Samsung Galaxy S3

[Tonyf33]

S3 batteries can be had for not very much so you can always have a fully charged one to hand http://www.ebay.co.uk/itm/2100mAh-Li-io ... 075f5282or if you don't trust ebay http://www.amazon.co.uk/Samsung-Galaxy- ... B007Y8VJP6
plenty of holders too
http://www.holdersandmounts.co.uk/samsu ... bar-mount/
http://www.amazon.co.uk/Cycle-Handlebar ... B008FXURDO
I had my 'bulky' GPS mounted and used it for a few years on various bikes without any problems

[Meshuga]

I have used the Proporta Beachbuoy whilst off-roading. works well but a little fiddly to get on the bike. Can also be worn around the upper arm more easily. Totally waterproof and have used it for my Nokia N8 for underwater shots. Only thing I can foresee is the S3 is huge.

Anyway, review here

http://www.allaboutsymbian.com/reviews/item/14649_Proporta_BeachBuoy_Bike_Mounte.php

[slogfester]

I had a S3 mounted to the stem (using Herbert and Richter universal bicycle mount) on my touring bike for a 4000 km on and off road tour of Europe last year. Never missed a beat. Spare OEM batteries weigh nothing and can be bought for c. £7. Run in 'battery saver' mode and use nav apps that have voice instructions and offline maps (e.g. OSMAnd) so you can turn screen off most of the time. Check you don't have wifi/Bluetooth on. (You will be amazed how many people leave on). Data connection also sucks battery life- with offline maps you can wait till coffee breaks to check your email! You'll easily get a whole day out of 1 battery.

[pete75]

Something like this for the phone on the bars

http://www.ebay.co.uk/itm/Black-Waterpr ... 58a9769a88

and this in you saddlebag for keeping the phone charged up
http://www.ebay.co.uk/itm/20000mAH-Batt ... 5400c8cbb5

[freeflow]

Try the topeak smarphone bag (android 4-5" version). A very neat handlebar mount. You may need to check for size. I think there are similar non topeak version that have a non reflective front cover.