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I have two 3.7 V, 900 mAh Li-ion pouch cells, which I connected in series to provide 7.4 V. But now I want 11.1 V. Instead of buying another Li-ion pouch cell, am I able to put these two Li-ion pouch cells and a 3.7 V, 900 mAh Li-on cylindrical cell in series to be achieve 11.1 V? Is it safe to power a brushless motor, ESC and a Arduino Nano with this connection?

Null
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cool bean
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    I don't think anyone can say it is safe with that information. It depends on so many things, are batteries protected, is there a BMS, how do you convert or replace a 2 cell BMS to support 3 cells, etc. The charge currents and voltages must also match, but you are mixing li-poly with li-ion. – Justme Jan 10 '23 at 14:28
  • I am not to sure about this information because I bought the lipo battery of Amazon, and I am using the LiOn charger to charge the lipo battey – cool bean Jan 10 '23 at 14:33
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    My advice is only buy from reputable dealers who can also provide a data sheet and, guarantee that the source of the component/device is manufactured to a recognizable quality system. This pretty much rules out amazon. ebay etc.. – Andy aka Jan 10 '23 at 14:49
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    1. All the warnings here are valid. || 2. The arrangement described is quite possibly safe for your intended use at sensible currents - eg if you have not mentioned the 200 Watt motor load all bets are off. || 3. Charging using a 3 cell BMS is required OR charge sensibly by suitable means. || 4. If you don't violate the various LiIon safety requirements all may be well. || 5. I'd do this personally if necessary BUT I'd ensure all safety aspects were covered. – Russell McMahon Jan 11 '23 at 11:51

2 Answers2

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"Can you" -- yes. In much the same way that you can put bald tires on your car and drive in the rain, you can certainly do this.

As Davide Andrea mentions in his answer, what you should do for a product is use a 3S battery management system. If, however, you're going to use model airplane rules to save cost and expense, you should use model airplane rules to build up your battery pack: use the same cells from the same manufacturer, then use a high-quality balancing charger -- and always store the pack somewhere that it can catch fire and burn off without burning down your house or shop.

If you're absolutely broke -- and you can store the cells where they won't catch other stuff on fire -- then you can probably get away with doing this if you use a balancing charger, if you're not demanding the maximum possible current from the cells, if you check the pack regularly for balance, and if you're prepared to dispose of it properly when it starts showing signs of distress.

Note that all of the above "ifs" are what you should do anyway if you're the battery management system for an airborne pack; you just need to be more careful than usual. In particular, start using the pack gradually, with each run taking more charge than the last. Check the cell balance carefully (your charger should do this, and if it doesn't you should make sure to have a battery checker). You'll see that the cylindrical cell will either be weaker or stronger than the two pouch cells -- you need to figure out how long and hard you can run your gizmo without dropping any one cell's resting voltage below 3.2V (higher is better) at the end of a run.

Note that when you use the cells this way you must expect them to be a wear item. Serious model airplane folks buy spare packs, monitor them for health, and dispose of them properly when they die.

Frankly, unless this is a one-time thing that you only expect to get a couple of dozen uses out of, I recommend that you find an actual hobby supplier (Horizon, A-main, ThunderPower in the US, or your local hobby shop). Then learn how to take care of battery packs (search the web -- "model airplane battery care" may be a good start). Then buy at least two packs and a good charger. Then pay attention, and don't burn your house down.

TimWescott
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    Here's a [longer read](https://electronics.stackexchange.com/questions/230155/why-is-there-so-much-fear-surrounding-lipo-batteries/230164#230164) on Lithium batteries. In short, unmanaged battery packs can and will burn your house down if you don't respect it. – Nelson Jan 11 '23 at 03:43
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What you call Li-Po IS a Li-ion cell. A Li-ion cell in a pouch, but a Li-ion cell nonetheless. They are all Li-ion cells. They are all 3.6 V cells. The only difference is the shape of the enclosure.

Therefore, as long as you use a 3S BMS to protect each and every cell, and these are truly plain cells (not complete batteries with a BMS), a battery with mismatched cells will be safe.

Will it be effective? That's a different question. No, it won't be very effective because the cell with the lowest capacity will limit the performance of the entire battery. The rest of the charge in the cells with higher capacity will go to waste.

Therefore, while not absolutely necessary, it is wise to use identical cells in a battery.

Davide Andrea
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    "as long as you use a 3S BMS to protect each and every cell" I suspect that's not being done. It sounds like the build is using hobby airplane rules, where bare cells are used and the user is expected to complete the loop on battery management -- and store the cells in a place where they can catch fire unexpectedly without starting someone's shop or house on fire. – TimWescott Jan 10 '23 at 15:40
  • pouch cells don't discharge as low as e.g. an 18650 will, not without damage anyways. so, not the same thing in practical terms. – Sixtyfive Jan 11 '23 at 08:36
  • @Sixtyfive - the chemistry of LCO pouch cells is exactly the same as the chemistry of small cylindrical cells. They have the same exact voltage thresholds. Yes, various cells (of either format) are optimized for either power or energy, but as far as voltage is concerned, all LCO (cobalt) cells are the same, regardless of packaging. – Davide Andrea Jan 11 '23 at 12:38
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    I have no clue about chemistry. All I know is that _in practice_ people (esp. from the R/C & FPV communities) pay a lot of attention to battery specs. See e.g. https://www.dnkpower.com/wp-content/uploads/2019/02/SAMSUNG-INR21700-40T-Datasheet.pdf (we'd call this a "Li-Ion" and might use it e.g. for a LONG-RANGE quad) and note "2.5V cut-off". I would strongly caution against trying to go that low with a pouch cell (what we'd call a "LiPo"). Some pilots of 1S ACROBATICS quads hammer these down to 3.0V but they KNOW they're going to waste them pretty quickly. 3.4V is more widely accepted. – Sixtyfive Jan 11 '23 at 15:11
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    @Sixtyfive Fair enough. Thank you. – Davide Andrea Jan 11 '23 at 15:14
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    Perhaps interesting to add that in recent years, for 1S applications, pouch cells with a nominal voltage of not the usual (3.6V for most 18650s, 3.7V for most pouch cells) but instead 3.8V have become popular and are being called "LiHV" or just "HV"=high voltage, which I find quite amusing. These are then nominally charged up to 4.35V. The difference can indeed be felt in terms of power and flight duration. – Sixtyfive Jan 11 '23 at 16:00