Greg, thanks for the response. I understand everything you are saying about the electronics aspects of this. I too would have been inclined more towards a 4S battery pack. But even the 3S might work fairly well. The rated voltage on a 3S pack is 11.1 and on a 4S pack is 14.8. But when these battery packs are fully charged they actually test out at about 12.6 and 16.8 respectively. Usually when they are run down to the point where they are ready for recharging they still have close to the rated voltage available. One potential problem that I was concerned about is you can't get away with getting anywhere near fully discharging them or you will kill them.

Both sizes are available in amp hour ratings as high as 6. But they get prohibitively expensive at that capacity at around $85 or $90. But for 3.3 amp hour capacities you can get them at around $45. It is the savings in weight and size over the alternatives that I am interested in.

I wasn't sure if there was anything about them that might preclude their use for powering a fish finder. It seems that most of them are designed for relatively high discharge rates. I didn't know if the slower current draw of a fish finder might cause any difficulties.

Thanks again for your response.