I am planning to buy a Anker battery pack but have a question. If we are using our Garmin 800 and the battery is almost depleted can it still be used by attaching the battery pack?
Thanks!!

Yes. I do that regularly.

Note that you might not need a very large pack (the 3300 mAh one would charge the 800 about 2.5 times).

(You don't have to wait until the 800 battery is almost depleted.)

All of the units from the 800 and newer should all work.

For what it's worth, my current battery is this one. It's smaller and cheaper than others I've used, and it has proven to be more reliable. The capacity is more than I need. I think it can charge an iphone 2 or 2.5 times. The size is very handy.

There are probably a dozen different brands of USB external battery packs that are equally good. I'm satisfied with the Jackery USB battery. Got one last year based on recommendations from a friend. Works great and it's solid. I've dropped it several times, mostly on pavement from handlebar height because I'm a klutz and the USB battery packs are shaped like bars of soap just asking to be dropped. Barely a nick in the cover, keeps on working.

So far my only USB rechargeable devices that can't be run and charged simultaneously is the Light & Motion Urban 500. Plugging in a battery pack puts it in standby recharge-only mode. Too bad, it's a great light otherwise.

All my other USB lights, smart phone, flip phone, video camera, etc., can run and charge simultaneously. I got a tiny, lightweight USB battery pack, about the size of a cigarette lighter, just to mount on my helmet to enable extended run time with my video camera when it's mounted on my helmet. When I run the video camera on the handlebar I'll hook it up to the Jackery and it's so efficient the camera is often fully charged by the time I get home while simultaneously recording my ride. The smaller, cheaper USB batteries aren't as efficient.

I think I've proven to myself that you want to start out on the external battery so that it doesn't have to charge the Garmin at a high rate. I have some 2600mAh lipstick batteries that I use to keep the garmin charged. One of them is going to get me through the fleche tomorrow. I'm not sure it will even be fully depleted by the end. I think on 4am starts, the 2600mAh is usually depleted at about midnight or later. But then the internal battery is fully charged and gets me through. The lipstick batteries are a nice size to mount to the handlebar. I use a twofish lockblock to hold the battery. I carry extras, they are light.