What timing. I bought the Gomadic and just received it a couple of days ago and I am going to pick up the 800 this week. I received this email from Garmin a while back:
I would be happy to assist you with this.
We have created a power pack specifically for long or extended rides.
https://buy.garmin.com/shop/shop.do?pID=86599
You will be able to stay charged while using the power pack.
Please let me know if you have any questions.
I can't believe you need that specific charger....will let you know when I get my 800!