I am currently building a large laser-tag system, and one issue that has come up is what sort of battery chemistry I should use. The whole system will probably end up drawing around 5 watts under normal use, and I'd like it to last at least 3-4 hours. My original intent was to use NiMH cells, but the cost and size (mainly size) of the capacity I need was getting out of hand.
With that in mind, I decided to start looking into lithium-based cells. At first the advantages of these cells looked awesome, and they're actually cheaper than NiMH cells of the same capacity. However, I've learned that most lithium-based cells are susceptible to thermal runaway and explosion if you do something they don't like such as overcharging them or drawing too much power. This is a huge issue because my system is going to be used by small children. Safety is of the utmost importance. There needs to be a virtually-nil chance of this happening.
However, I learned that there exists another lithium-based battery chemistry that doesn't have these problems: LFP or LiFePO4, also known ambiguously as Lithium Iron cells. These are what I'm currently leaning towards using in the design. Unfortunately, information on these cells is scarce as they have only recently started to be widely adopted.
So the first part of my question is, can lithium-ion cells be made safe enough for use in this system? I'm willing to put in a fair bit of effort for safety features, and I do have a microcontroller handy for doing some heavy-duty monitoring. But is there any reason beyond the slightly lower cost to go with Li-Ions over LFPs?
The second part of my question is, whichever chemistry you recommend, what are some good brands for cells? I need a total capacity of at least 15WH in no more than 4 cells, and as much of a cell capacity lifetime as possible.
Thanks for your help!