db, which fish finder unit do you use? You mentioned, no frills, so I'm guessing it might be a black and white unit. Those do draw a lot less power, maybe 1/4 what my Hook 4 uses. This unit replaces a Lowrance Elite 4x, which consumed about half as much power. The x in the unit name for Lowrance units means it has no chart plotter mapping/GPS function. Color and GPS and screen size all contribute to power draw.
The Ryobi lithium battery is certainly rechargeable and that is part of the beauty of using it. It has protection built in, so you cannot over discharge and ruin the battery. I have a number of tools that use 18V Ryobi batteries. These bigger ones don't get much use though, I use them mainly in my weed wacker and a brushless chainsaw. To be honest, if I didn't already have these batteries I probably would use a LIFePO4 battery as they are less expensive.
I have one of these 16Ah LIFePO4 batteries which I've used though about a half dozen charges so far and it has preformed well. It's hard to beat for the price, but it is physically larger with almost twice the Ah. It's an option if you need some of the added frills of a power hungry fish finder and don't mind the weight.
(You need to Login or Register to view media files and links)I was using a 12.8V 10Ah LIFePO4 battery for the last 8 years, but last winter I noticed it wasn't charging fully. I took it apart and found one cell of 3 batteries were leaking. That forced me to start looking at other options.
The Lowrance Hook 4 unit is no longer in production, having been replaced by the Hook2 and then Hook Reveal model. The Hook 4 is superior in that it has the ability to display Raster as well as Vector map images. Lowrance turned off Raster image mapping in their lower tier units, namely the Hook2 and Hook Reveal models. I had been using a Garmin GPSMap 62s in addition to my Elite 4x unit, but it always seemed like a disjointed system for fishing. The Garmin is definitely they way to go if you are bushwhacking or using seldom used portages, as it's a hiking GPS. Yeah, I could mark waypoints on the Garmin, but it wasn't on a detailed map. The Hook 4 unit allows me to view Navionics (Vector) maps if the lakes have been mapped(a few in Quetico), or as I have shown it can display Raster satellite imagery.
I have heard that it is not good for the transducer to run it out of water, so I try not to do so intentionally. I'm not sure if it causes overheating of the element or if it just needs a denser medium to flex against to reduce wear. I know some people epoxy or silicone their transducers in their boat, so perhaps it's not a cooling issue. I don't think running the transducer in or out of water would affect power consumption, it is pinging either way. Sound travels faster underwater, so perhaps it pings or listens harder out of water, I'm not sure.