As it turned out, I was in the market to replace my 12V AGM/Gel batteries for both my Vex and underwater camera, which were pretty much done.
I did a bit of research, then went back and bought 2 packs. Went to Radio Shack and got a couple 4 cell AA battery holders, about $5 for the pair. Taped the 2 battery holders together and wired the leads in parallel to make an 8-cell, 12V, 1.2Ah battery pack. Added shrouded male QD tabs, so that both my Aqua-vu and Vex would plug in, and I had a nice solution to my battery problem.
I had previously measured the current draw of my camera at 450mA, and the sonar at 210mA. I tried running the camera on a single 4-battery pack right out of the package, and it ran for 45 minutes; after recharging the batteries, it ran 50 minutes. In other words, the batteries were at about 90% charge even after sitting on the shelf at Walmart for however long. Pretty impressive, for a rechargeable battery.
The 8-battery pack runs the camera for 110 minutes (measured). It should run the sonar for about 4 hours.
The major advantage is that the battery pack I built weighs 7 ounces including the foam insulation; the battery it replaced weighs 5 lbs. Since I mostly only use the sonar and camera for locational purposes, the run times are plenty long for me.
It is handy that I can use the very same chargers that I used for the 12V 7Ah Gel/AGM batteries. These are “wall-wart” DC supplies that put out 400-600mA, and top out at about 14.5V. They were made for safely being connected to a Gel/Agm battery for indefinite periods, but they also work great for fast charging LiFePo4 battery packs, getting a full charge done in about 3 hours.
These lithium batteries are a different chemistry from the lithium-ion batteries in your phone, laptop, and car. They share a lot of the good characteristics – fast charge and recharge, low self-discharge. They are not quite as energy-dense, but they are also apparently extremely safe – they are not susceptible to the thermal runaway issues that plague lithium ion. Oh, and they are rated for 2000 cycles as well…….
I will add a post to provide more detailed information on these batteries, as it can be challenging to find. Please note you cannot use these to replace the AA batteries in your GPS or other product, as the cell voltage is over 2X, and you will fry your toy.