Fred Whitton - Garmin battery life......

Did it two years ago. Your Garmin should be fine, I was using a Mio 505HC without bluetooth and I still had 40% battery left at the end. The feed stops are really good. Proper food, sandwiches and stuff on offer as well as sausage rolls, cake and jelly babies. 

Main advice would be to try and find a group on the flats as it can be quite windy and exposed if you're solo and try and get up near the front before a climb, can be a bit of a nightmare if you're at the back due to people, walking, stopping and slowing down. I found myself weaving through the groups quite a bit which was a nightmare.

Back to the original question, my experience of using an Edge 800 is that only using the map screen when really necessary makes a significant difference to battery life. I have also used a USB portable charger with good results: http://amzn.eu/8N6WM5q I've found this (and other products with the same brand) to be good quality. I'd probably wrap it in something soft before taping to stem/top tube, as they probably aren't designed to deal with road shocks/vibration.

I tried - and failed - some years back on a 34/29. There's no shame in walking, plenty will be up hardknott. it's 30% + and not just at the hairpins. If you've managed 34% gradients before on 36/28 ??!! then in theory you will be OK.

Then again after 90 miles + the longer ascent of hardknott, it would be unnecessarily hard to try it on your current set up.

As others have said definately get a compact for this ride and good luck.

Thanks for the advice re gearing. Will see if I can get a 32t cog in there and try and borrow a 34t cog from the wifes bike. I think pacing will be the most important factor. And fuelling. Any idea what the food stops are like, and the best fuelling strategy for the ride. My mate says take some real food ( Ginster's pasty) for about half way point for a psychological boost

You want to save your legs as much as you can. Hold back until you get to hardknott then if you've got something left you'll get up it and finish strong. Compact with 32 is best. Also I wouldn't load route on Garmin. Star all the major climb segments in strava and then when your approaching the climbs it will automatically prepare you for what's coming. Also you'll get the gradient map so you can pace yourself. Don't think stared segments work when following a route.  Fred Whitton climbs strava segments:

5.5 miles - Kirkstone Pass from Holbeck Ln (full climb) - 4513289

20 miles - Ullswater-Matterdale climb - 1144239

27 miles - A66 climb – 1.2 miles - 677912

42 miles - DP Honister climb – 5956789

47 miles - OFFICIAL 100Climbs No81 Newlands Hause – 6677326

55 miles - Whinlatter – 1.8 miles - 2646437

66 miles - Fangs Brow – 651758

69 miles - Cogra Climb to Croasdale – 3778055

74 miles - Another 100 Climbs No180 Burn Edge – 7218056

81 miles - Stanley Arms to the Red Admiral –1088740

86 miles - Climb out of Stanton Bridge – 5892499

93 miles - Hardknotts Pass – 637403

96 miles - The Whole of Wrynose – 1807845

102 miles - Little Langdale little climb – 4490357

Slightly off topic as you already have a great computer but if anyone was looking for a new one the Wahoo Elemnt Bolt is well featured, navigates, takes texts and phone calls when paired to a 'phone  and (probably due to the black and white screen) has a battery life of 15 hours even with turn by turn navigation on.

I don't work for Wahoo, but I did buy one and first impressions are really good.  I upgraded from an Edge 500 though, not one of the newer ones.

I tape a USB power block under the stem and run a charging cable up for events where it is potentially going to run out. Turn off Bluetooth and put the timeout to the lowest, brightness down etc. I can get 8 hours out of a 3 year old well-used 810 this way.

There are a few vids on youtube showing you how to get 11-32 to work on SS mechs. I think Durianrider has one if you can put up with the swearing. Gavlee is right it is a matter of maxing the b screw.

Used compact crank and 32 rear on short cage ultegra. Best to save the legs as much as you can for the grind fest at 99miles. I'd imagine a 32 would work on di2 also as it just needs the bscrew adjusting to clear the top jockey on the 32t. I had more adjustment left if needed but didn't fancy trying a 34t. Chain needed 2 extra links from prev set up similar to your current one.

Sorry to hi-jack this thread a little, but I'm also doing the Fred Whitton. I was just wondering if I need to change gearing. I'm currently running 36 x 28 as a lowest gear currently. i dont think I can swap out the cassette to a 32t, as it's Di2 short cage. i could possibly buy a 34t front cog, but am wondering if it's worth it. I can currently get up 34% hills (Mytholm Steeps) with this set-up and have a watts/kg of 3.6ish. Any thoughts?

turn off 'turn by turn' navigation. should last 12+ hours. 

The 810 will last 6 hours no problem.

So don't worry.

As thereverent says this will be more than enough to get around the fred within the cut off times and the garmin to be on. First attempt my mate used a 1000 with backlight on and WiFi. It turned off 3/4 of the way round. Following year he bought a usb battery back up but didnt need it once he had done the above. I used 510 both times turned everything off apart from sensors and route (basic on 510) with 50 odd % left on it at the end (8hrs). Enjoy the Fred, great but tough event, your 810 will be fine my legs gave up much sooner than the garmin.

Backlight and Brightness are the main killer for my Garmin 1000 - two separate settings on mine. Brightness down to 1/10 and Backlight timeout to 15 seconds.

For external battery, I use an Anker PowerCore  - big capacity - 3350mAh but is small enough to cable tie neatly under my stem - 95mm long by 23mm dia. I've done 250+ miles over 23 hours with such a setup, using the battery to charge the Garmin as I go.

Get yourself a small USB battery charger and short mini USB cable - I looked around recently, and the best option seemed to be a Veho Pebble Ministick, slightly smaller and slightly more durable looking than the thousands of similar options on ebay. You can tape to your top tube and connect to charge while running the Garmin, or leave in a back pocket, and recharge while stopped.

Otherwise, turn off bluetooth and turn backlight to low, which extends life quite significantly. Download the gpx to follow as a course if you want, but only follow that if necessary (this doesn't make a huge difference in my experience on the 810, but I believe the 820 drains particularly quickly when following a course).

Swap to a Fenix 5

Turn the backlighting down as low as possible.

Turn bluetooth off.

Turn Wifi off.

Turn Strava live segments off.

These should give you more battery life.