What goes into the design of a "safe" lipo battery pack & circuitry?

[monster_of_electronics]

Is there a detailed video about this?  A guide?  A book?  I know there's temp sensors, current sensors, fuses, breakers, etc. normal stuff like that.  But specifics as well as materials and things that would make it almost good enough for UL.

[Cliff Matthews]

If you don't plan to provide an internal PCB with charge controller, at the very least I'd say a fire retardant rigid plastic case, lots of insulators around the cells (eg: at the ends of typical 18650 cells), and a series connected thermal cutout switch glued to the side of every cell. If you want some hints, plunk down $5 on a junker laptop and carefully, peel apart the battery pack in a well ventilated area (not as easy or safe as one may think).

[Don Hills]

... carefully, peel apart the battery pack in a well ventilated area (not as easy or safe as one may think).

A method I have found to work reliably:
- Clamp the pack in a vise so that one of the seams faces up. Tighten the vise enough to hold the pack still, but not enough to start to squash it. Place a block of wood under the pack if required so it can't slip down.
- Hold a wood chisel against the seam and tap with a hammer - gently at first, then a little more force if required. Pretty soon you'll hear a crack and the seam will open slightly. (Repeated light taps work better than a heavy blow. If you hit too hard, you risk the chisel going through into the pack.)
- Repeat along the seam, and rotate the pack as you go.

This also works for other welded cases such as laptop power bricks.

Usual safety precautions apply. Well ventilated space, safety goggles / glasses, gloves (leather gardening gloves work well and are cheap), metal bucket with some sand or soil to drop the pack in if it starts to "brew up".

[stmdude]

Lets keep the chemistries apart here.

LiPo (as the title states) are _soft_ foil batteries, typically found in cell-phones and the like.
18650 are LiIon, which have a _hard_ metal casing.

For LiPo batteries, you would need some kind of protection to prevent accidental puncturing. The outside of your device is usually enough to accomplish this.
For LiIon, there's very little you need to do mechanically to make it safe. You could go the extra mile (as suggested above) to make sure that if all your protection circutry fails, and the cell de-gasses, that the hot gasses can't harm the end-user or damage the product. However, I doubt that would be a requirement for UL.

For the electrical, it seems you have most of the parameters figured out, but I'd like to mention some particulars that a lot of people seem to forget.
Temperature. This is very important. If you go below a certain temperature (datasheet says which), the device should stop draining power from the battery. Same goes for too hot.
This also applies to charging. Going outside those temperature ranges will at best shorten the life of the battery, and in worst case result in de-gassing or swelling.

If the battery is end-user replacable, you'd might want to design in some rudimentary identification circutry, so that the user doesn't replace it with something just similar, but something that actually works.

Also, remember, there are many different cell-voltages, even within each chemistry.
Li-Ion is available in 4.2V for the most part, but others are available
Li-Poly is available in 4.2, 4.25 and 4.35V

Last little comment would be that after charging completes, charging should _stop_. It shouldn't have the current slowly peter down to near nothingness, as that would "polish" the cell and also reduce its lifespan. Charging should stop, and only recommence when the cell voltage has dropped to a certain level (for example, 4.1V for a 4.2V LiPo cell).

But, it all comes down to your own decisions on what's important for your product, and how much effort you're willing to put into it.

Source: I worked with batteries and charging in cellphones for quite a while. We did it all, and then some..

[eas]

Testing/verification, among other things.