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I have several 18650 Li-Ion batteries, many with different capacities. Can I charge them with the same charging circuit (5 to 3.7 V 1 A buck converter)? Can I put them in parallel and use the same step up circuit to make a 5 V USB bank?

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  • \$\begingroup\$ Have you done any research on this yet? \$\endgroup\$ – Bort Jun 19 '18 at 17:30
  • \$\begingroup\$ I have tried, but I can't really find anything on this. Most of the time its just about one type of battery, and when it isn't, its about different chemestries, or ages of batterys. \$\endgroup\$ – Snow_Cheetah Jun 19 '18 at 17:32
  • \$\begingroup\$ You shouldn't be using just a buck converter to charge Li-Ion batteries, and 3.7V 'nominal' batteries are seldom (if ever) charged at 3.7V (4.1V or so would be more common). Are you using a charge controller? \$\endgroup\$ – brhans Jun 19 '18 at 17:40
  • \$\begingroup\$ Yeah, I was just going to buy a 18650 charger off of ebay or something \$\endgroup\$ – Snow_Cheetah Jun 19 '18 at 17:52
  • \$\begingroup\$ Should be fine if you can balance their voltages first. \$\endgroup\$ – winny Jun 19 '18 at 18:10
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If you mean to charge each cell individually, then it should be fine, provided that the charger doesn't exceed max charging current for the smaller cell. But if you mean to make a battery out of two different cell in parallel, and charge-discharge them as a battery, then the following apply:

One should never charge different Li-Ion batteries in parallel. It is true even if the cells are of the same rating, but their charge-discharge characterics do not match to high (1-2%) degree. Charging cells in parallel is unacceptable for three reasons:

  1. Li-Ion cells with different capacity will take different time because a smaller-capacity cell will/might be charged first, and then will be overexposed to charging voltage (while the bigger cell still gets the charge). This will result in "overcharging" of the weaker cell, it will grow some bad chemistry inside, its capacity will fall more, and it will die being bloated or worse.

  2. On discharge cycle the process is also bad - weaker (less capacity) cell might be depleted first, and fall into "overdischarged" state, with similar bad implication for internal chemistry.

  3. On charge, a stronger cell with less internal impedance might take more current than the cell design can afford, which can lead to "fast venting including fire".

In short, it is a very bad idea to make a battery from different Li-Ion cells in parallel without either perfect matching, or using some electronics to manage each cell individually.

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  • \$\begingroup\$ If they are in parallel, the voltage across must be the same. So, for #1: how would the smaller capacity cell be charged sooner? During charging, 3.5V is 3.5V. One cell cannot reach a higher level than another. #2: same as #1. How can either be more depleted, percentage wise? Perhaps only if the state of health had changed significantly. #3: Agreed, though it can be accounted for. \$\endgroup\$ – Bort Jun 22 '18 at 16:53
  • \$\begingroup\$ @Bort, Li-Ion chemistry gets its predominant charge during constant-current phase. The voltage is creeping up slowly during the process. When the internal chemistry gets charged to certain degree, the cell starts to consume less and less current, which is a precursor to end the charge. First, no one can guess how the input current will be split between two cells (#3), then a smaller cell will/might charge faster, its current will drop, while the other is still charging (#1) and consuming current (so the charger won't stop) thus overexposing the smaller cell. \$\endgroup\$ – Ale..chenski Jun 22 '18 at 18:41
  • \$\begingroup\$ It sounds like the only issue is overcurrent (and not voltage), would you agree? If, say, the paralleled cells are charged by a 1A charger, and 1A is not above the max for any individual cell, then there is no issue with current, yes? For discharge, I am assuming the draw is also managed to be less than any particular cell's max. No offense, but I am still doubting your #1 and #2. I have been told be colleges in the Li-ion field that there are battery packs used in consumer products, such as macbooks, that actually use batteries in parallel. (I have to verify that.) \$\endgroup\$ – Bort Jun 22 '18 at 21:21
  • \$\begingroup\$ @Bort, the voltage was never an issue, assuming identical cell chemistries. In almost all cases (before proliferation of small tablets and smartphones) all laptop batteries were made in mixed configuration, 2P or 3P (in parallel), and 3S (11.1V). But all cells in the parallel sections are carefully selected/matched by manufacturers with nearly identical characteristics. \$\endgroup\$ – Ale..chenski Jun 22 '18 at 21:53
  • \$\begingroup\$ @Bort, the "overcurrent" is only one concern. The major issue is with "overcharge". It is not about too much current, but rather "too little current", when a cell is full and consumes no more current, but is still exposed to voltage. This causes so-called "overcharge" condition, which leads to degradation of almost all elements, lithium depositions in metal form, degradation of electrode surfaces, membrane damages, etc. Please examine some scholar articles on the subject, as sciencedirect.com/science/article/pii/S0921510714002657 \$\endgroup\$ – Ale..chenski Jun 22 '18 at 22:00

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