If you buy a new battery for your phone, it is expensive and I don't know if it would work in a new phone you might get next year. If you buy a new i-phone, you'll suddenly be dealing with something new on your up-coming trip. Those were things I considered with my 3-year-old Android.
However, if you don't have a big screen phone already, you would probably love the big one available on iphone now. Definitely worth its weight in gold.
I found that the best way to conserve battery was to use Airplane Mode. The big drain on battery is when the phone is searching for phone signals, and checking for notifications, etc. You can turn on WIFI even when you are in airplane mode, but I would keep it off unless I was using it. If you are getting low on battery, you can turn OFF Location until you need it. The GPS generally still works.
What
@jmcarp said is true. Mine has a small USB cable that you use to charge the charger (which is a battery), and then you reverse the ends when you want to charge the phone. It is very obvious how you do it. You can carry and use the phone while it is connected to the portable charger.
My brand is a cheap one with no label remaining. For my particular phone (not i-phone) I should have a charger for 3100 mAh (because that's what is written on the existing battery). I can't remember why I got the lesser size (maybe I was too cheap, paranoid about weight, or it was out-of-stock). Nevertheless, this still gives my phone almost a 50% charge, which is fine to get me through the day, especially since I am more careful after I resort to the backup charger.
You can go into an electronics/accessory shop (maybe your phone store won't stock them, because they'd rather sell you a new phone). Just show them your phone and ask for the options. I have seen 2 types for i-phone:
- My cylindrical one weighing 73 g that must be recharged each day (or maybe 2 days if you don't use it; the charge does disappear in time). This is the identical charger that my companion used for her old i-phone and got one full charge out of it.
- Heavier and more expensive type like a case, that your phone fits into. I really don't know, but these might give you more than one full charge. The disadvantage here might be that if you get a new phone next year, the casing might not fit. (Just something to consider.)
I am not very knowledgeable about GPS. My companion knew more and she had the route downloaded onto MapMyWalk on her i-phone so we could follow the dancing blue dot. It was very helpful on the VdlP and I will definitely figure this all out before the next walk. You have a head start on this, as you have used a GPS device.
Bottom line: I would not go without a big-screen smartphone, with GPS track downloaded, and a portable charger on hand.
