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I will have soon an electric scooter with a 60v (top 67.2v) and 35ah battery pack. As it is designed, it can be charged by one charger which pumps 2,5 amps or it can be charged by 2 chargers in parallel, pumping 5A. That's ok for city use, it can charge over night, there is no problem waiting few hours for a full charge. But from time to time I plan to take trips into forest, between villages, etc. and I may need to charge it on my way, much faster, in an hour or so. For that I can purchase a 60V and adjustable 5-25A charger to take with me and plug it where I can find a wall plug, then continue my trip. But searching on aliexpress for similar sized battery packs, I discovered something strange in the specifications. For a 60v 20ah pack, the maximum continuous discharge current can be as high as 50 amps, but the charge current is max 5A. Why?? The connections between cells clearly can support high currents, otherwise it cannot discharge with 50A without damage. Why is the charging max so low and what happens if I push 25A with a powerful charger? Thank you.

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    \$\begingroup\$ The cells themselves can't handle the higher current. If you push 25A through with a powerful charger, you'll probably end up with your battery catching fire. \$\endgroup\$ – Hearth Jan 17 at 17:46
  • \$\begingroup\$ Are you sure? How can it handle high current when discharging but not when charging? A 60v pack is made with 16 series of 3.7v li-ion cells, in my case all are 18650 format. One cell, depending on its manufacturer/model, etc can have capacity around 2-3 ah or so. Afaik, there is no problem to charge one cell of li-ion or discharge at 1C. To achieve 20ah, each 3.7v stage must have around 10 cells in parralel, thus handling 20 ah without problem, that's 1C for them. So why a pack of 16s10p cannot handle more than 5a when charging but can handle 50 when discharging? \$\endgroup\$ – Florian Chende Jan 17 at 18:22
  • \$\begingroup\$ I just looked at a random 18650, 3ah element from TME. It can be charged with 5A, that's 1.6C and discharged with 15A, that's 5C. By these figures, a 60v 20ah pack should withstand 30a charging current. But by specification, it can be charged only with 5a, that is 0.25C. \$\endgroup\$ – Florian Chende Jan 17 at 18:24
  • \$\begingroup\$ This is a good question. Did you try any academic research articles on this subject? What did you find? \$\endgroup\$ – Ale..chenski Jan 17 at 20:33
  • \$\begingroup\$ I am still digging for information. I have some hints from here and there so far. Every battery pack have a BMS which, in its simplest form must provide some protection for overcharging and overdischarging as well as cell balancing. Due to cost reasons, I assume that almost all of them are using passive balancing and I think the low maximum charge current has something to do with the power dissipated on the balancing resistors, rather than with the cell itself. But I do not have a clear picture yet, still digging. \$\endgroup\$ – Florian Chende Jan 17 at 20:45
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Okay, let me offer this hand-waving explanation. After quick Google for [what limits charge rate for LiIon batteries?], several articles came up, like this one. In simplistic and very general therms of Physics, I would say that there is an asymmetry in solid-state chemical processes during charge and discharge. Essentially the charge is a process of some "order" in lithium ions deposition. So a higher temperature works against this process and slows the ordering down. The discharge process is sort-off "disordering", and goes more intensively with increase of temperature. The Joule heat, however, warms up the Li-Ion cell in both cases. So the charge must go slower to get the final state of charge, while release of energy is not limited. That's why the asymmetry exists between recommended charge and discharge process for batteries.

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  • \$\begingroup\$ Thank you. Yes, that I understood, perhaps I was not clear enough in my question. My question is related to very big difference charging versus discharging in packs. In other words, the cell I talked about above, one 18650 and 3Ah, can be charged with 1,6C=5A and discharged with 5C=15A. A 20Ah 60V pack built with these cells should be 16s7p. This is actually 21Ah but ignore that for a second. Making calculations, this pack can be discharged by 7*15A=105A and can be charged by 7*5A=35A. But a random pack of 60V 20Ah from aliexpress have max charge at 5A. About this I am talking about. \$\endgroup\$ – Florian Chende Jan 17 at 21:04
  • \$\begingroup\$ Right now I am tempted to believe that if I charge with 25A a pack like this, the balancing resistors+mosfet that controlls it will melt at first cell that hits 4.2 volts. \$\endgroup\$ – Florian Chende Jan 17 at 21:11
  • \$\begingroup\$ The battery cell has a limited temperature range in which it can be charged. While charging, the battery is getting very hot and will exceed the allowed temperature window very fast if charged with too many amps. Also the battery management system propably included in the battery will open the charging fets if the charging current is too high. \$\endgroup\$ – A.R.C. Jan 21 at 13:00
  • \$\begingroup\$ @A.R.C., "While charging, the battery is getting very hot" - no, under normal charge the most incoming energy goes into chemical transformation, so the batteries remain relatively cool. The warmth only comes from parasitic dissipation over battery's ESR. This is one of factors that sets the charging limit. \$\endgroup\$ – Ale..chenski Jan 21 at 20:36
  • \$\begingroup\$ @FlorianChende, what do you mean "at first cell that hits 4.2 volts"? I believe you need to research how BMS balancing and protection works in more details. \$\endgroup\$ – Ale..chenski Jan 21 at 20:43

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