I have a depth finder I bought for another project and considered how to mount it in my kayak. The first thing I was up against was finding a battery (and charger) to power it. I considered one of the AGM batteries but with the cost of the battery (≈ $30) and a charger (another ≈ $30) the cost became prohibitive. While in my shop I noticed my battery powered drill batteries in their chargers and it hit me. I already have a 12V battery and charger. All I had to do was figure out a way to connect my drill battery to my depth finder. That turned out to be easier than I thought. My drill is a Dewalt and the connections on the battery are two straight spades set down in the connector. I checked my electric junk drawer and found two straight terminals called Female Spade Connectors. I found two that were close and all it took was to insert a small screw driver to expand the width of the terminals so they would accept as the spade part in the battery as it is considerably thicker than the intended spade. The battery was not marked for polarity, so I had to use my multi-meter to determine that. Once I had the female connectors attached to the wires of the depth finder, a quick check showed that the kluge works. I am off to the local hardware store to find a plastic tool-box large enough to hold one battery and the excess cable and with a top flat enough to mount the depth finder display, and I will have a portable unit I can take in my yak or my Jon boat. I have the transducer mounted on a small float.
If you have a different brand drill make sure that whatever connectors you find to work that the connectors are firmly attached to the battery. And make sure you are certain of the polarity as reversing them can often cause damage to the depth finder and if left connected to the battery it will be harmed also.
Below are some pictures of the "almost" finished project. I opted for a small snap lid box. The second picture is the battery and excess cable inside the box and the third is the underside of the top. I used a thin piece of aluminum clad PVC sheet (Alucabond - locally made) to make the top stiff enough to take the screws and nuts to secure the bracket for the display unit. Not shown is the transducer which is presently mounted on a chunk of 2X4 as a float.