It's nice to see someone caring about saftey for once. Batteries can be dangerous things.
Built in protection in the cells is usually very limited, often just an unreliable thermal fuse. A best it gives very little protection, but I find it gives a compeltely incorrect sense of saftey. If a cell has built in protection, my response is to say that's nice, and then treat it the same as if it didn't have any.
A BMS is good idea. But BMS can be at any level. Therortically they should:
- Stop over chargining (saftey)
- Stop over dischargining (saftey)
- Control charging (from a practical point of view)
- Protect against over (and under) temperature events (saftey)
- Balance all the cells in the pack (for saftey, cell life and perfromace)
- Monitor pack usage for general information and for state of charge But I am aware of may commerically avalible ones which do not do all these things. I have tried to put these features in comerical packs I was working on, but was told it was too expensive, and not required as we had already covered the bare minium.
Fuses should always be fitted. But they are the final level of protection. Make sure you fit the correct one for your system, but make sure you design the rest of the system (including the BMS) so that you never need it. A fuse is for when other things go wrong. Fuses will not save the pack in case of over or reverse charging. Thermal fuses are a nice idea, but are not to be trusted any more than normal fuses. I would fit a thermal fuse as well as a standard fuse.
All decent BMSs will monitor cell temperature. It is required of pretty much all saftey regulations CE, UL etc.
Li-Po safe bags is a nice idea, but again it's a thing that should help if things go wrong, but you should have a BMS which stops this ever being required.
A fuse per cell shouldn't be required as the mechanics of pack should not allow access to indervidual cells.
You appear to have missed off the important box for the pack. The pack should be in a nice self containted insulating box so that there is a physcial barriear between the cells and the world. The power comes out via a couple of contacts (and maybe some method of talking to the BMS if requried, but that could be only one more contact).
In summary: put in everything you can. But make sure the cell pack has some phsycal protection between it and the outside world. This stops it hurting the outside world, and the outside world hurting it. A decent BMS, which does everything I listed above, would be a good place to start. There is no such things as too much protection.