Back on the topic of power source.  I did a little research and ordered a Lithium Iron Phosphate Rechargeable Battery (LiFeP04) to power my Depth Finder.  It's the same size as the Lead Acid batteries many use for Depth Finders while ice fishing, but instead of 5lbs, its only 2.6 lbs.  I ordered a 10AH model with charger from here:
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(used a 12% off coupon found on the internet- battery12)
Pricey for sure, but I'm hoping the superior performance, capacity and weight will make up for it.