Cool project... I'm in
One thing I've found with the battery life relating to GPS is that the GPS feature activated means the phone is sending and receiving signals. That could mean to the cell site as well as the GPS satellites.
If I want to limit battery drain I turn off the GPS feature. Using my phone I access that by going to settings - location. Then select or deselect wireless networks and/or GPS satellites.
You should also be able to revisit the map you downloaded in all the detail you had viewed previously. what you won't get is turn by turn updates unless you have the GPS and/or networks location features activated