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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?

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For applications like this better choice is charger chip with power path, something like BQ2403x, BQ2407x etc.

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.

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  • \$\begingroup\$ Ooh, I wasn't aware those types of chips existed, the datasheet makes them sound amazing! Do you know if they make 2s or 3s variants? Also, do you know if the chip has low battery voltage protection, or does that need to be designed separately? \$\endgroup\$ – Blaine Jan 19 at 7:53
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    \$\begingroup\$ I never looked for more than one cell chargers, so I can't say off-hand if those variants exist. But if you look through that app note you can see that power path can be built using discrete components external to the charger. These devices do not control discharge, usually. Low battery is only of interest to them as a special case where charging profile should be adjusted. \$\endgroup\$ – Maple Jan 19 at 8:12
  • \$\begingroup\$ Alright, well apparently the bq24133 chip does the same thing but for 3 cells. Only problem is that it's freaking complicated, so I will be looking for a different solution... Good to know those things exist though! \$\endgroup\$ – Blaine Jan 19 at 20:23
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    \$\begingroup\$ Looking at the charger on your photo I'd say bq24133 with all the external parts is about the same, complexity-wise. I guess your problem is not in chip complexity but in lack of cheap off-shelf module based on it. Although, if not going for cheap, the TI does sell evaluation module and charger module \$\endgroup\$ – Maple Jan 19 at 22:06
  • \$\begingroup\$ Thanks man, you're the best! I'm kinda surprised there aren't more modules that implement that or similar chips. Surely this is a very common thing to need for mini-projects \$\endgroup\$ – Blaine Jan 21 at 22:35
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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.

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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)

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