If you're at a point where a handheld GPS is saving your bacon, things have already gone seriously wrong in your life. If you're seeing this on a regular basis, you need to rethink the way you're doing business.
A paper chart never needs batteries. Your GPS should be powerable from the aircraft, and your handheld GPS shouldn't really be something on which you're relying. Using, sure...but not relying. Accordingly, it should never be a bad day if the batteries on your handheld GPS fail.
It's very possible to fly without one at all.