Guys, doesn't everyone here already know that anything with a high energy density is dangerous? If you are pushing the design envelope of these things, you will get the occassional mishap.
Buy yourself a steel ammo box. Line it with something to prevent any risk of a short. Put your batteries in that, and charge away. If you flip the lid back over, you will have a tiny gap for the leads and for venting should it be required.
However, this applies for almost anything where you are stuffing huge amounts of energy into something. If you build a gyro, it is safe. Start pumping energy into it, and you might want to think about the failure protection. Go 30% beyond the design threshold, and you really, really want something there to stop the shrapnel.
Make nitrogylcerine, it's the same. Load ammo, it's the same. Pump air or other gas into a pressure vessel, it's the same. It isn't a secret, it's in fact fairly obvious, in the same way you don't leave a petrol tank sat in the hallway for months. That much energy stored up, unless proven stable (almost impossible, it can only be at a local minima until exhausted) should *never* be left in a place where major and catastrophic failure will be dangerous.
Don't store loose rimfire rounds in your front jeans pocket, same as you don't store your Li-Po flashlight in your front jeans pocket.
For info, I use a set of reclaimed from a laptop rechargable Li batteries. They are probably PTC protected, but they aren't circuit protected, as I removed all that when I split the pack. However, I run them conservatively, and it is two parallel matched cells. I don't expect to have any issues. They charge using the "dangerous" charger shown above (which clearly isn't - if you set it to the wrong setting, anything is dangerous) in a steel box to limit the impact should anything CATO. I don't expect any issue, but I'm smart enough to know that one might arise.
