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Paralleling 18650's is relatively easy. You need to match voltage to within a few mV and make sure the connection is really solid (welded) to ensure they stay paired perfectly. Flaky connections, putting cells in series, impact damage, bad chargers etc are the risky bits, a solidly connected pair of 18650's is to a close approximation just as safe as a single cell, but it does have twice the short circuit current so you are going to have to be more careful around them. But at least the casings will be at the same potential.

I've built a 17P10S pack which was a pretty interesting (and scary) effort but it has been working flawlessly for years now with just one inspection of the guts after two years to make sure that nothing was coming loose (it's on an s-pedelec e-bike). In a big pack like that it's the spaces between the alternating blocks of cells and on top where the interconnects are that the real risk lies, besides the fact that the short circuit current of that pack is just shy of a kilo ampere so you really don't want to drop a tool or a piece of interconnect strip on that.

> Paralleling 18650's is relatively easy. You need to match voltage to within a few mV and make sure the connection is really solid (welded) to ensure they stay paired perfectly.

These requirements are already not easy, and there are still plenty of things to consider for using LiPos in parallel (e.g. identical health, preferably batteries from same batch, to increase chance they age identically)

> These requirements are already not easy,

Get them reasonably close, then leave then connected by a moderate value resistor for hours. Then you're within mV.

If you're just concerned with getting 95% capacity from each battery, "close" is good enough for the rest.

OP exaggerates the difficulty. You can trivially parallel the vast majority of lithium batteries so long as their voltage is reasonably close (I personally wouldn't fuss much over a 100mv difference, or even more in most cases, unless it's a massive battery or a power cell capable of delivering and accepting very high currents - charging most cells will often involve raising the voltage 200-300mv during the constant current phase, so you can safely parallel with a difference like that)

You can match up pretty different batteries in parallel as well. One will take more load etc, but this is not usually a problem. It's not ideal, but I think people often exaggerate the dangers.

Series is much more problematic, since most balancing circuits have very limited capacity to balance mismatched batteries.

Many devices have parallel lipo cells, from powerbanks to electric cars, nothing special here.

If one cell is weaker, the other provides more current, there is no "one discharging/emptying into the other" during normal work (read below). No real need for any proper matching either, if you only care about capacity (if you care about current, you don't want to get into a situation where any of the cells has to provide more current than designed for and safe.

The only "problematic" part of parallel batteries is making the first connection, where one might be at a much higher voltage than the other. Usually this is mitigated by equalizing voltages (either dis/charging to a fixed voltage, or do a parallel connection through a proper resistor), and after they're safely connected in parallel, it doesn't matter.

On the other hand, two cells, user removable and replacable can cause exactly this issue, where the user removes one, recharges it in an external charger and replaces it (while the other, empty one, still stays inside)... but maybe there's a diode somewhere that prevents reverse currents.

Li-Ion chemistry is pretty happy with 1S2P configurations. (That doesn't necessarily mean you should do it.) 2S1P is where the fun starts.

It doesn't matter what happens in the 10 to 90% range, if one discharges before the other, it's perfectly fine. It's not like this is an application that needs the combined current capability of both cells. What does matter is that neither cell is overcharged nor deep discharged, and the [dis]charge curve is absolutely not flat in those areas.

The GP is talking about the case where you plug in two batteries of varying charge levels or health, which I agree is not an amazing thing to do.

That's not obvious from the comment and logically inconsistent with parts of it; if the trace between the cells is small it would act as an auxiliary fuse, and there is that balancing resistor (whose value I can't read because whoever drew that schematic didn't bother repositioning overlapping labels.) I'm also a bit confused about the 2 polyfuses.

That said you're right and I was focusing a bit too much on my reading/interpretation of the GGP post. I'm not sure I've ever seen a 1S2P LiIon configuration with individually user swappable cells. In the 2-cell design I did, I specifically decided to go for 2S1P and have the balancing circuit, to avoid this exact issue. It does have the downside that you need both cells, the WHY design works with only one populated... (which is what I'd recommend doing in any case.)

[ed.: the balancing resistor seems to be 200Ω. The polyfuses are 15mΩ. So I guess it's designed to trip one or both polyfuses if the cells are imbalanced. That's an... "odd"... design.]

> That's not obvious from the comment and logically inconsistent with parts of it

I agree, but personally I decided to go with the most charitable interpretation, and that's the one that made the most sense to me.

It’s what I thought as well, but I’m not too much into electronics to hold an opinion. It looks like there’s a balancing resistor between them: https://gitlab.com/why2025/team-badge/Hardware/-/blob/main/C...

With floating grounds due to those MOSFETS adding 50 milliohms or so (on the order of the internal resistance of the batteries!)!

YIKES!

Putting the protection circuit* on the battery's negative pole is standard best practice (due to NMOS efficiency, and it not being a problem in the slightest), and the 50mΩ actually improves balancing. Please avoid making comments like this based on half knowledge.

[*] I do wish it were an actual full protection circuit. It isn't. Then again a run of the mill protection circuit commonly doesn't cover reversed polarity [between protector and cell], which is rather important for this specific appliation.

> Putting the protection circuit* on the battery's negative pole is standard best practice

Pointer? Especially since LiPol paralleling seems to want to use bus bars to minimize wiring resistance.

Admittedly my experience is all about avoiding parallel LiPol batteries ...

https://www.ti.com/lit/gpn/bq77908a

https://www.diodes.com/assets/Datasheets/products_inactive_d...

Look at the reference circuits, it's a pair of antiserial NMOS on the negative pole.

(Those 2 protection circuits are at the opposite ends of complexity & features)

To be clear, using 2 PMOS on the positive pole is also quite common, my choice of words with "standard best practice" might be a bit misleading.

> use bus bars to minimize wiring resistance.

Those come after the protection circuit, there should always be 2 MOSFETs in series with the individual Li-Ion cell in a design like this (specifically: user swappable cell).

(Protecting paralleled cells together is kinda nonsensical because you also want to protect them from each other, I don't think I've ever seen a 2P combined protection circuit.)

Those datasheets show creating a series pack/cell. They don't show the circuitry to then parallel the packs together.

I guess I need to do more research on this.

> Those datasheets show creating a series pack/cell.

You seem to only have looked at the TI one, the Diodes one is for a single cell.

& if the cells are "permanently" connected in a pack, you wouldn't have individual cell protection and just have them properly balanced before connecting them in factory.

> parallel the packs together

You parallel cells, not packs.

Is it just me or is that schematic hard to read due to bits of text being on top of each other? Also "LED will burn when battery wrong way round" .. how about fixing this problem which you have acknowledged? What happens to your balancing resistor when you put one battery one way round?

"LED will burn when battery wrong way round" .

I don't know about the rest of it, but I think this is just an idiosyncratic translation of "LED will light when battery wrong way round" - IE it's a warning LED.

> Is it just me or is that schematic hard to read due to bits of text being on top of each other?

This is extremely common in products that are open-source community adjacent. I assume it's some sort of stylistic choice as almost none of these labels are in their default positions and many of the default text sizes have been changed, the designer has put in additional work to make this less readable.

The purple text comes from additional fields added to the symbols (specifically these look like LCSC part numbers). When adding properties there's a checkbox to choose if they're visible on the schematic or not and normally you'd leave in unchecked. Again in this case I can only assume it's some kind of stylistic choice that I don't understand as it's a common thing to see.

> These don't look like they're matched in any way.

How would you tell?

The problem you're describing is real, but it's only when installing the batteries. And you can avoid it by only inserting batteries that are both empty or both full.