Becca,
I have a non-Android phone that uses the same battery as my Android phone. So I charge that non-Android phone in the car while I use the Wherigo app on my Android phone.
If the battery gets low, just a matter of saving my progress, go back to my car, swap out the batteries, and continue with the wherigo app.
In most cases, a fully charged battery should be able to handle an hour’s worth of using the app if not more, depending on the phone model.
So as long as the wherigo cache in question is not one that is super long and extensive, you should be able to handle it easily on one full charge.
Otherwise, see if you have a friend with an old phone that uses the same battery as your current phone so you can swap out batteries if needed and not miss a beat – or –
Buy an extra battery if you have no spare phone. Charge both batteries in your android phone before you set out in the field and then carry the spare battery in your pocket so it’s just a matter of saving your progress then a quick swap in the field if the first battery gets low.
Hope this helps.