Lithium-Ion or LFP?

[Broodoobob]

Hello,

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!

[Robin24k]

Have you looked into K2 Energy? Lithium phosphate batteries will be much easier to maintain, and you can use 18650's, 26650's, or go with a rectangular 12V SLA replacement.

http://www.k2battery.com/products.html

[Shadowww]

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.

For LiFePO4, A123Systems and K2 Energy.
For Li-Ion, Panasonic, Sanyo, LG, Samsung.

If you're aiming at 15Wh you can get 4 A123Systems' APR18650M1A's, they're ~3.7Wh each, so 4 of them = 14.8Wh. If you have enough space, you can get double of that, though, by using their long-C sized cells (ANR26650M1B's - 2500mAh)

[hellokitty[hk]]

LiFePO4 is considered very safe, but their energy density is not as high as normal Li-ions.
http://www.youtube.com/watch?v=rb_J2QQ0k-4
Prismatic LiFePO4's also exist if you don't want to use cylindrical.

[snakyjake]

LFP won't have a thermal run away (you can double check with K2, as they would be concern about safety of children too). Glad you are being cautious and have the safety in mind of others.

[Broodoobob]

Thank you so much for all of your help everyone! Can anyone refer me to a place I can learn about the safety issues involving Lithium-Ion batteries and how to avoid them? Even if I don't use them in this system there's other projects that I'd like to use them in. Specifically, I want to know what you'd need to do to charge and discharge them as safely as possible (with a self-made charger).

Thanks!

[Shadowww]

Just three things, really

1. voltage limits (never discharge below 3.5V OCV / 3.0V under load)
2. temperature control (cut-off charging or discharging if temperature exceeds 130°F)
3. proper charging profile (CC followed by CV, terminating charge at either C/50 (or lower value, e.g. C/10 to extend cell's lifespan) or 3 hours, whatever is reached first).

[jasonck08]

Lifepo4 doesn't have much more energy density than NiMh. You could take a look at Lifepo4 pouch / prismatic cells. If you could find a single cell that will do the job, that may be ideal, and would keep things simple.