I'm creating a portable device that can be powered by a battery pack, or plugged in and used while charging the battery. At 4:18 in this video (shown below), it shows that the circuit can be powered by the battery charger while it is charging the battery pack through the BMS. I feel like the current draw from the device would interfere with the charger's CC/CV modes and battery state monitoring, causing incorrect battery charging, or insufficient power for the device. Am I missing something, or is the video wrong?
You are correct that this setup will change charging profile, potentially causing charger overheating, undercharged battery or reduced battery lifetime. Or as simple as device not working if charger cannot supply enough power.
If you want to use these components somewhat better solution would be to create additional path from voltage regulator to the load using diodes or mosfets. Check this application note form Microchip for many good ideas.
It will just take longer during CC mode but may never finish in CV mode as the load current may exceed 5 or 10% of CC current used for cutoff. If this is the case, there should be a timer with CV time limit and cutoff minimum time in order to reduce aging effects at 4.2V. I would just leave it float at 3.8V if you do not have a separate battery current sensor.
Normally the device senses battery current but here it is the charger.
You are correct, but the question is how big a deal it’s going to be.
With the device powered, the charge will take longer. The charger switches modes based on the battery voltage at the charging current, but in this case, the charging current will be off.
I suppose if the charging circuit were super optimized for a particular battery, you might feel bad that your design was no longer optimum. Then again, what if the battery life were reduced by 10%? Would it be worth the extra design trouble to get this life back? As the designer, you can decide. (By the way, the 10% number is just a gut feel guess. If you are designing something really important or critical, I guess you would have to experiment to find the actual number)