I tried the solar cell and rechargeable battery approach for a couple of seasons a few years back and wasn't very satisfied with the results.

I've since changed over to lithium batteries.  They are MUCH lighter than rechargeables (and even standard batts) and, with their improvement over the past couple of years, have a substantial service live.  They are, of course, expensive.

I've been running my depth finder with AA lithium batteries for the past 2 seasons and I've very satisfied.  I don't run the unit daily (I'm not typically pinging the bottom on travel days) and my use is probably on the order of 6 hours per day when fishing.  This season I did a 10 day solo and 15 day solo (with company).  One set of batt's was all that was needed for this season.  In fact, my GPS unit batteries died on the last day out and I yanked a couple of the depth finder batts (because they were more handy than other replacements) and they were ample to finish my last day.  I haven't thrown a volt meter on them yet to see what "signal strength" remains, but I'll swap them out for fresh ones before the next season anyway.  BTW:  I think the GPS batts were the lithium batteries from LAST year's trips.

dd