sort of additional lighting to allow me to see the screen, an additional complication.
In an effort to reduce the likelihood of recording errors I cleared the majority of the stored rides from the unit which had all previously been downloaded anyway, and I recorded to the internal storage of the unit rather than a storage card. I disabled the mapping to reduce CPU load but whether this actually made any difference I don't know, I just knew that I didn't need the mapping to be active so there seemed no point in using it. The file size at the end of the 24hr ride was only 2MB and there is around 105MB of internal storage in the unit, about 12MB of which is allocated for system "stuff" so there is plenty of space.
The Garmin External Battery Pack is essentially a Garmin version of the PowerMonkey Explorer and is intended to either power or recharge a Garmin unit. The stated battery life of the Garmin 800 itself is I think about 10 hours and the Garmin External Battery Pack is intended to provide power for 20 hours. The total recording time for the setup should therefore approach 30 hours in decent conditions, long enough for a 24hr TT.
One the Garmin External Battery Pack is fully charged and mounted on the bike it's easy to use but there is one option which confused me which is the mode of operation which can be either normal or "Always On" mode. The details of how to select the modes are in the manual which I know we all read carefully! The bottom line is that I used the unit having selected "Always On" mode and it worked fine for me so that is what I recommend. What "Always On" mode does is ensure that the External Battery Pack will continue to power the Garmin Edge after it is fully charged, according to the manual.
This means that in "Always On" mode when you set off the Garmin External Battery Pack is continually charging the internal battery of the Garmin 800 whilst it is in use, so as time passes the charge in the Garmin External Battery Pack falls and the internal battery of the unit remains full. The display on the Garmin 800 will show a small "lightening bolt" to indicate that it is connected to an external power source. After approximately 18-20 hours the Garmin 800 will beep and show a message "External Power Supply Lost" - this means that the Garmin External Battery Pack is exhausted. You do not need to do anything as the Garmin continues to operate from the internal battery though it is no longer being continuously charged by the Garmin External Battery Pack which is then empty.
I found that at the 24 hour point, the end of my race, the Garmin 800 was still showing that it had 60% of it's internal charge available so almost certainly enough to run for another 5-6 hours.
I hope that saves someone some head scratching! I am grateful to Stuart Birnie who generously shared his enormous experience with me when I was trying to work out how to approach this problem. Unfortunately I was unable to borrow his legs for the event, maybe another time!