Charging two 18650 in series with USB.

The first link appears to be a 2S charge / discharge controller - so far so good. As you're creating the battery pack from 2 x LiPo batteries (at a guess) then that should be fine.

As for using the buck / boost convertor - that's usually ok too, I recently bought a bunch of those from eBay and they should work just fine.

Regarding the specifications for the buck / boost, 2A is fairly generous I'd suggest - I'd figure about 1A output will be about right (2A for short periods but not really continuous unless rated as constant), output ripple is a bit crappy usually but for your application I'd think it should be fine.

One final question, do the 18650 cells you're using have a built in protection circuit? (If they do then they're usually slightly longer than the non-protected cells).  If they do have protection then are you intending to leave the charge controller connected too during use? I honestly don't know how that will work out (possibly fine but no idea).

Yes, the 18650's do have protection.

I probably won't be charging with more than 1A, so I hope the buck will be up to the task.

I don't understand your last question. I intend to leave all hardware connected yes, and charge it through USB when necessary. I'll draw a diagram:

jacobtc I was just wondering if you would be supplying the load through the BMS as shown in the diagram within the aliexpress link.

The reason is, the BMS has cell over discharge protection but so do the cells, I don't know if the two will work together - it may be fine but I honestly don't know.

If you're supplying the load directly from the batteries then the cell protection prevents over discharge and the BMS prevents over charge.

Hopefully this clarifies.

I will probably discharge the batteries directly from the battery terminals, as my diagram shows.

Then you should be good to go. BMS for overcharging protection (and of course balance) and inbuilt cell protection for over discharge by the load.

Sounds good!

I will be 3D printing an enclosure for everything in the end.

One final thing I've just thought about, re: the USB charging, is it from a PC USB port or a 'wall wart' type USB mains supply? The only reason I ask is because a PC USB port supplies around 500mA (and around 900mA for USB 3) so this would limit available charting current, an external (mains powered) supply opens up your current supply options considerably.

The charging will be from computers, wall adaptors and the like. The battery pack I'm making is for when I don't have access to an otherwise larger powersupply, which would normally charge the battery pack which came with the product.

Charging will be quicker when using the wall adaptor obviously (unless it's 500mA or lower of course, be sure to post back here when it's done, it'd be nice to see how it turns out.

Charging will be quicker when using the wall adaptor obviously (unless it's 500mA or lower of course, be sure to post back here when it's done, it'd be nice to see how it turns out.