Lithium Iron Phosphate batteries for Ice Fishing
Yes, your Vex charger will probably work fine. The one caveat would be whether it can withstand the charge current that these 18650-sized cells will draw. It is possible that you might break the charger; you will find out when you try it.
I would not shy away from the higher capacity cells for this application. The cells you referenced are 1.5Ah, you can find 2.6Ah in the same size battery. The relatively low discharge currents needed by the Vex are ideal for those larger capacity cells. (But, the higher capacity cells will stress your charger more...)
A little more money into solution for PCB to not have to worry about battery damage because of over use or poor charging habits.
I read alot of the Ebay batteries are over stated in capacity, and that it is better to go through a real place. They also have a wider operating temperature better suited for ice fishing then what you reported.
I believe with this setup I can use the same charging system I have on my Vexilar now.
Here are the rules for LiFePO4 battery use: Nominal voltage, 3.33V fully charged, rested. (x4 = 13.32V on the battery) Do not discharge below 2.0V per cell (8V on the battery) or else permanent damage can occur! Do not overcharge above 4.2V per cell (16.8V on the battery) or else permanent damage can occur!
Charge 32F-104F only. Discharge 15F-140F. Capacity 85% at 32F, 50% at 15F. Perhaps as low as 20% at 0F. Keep these batteries warm!
The wall-wart battery charger you used for the 12V 7AH Gel battery will work fine to charge these lithium battery packs, as long as you monitor the battery voltage and disconnect when the battery reaches the required voltage:
While charging, 3.50V per cell (14V on the battery) is 90% charge; 3.65V per cell (14.6V on the battery) is 100% charge. Disconnect the charger when you see anything above 14.6V. The MOST time the entire charge cycle will take is about 3 hours, at least with 600mAh cells.
The energy storage of the 600mAh LiFePO4 cell is about 2.6W-h; this is about identical to the energy storage of a Duracell Ultra Power alkaline battery, which is non-rechargable.
Scanning Ebay reveals that 600mAh, 700mAh, 1200mAh, and 1500mAh AA cells are available, along with chargers. I saw 1200mAh cells selling for $13.29 for 4 pieces; this is an even better deal than the Walmart price, from a capacity point of view. 8 of these cells will make a 2.4Ah battery; that would power a Vex for 8+ hours easily, and weigh considerably less than 1#.
4 cell battery holders are available via ebay or Radio Shack; don’t be tempted to buy an 8 cell holder unless you want to build 24V batteries. Paralleling 2 4-cell holders is the way to go; the #22 leads can be twisted together, and the parallel #22’s crimp in the tab connector better than a single #22 does anyway.