I've found that my Garmin will run for about 12 - 14 hours in record mode, I'd like to be able to run it for longer for 24hr events.
Looking at the spec it would appear to have a 1000mAh battery, so if I plugged one if these in http://www.sportpursuit.com/catalog/product/view/id/475678 with its 2800mAh charge facility would it be reasonable to assume that 24hrs would be a breeze?
They had issues with external power supplies resetting the device as it assumed it was being plugged into a PC. IIRC there was a firmware fix.
Either way, I'd test it before you plug it in 11 hours into a 24h race as if it's not on Strava it didn't happen* :-p
*actualy tempted to sell my 800, I do strava from my phone and remembering to charge the garmin is a PITA. Only reason I've not sold it is a decent computer that does speed, cadence and HRM is almost as much as a 2nd hand 800's worth, and the mapping is better than nothing.
I think that it depends on the kind of lead you have. If you use one that only connects to the power then you will have no problems using an external battery. I think that you can run into problems as described above if the lead has connections for data.
I've run an extra powerpack on my 800 without problems, I tend to use it from the outset rather than adding it as the battery on the 800 fades. However, you will have to think about how you will waterproof the connection when riding with the battery plugged in.
Apparently the Edge 800 can crash after about 20 hours. Seems to be a limit on how long a track it can record. Best to avoid this by resetting it and clearing the track before this happens.
I run mine off an external power suply occasionally. Just a generic battery off amazon. It works fine for any of the rides I've done. Max I've taken it to is 16hours and it didn't crash. Might be worth resetting mid ride on longer ones. I mainly use it mapping mode so will have a spare battery on anything I expectto take longer than 8hours as it eats battery.
Run it off the external supply first as it behaves better that way than waiting for it to run down.
I did 302km on an edge 800 no issues, however there is a glitch and for large rides now I save and then use the following
That's what I was thinking, use the external and then when that goes flat rely on the inbuilt battery
have a look at powermonkey stuff https://www.powertraveller.com/
something like https://www.powertraveller.com/en/shop/portable-chargers/back-up-business-travel/powermonkey-discovery/
I got the extreme, solar panel works too even in british sun! Loads power plus I use it from bar bag for long rides no issues with connections.