Portable Battery Pack
Assuming you have a small black and white locator, your battery will probably last close to a week. Color or larger locators use much more power.
I just got back from Canada and I charged my Garmin 200 mid-way during the week to be sure. It may have lasted the week, but I wanted to be careful. Be sure to turn off whenever you can.
You may wish to consider a locator that runs on D or triple A batteries. Guys I fish with use such units and carry extra batteries. If weight isn't an issue you can take an extra battery along for your locator . Also, if you go on internet, you can buy longer lasting 12v batteries. I did.
easy to remember for series,
to connect to device, use pos(+) terminal of 1 battery, and the neg(-) terminal of the other battery...
to complete the circuit , use a jumper to connect the remaining pos and neg terminals
example of 12v 7ah battery use by portable finders
try googling "wiring two 6 V. lantern batteries in a series to make 12 V." (not trying to be a smart ass, but the answers I saw are too long for me to remember to type here) and I don't know how to share the link, sorry.