"Proper" ways to integrate a rechargeable LiPo battery?

[ebastler]

For a gadget I might want to sell (in small quantities) to hobbyists, either as a kit or as a ready-made device, it would be nice to integrate a small lithium polymer battery, 3.7V and on the order of 1000 mAh. This works nicely in prototypes: A pouch cell with integrated protection circuit, MCP73831 charger IC, polymer fuse, USB jack -- done.

But having the integrated battery raises some safety and compliance questions. While one might accept a kit that is a gray area regarding EMC compliance, I would feel less generous when it comes to fire protection. UL-rated and/or IEC 62133-compliant battery, single-fault tolerant charging circuit, fire-proof enclosure... It's quite a rabbit hole.

What do you do to integrate a LiPo battery "properly"? Do you address all the requirements in a bottom-up design? Ignore them? Are there prepackaged modules which make life easier?

(Something like a "pre-certified integration-ready powerbank" would be nice: LiPo battery, charging circuit, optionally a boost converter, all in a flame-retardant enclosure. I.e. all the ingredients of a small USB powerbank, but in a package meant to be integrated e.g. into the back panel of a device, exposing the charging port. I don't think such a beast exists?)

[voltsandjolts]

If you want to sell and ship, my advice would be to sell "batteries not included". Your customer can then buy suitable battery online or whatever. That way you don't have to deal with shipping batteries which, believe me, is becoming more onerous every year.

[ebastler]

Right, shipping is yet another consideration, and probably best avoided by not including the battery. But that still leaves the requirement that -- once the user has added a battery of the recommended type -- the product should be safe in use. So it does not avoid the design and integration questions, I think.

[voltsandjolts]

So it does not avoid the design and integration questions, I think.

That's right, but it may shape the design decisions, e.g. must be a user accessible battery that doesn't need soldering to fit. Since you only need low power, I would still consider good old AA or AAA NiMH. Mainly because they're safe, ubiquitous and you're more likely to find an off-the-shelf case for your product with suitable battery compartment. If that's a consideration.

[studiot]

Seems to me that the majority of laptops these days come with internal fixed in Li-ion batteries, especially the 'slim' ones.

[ebastler]

Seems to me that the majority of laptops these days come with internal fixed in Li-ion batteries, especially the 'slim' ones.

Obviously integrating Li-ion batteries is a solved problem... But I don't have the engineering and manufacturing resources of a large OEM supplier of laptops, and won't quite reach their unit numbers. As mentioned in the OP, this is about a hobbyist endeavor, a niche product to be sold in small quantities, and I am interested to learn how others handle designs of a similar caliber.

[studiot]

Seems to me that the majority of laptops these days come with internal fixed in Li-ion batteries, especially the 'slim' ones.

Obviously integrating Li-ion batteries is a solved problem... But I don't have the engineering and manufacturing resources of a large OEM supplier of laptops, and won't quite reach their unit numbers. As mentioned in the OP, this is about a hobbyist endeavor, a niche product to be sold in small quantities, and I am interested to learn how others handle designs of a similar caliber.

What I can tell you is that my current project is restoring a Taylor model 44 transistor tester.
This incorporates a 12v, 1 amp battery, tapped at 1.5 volt intervals that is no longer available.
I have found that a bank of 1.5 volt Li ion rechargeables does the trick nicely, using one of the modern bank rechargers/storage boxes.

I also belong to a vintage radio forum where similar projects have occurred.
There has been much discussion of these as some Li ion batteries (1.5 v and 9v) have internal circuitry that can lead to much unwanted rf interference.

[thm_w]

A pouch cell with integrated protection circuit, MCP73831 charger IC, polymer fuse, USB jack -- done.

Yes thats all you need, maybe reverse polarity protection if there is a chance customer can install the battery backwards.
You can also double up and put a DW01 or equivalent on the PCB if you think there is a chance an unprotected cell can be installed.

If its a premium product the case can be metal. If its not, ideally the enclosure would be UL94V1/0.

[ebastler]

Yes thats all you need, maybe reverse polarity protection if there is a chance customer can install the battery backwards.
You can also double up and put a DW01 or equivalent on the PCB if you think there is a chance an unprotected cell can be installed.

If its a premium product the case can be metal. If its not, ideally the enclosure would be UL94V1/0.

What is the requirement on temperature monitoring (while charging)? In my understanding that's not something the standard protection circuits that come integrated into pouch cells provide -- and neither do the basic charging ICs like the MCP73831. Is it a requirement in the safety standards?

Since I need a custom case for my project, one option would be to let kit builders 3D-print their own outer case for the device. In that scenario I would want to provide an internal enclosure around the battery which meets UL94V1/0 requirements. Has anyone come across suitable small plastic or metal boxes (made for this specific purpose or as EMC shields maybe) with UL94V1/0 classification? I'm thinking of a small box with an open bottom that gets clipped or screwed onto an FR4 PCB (UL-rated) which carries the battery.

[thm_w]

What is the requirement on temperature monitoring (while charging)? In my understanding that's not something the standard protection circuits that come integrated into pouch cells provide -- and neither do the basic charging ICs like the MCP73831. Is it a requirement in the safety standards?

Since I need a custom case for my project, one option would be to let kit builders 3D-print their own outer case for the device. In that scenario I would want to provide an internal enclosure around the battery which meets UL94V1/0 requirements. Has anyone come across suitable small plastic or metal boxes (made for this specific purpose or as EMC shields maybe) with UL94V1/0 classification? I'm thinking of a small box with an open bottom that gets clipped or screwed onto an FR4 PCB (UL-rated) which carries the battery.

Three terminal pouch cells are not very common.
You can see this one on digikey passes UL1642 but has no temperature monitoring in the BMS: https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/5336/USE-702528-500PCBJST.pdf
Part of the test is heating up the pack to 130C so having a temperature sensor isn't going to make any difference there.
It does show R3 NTC in the schematic but its either not connected or not populated.

If you wanted an enclosed cell with temp sensor you can look at gopro or similar compact consumer product batteries, they'll often have cheap aftermarket clones.

Making an extra enclosure seems like too much effort.
Sparkfun/Adafruit/etc. sell tons of li-ion powered products and don't bother with this.

[ebastler]

You can see this one on digikey passes UL1642 but has no temperature monitoring in the BMS: https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/5336/USE-702528-500PCBJST.pdf
[...]

If you wanted an enclosed cell with temp sensor you can look at gopro or similar compact consumer product batteries, they'll often have cheap aftermarket clones.

Thanks -- that is a refreshingly detailed battery specification; much better than what I found from other vendors. Not an actual certification or guarantee that the battery meets all applicable requirements from the standards, but good reassurance that the supplier at least knows what is required.

Using a battery pack for a camera etc. had occurred to me too. It might address both, the NTC and enclosure aspect. Although one probably won't get any certifications or specification with the third party replacement batteries. (Is their enclosure actually specified to provide fire protection, or do they still rely on the outer case of the product?) I will look around a bit for a suitable form factor and connection style.