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I'm currently building my own electric longboard and even though I looked through some resources considering this, I am not a 100% sure that I am on the right path.

I want to use a 10S2P battery pack, consisting of Samsung's 30Q 18650 and I bought a 10S BMS with balancing capabilities.

Now I need to charge the battery pack. Since it is 10S the charging in the first stage would be 42V for CV and below 6A for CC. I saw videos online where they simply use a Buck-Boost-Converter, which is capable of CC-CV. To my understanding, that would only cover the first stage of the charging process.

Questions:

  1. What happens afterwards, if the CC-CV converter is still connected, even though first stage is over? I somewhere read, that CC-CV is from an electronics point of view simply CC, so the voltage might vary. Are there fatal consequences to be expected, if the converter is connected for too long? What happens in that case?

  2. If the first stage is over, there might be a chance that the CC-CV converter (which is really only a CC as I read, in case that's even correct) puts out more than 42V which is definitely a hazard for the Li-Ion cells. But since I am using a BMS (with overvoltage and balancing capabilities) I should be safe from this, right?

  3. Are BM systems in general capable of doing overcharge protection? voltage- and current-wise? So simply put: Can I connect the BMS to CV 42V and it will take care of everything?

I could not really find specific answers to these questions, but hopefully you can enlighten me.

Thanks in advance.

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  • \$\begingroup\$ Lithium batteries require the CV to be removed when CC drops below 3~10% of the CC rate to prevent cell damage long term. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 16 at 0:23
  • \$\begingroup\$ Depends on BMS specs... maybe ok. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 16 at 0:29
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I somewhere read, that CC-CV is from an electronics point of view simply CC, so the voltage might vary.

CC-CV stands for Constant Current and Constant Voltage, which means both voltage and current are regulated. If the battery does not accept the set current then the voltage will be held constant and the current must go down. This is the standard charge profile for Li-ion.

Are there fatal consequences to be expected, if the converter is connected for too long? What happens in that case?

The booster should continue to hold the battery voltage constant forever, and the battery should handle this. However at maximum voltage the battery's lifespan is reduced, particularly at high ambient temperature. An hour or two at full voltage is fine, a week is not. If you have to float continuously then reducing the voltage from 4.20V to 4.15V or 4.10V will increase lifespan with some reduction in capacity.

To check that the booster maintains a safe voltage, simply set it to the correct voltage with no load. Then charge a battery with it and measure the voltage regularly as it gets close to the end. It should not go above 4.20V per cell.

there might be a chance that the CC-CV converter... puts out more than 42V which is definitely a hazard for the Li-Ion cells. But since I am using a BMS (with overvoltage and balancing capabilities) I should be safe from this, right?

Yes, in the unlikely event that the charger malfunctions the BMS should disconnect the battery before any serious harm occurs. However to avoid interfering with charging the BMS must cut off at a higher voltage than the normal peak voltage, so there is still a chance of cell damage occurring. Also some BMS circuits have an uncomfortably high cutoff voltage that may not be very accurate.

Are BM systems in general capable of doing overcharge protection? voltage- and current-wise? So simply put: Can I connect the BMS to CV 42V and it will take care of everything?

Most BMS circuits protect against over and under voltage and over-current. This should protect the battery from catastrophic failure of the charger or device being powered. It won't prevent long-term damage due to consistent floating at maximum voltage, drawing high current, discharging below the normal cutoff voltage or charging at a high rate when the battery is deeply discharged.

If the BMS has balancing then it should be able to maintain balance provided that the cells are fairly well balanced to start with, and the charge current is not higher than the balancing current. For the first charge you should measure cell voltages regularly and reduce the charge current if they are not all within +-0.03V of each other. Balancing may take several hours if the imbalance is more than a few tenths of a volt.

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  • \$\begingroup\$ Ohh ok, so while charging Li-Ion cells, CC-CV is CC in the first stage, voltage may vary. But when the cells does not pull full current, the CV kicks in and limits the voltage to 4.2V. And after a while the current drops lower and lower and NEEDS to be cut off by the charger, which does not happen if you simply use a dc converter. Otherwise you damage the cells. Ok and if I put the voltage limit to 4.15V. What happens to the current then? Will the drawn current drop to 0 without damaging the cells? \$\endgroup\$ – timothy3001 Sep 16 at 12:35
  • \$\begingroup\$ That's about right, though the current doesn't need to be cut off if you charge to a slightly lower voltage. A fully charged cell will typically drop to 4.15V at rest, so charging continuously to that voltage is no worse. \$\endgroup\$ – Bruce Abbott Sep 16 at 12:41
  • \$\begingroup\$ Thanks a lot! Finally I have an idea what's going on. \$\endgroup\$ – timothy3001 Sep 16 at 12:54
  • \$\begingroup\$ @BruceAbbott You answer is mostly correct and useful BUT dangerous in one key area. "Leaving a bnattery at 4.2V for a few hours" is SUBSTANTIALLY different than proper CCCV charging wit termination. In the CV mode I tails down to some chosen termination fraction of I max. If Imax was at C then a C/2 termination gives a relatively full charge, C/4 gives a very full charge and C/10 is "Road Warrior" level where you want every mAh of capacity you can get and don't care that the battery is being thrashed to death. ... \$\endgroup\$ – Russell McMahon yesterday
  • \$\begingroup\$ ... Just leaving it for a few hours at CV where "few" is poorly defined risks Road Warrior mode or worse with minimal and uncontrolled gains and substantial reduction in cycle life. || Whereas setting CV at say 4.1V does allow essentially floating as the cell assymptotes to a lower level of max charge. Actual 'safe' max CV is temperature dependant and some newer chemistries allow eg 4.35V. Stand clear :-). \$\endgroup\$ – Russell McMahon yesterday

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