Had an interesting situation the other day out fishing at one of the lakes that is trolling motor only. My battery, which is getting toward the end of it's life, just decided to die in a matter of minutes. I was about 1/4 mile from the ramp and the sun was going down. I started rowing while nico steered with the net. It was pretty lame. After a bit I decided to try my graph battery by touching the wire to the posts. Sure enough it worked fine and got us back to the ramp.

The battery is this little guy:
http://www.basspro.com/servlet/catalog.TextId?hvarTextId=30289&hvarTarget=search&cmCat=SearchResults

It's a good graph battery so you don't have to run your graph off your main battery (possibly getting interference) and it's 20 bucks that will get you back to the ramp in a pinch.