Li-Po battery - discharging

[bipolunipol]

Hello!

I decided to use a Li-Po battery in my project. I have read a bit about the principles of designing devices with these batteries. These batteries should not be overcharged above 4.2V as they may be damaged. But that's what the charger chip takes care of - I decided to use the MCP73831T. The discharging problem looks worse. My device will use a microcontroller. At the beginning I wanted it to turn itself off when the battery voltage drops below 3V. But there are other components in the device, so I decided to use the PCM chip AP9101CK. But I have doubts if this schematic is OK (the schematic is attached in the attachment). If the battery has its own PCM, will it not interfere with the AP9101CK? Is this combination of IC correct?

I would like to use this type of battery: https://aliexpress.com/item/4001345104154.html

[sleemanj]

Your schematic is not correct.  Pay careful attention to where VSS (of AP9) goes.

[bipolunipol]

Ah, of course. And now?

[daisizhou]

Why not use TI’s BQ series chips, BQ series can use the BMS system to manage lithium batteries

[bipolunipol]

Why not use TI’s BQ series chips, BQ series can use the BMS system to manage lithium batteries

How will such a system be better than the proposed one?

[tooki]

Why not use TI’s BQ series chips, BQ series can use the BMS system to manage lithium batteries

This comment is meaningless: the bq series spans from simple chargers comparable to the MCP73831T, all the way to ultra-complex ones with full-fledged USB-C support and encrypted communication with in-pack BMS ICs. The bq line also encompasses the ancillary chips like fuel gauges, protection ICs (I dislike the term “PCM” because that abbreviation is already squarely spoken for by pulse-code modulation), and power switches.

And the OP’s battery pack doesn’t contain any kind of onboard chips with communication.

Why not use TI’s BQ series chips, BQ series can use the BMS system to manage lithium batteries

How will such a system be better than the proposed one?

Impossible to say, since they didn’t specify what they meant.

Anyhow, I believe your revised schematic should work.

Redundant protection works fine; in my current major project, we used protected cells during development just in case, but once the charging circuit (with its own protection) had proven itself, we used unprotected cells as envisioned.

P.S. The main purpose of the protection is against overvoltage (what if the charger malfunctions? Or if you accidentally miswire it? Or if a bit of wire shorts the DC in directly to it?), and overcurrent, especially short circuits. Undervoltage is a secondary function in a way.

[Peabody]

Will you be charging the battery and powering your project at the same time?  If so, then the load current will flow through the charger.  I believe the MCP73831 terminates charging when the "charging" current falls below 10% of the maximum charge current.  Will your load draw more than that?  If so, then you may need to add a load sharing circuit, or possibly use the MCP73871 instead, to prevent continuing to charge a fully-charged lithium battery.  An alternative is to periodically put your processor to sleep so it draws very little current.  Then charge termination could take place properly.

http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf

[bipolunipol]

Will you be charging the battery and powering your project at the same time?  If so, then the load current will flow through the charger.  I believe the MCP73831 terminates charging when the "charging" current falls below 10% of the maximum charge current.  Will your load draw more than that?  If so, then you may need to add a load sharing circuit, or possibly use the MCP73871 instead, to prevent continuing to charge a fully-charged lithium battery.  An alternative is to periodically put your processor to sleep so it draws very little current.  Then charge termination could take place properly.

http://ww1.microchip.com/downloads/en/AppNotes/01149c.pdf

No, charging and powering at the same time is not necessary, but the device will consume approximately 10mA and the charging current will be 200mA. Only that the charging current probably drops towards the end so there may be a problem with the completion of the charging (if the device is accidentally turned on). What if I will connect the STAT pin to the input of the microcontroller? If the status is LOW, it means charging is in progress and it should be asleep. If the status is HIGH, then charging is complete. Microcontroller can detect it and will respond appropriately.

[Peabody]

If the full charging current is 200mA, then the termination current will be 20mA.  And if your load is only 10mA, you should be ok even if charging and powering at the same time.  Of course you could monitor the STAT ouput, but you would need to enable the internal pullup resistor on that processor pin.  But if you're really only drawing 10mA, I think your circuit should work fine.  No need to make it more complicated unless you really need to.

On the subject of having two protection circuits, I think that typically works fine, with no interference.  But you will have two protection chip operating, each drawing a few microamps, so that power will be used up with no benefit.  I think almost all the lipo batteries you will find of the type you linked to will have protection built in.  18650s come protected or non-protected, but I think those LIPO flat packs are almost always protected.  I've never seen one that wasn't.  So you might want to consider eliminating your protection chip and the dual mosfet chip, and just always use protected batteries.