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I have to ask about protection circuit module, like BQ297xx, AP9211, AP9101 etc.

As I see, protection circuit module is used to protect li-ion cell from deep discharge - so if the li-ion cell has, for example, 2.5 V already, the protection circuit module stops discharging. But it is a deep discharge.

But now, for example, if our li-ion powered device is a device with STM32 powered from 3.3 V, this microcontroller should be turned off (shutted down) earlier - I mean, it should be done by software on STM32 to shut it down safely - and then, if li-ion is somehow discharged to 2.5, the protection circuit module is the "the ultimate disconnector" of a power for our microcontroller.

But I like solutions made with the principle "less code, more hardware". So my question is: If we choose, for example, AP9101 with cut-off voltage at about 3.2 V (for example), can we make a "hardware turn-off system" to cut-off the whole circuit with the microcontroller? Is it possible to do it so that you don't have to handle stm32 shutdown with lines of code, just do it purely hardware, but safe? Would this somehow affect the pcm, which from now on would not only be used for deep discharge but for regular discharge?

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  • \$\begingroup\$ In my opinion, your idea is just useless. \$\endgroup\$ Commented Aug 20, 2021 at 22:04
  • \$\begingroup\$ Could you explain that? \$\endgroup\$ Commented Aug 20, 2021 at 22:05
  • \$\begingroup\$ A shutdown implies of storing some data in EEPROM, so it needs the same amount of code. If this kind of shutdown isn't needed, then it makes no sense to bother of it. \$\endgroup\$ Commented Aug 20, 2021 at 22:14
  • \$\begingroup\$ Ok, but until now, it seemed to me that devices for lithium-ion cells require some protection against discharge (unless we have a cell that has its own built-in circuit protection). Does this mean that implementing shut-down system only makes sense only with code? \$\endgroup\$ Commented Aug 20, 2021 at 22:27

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You are right to be concerned. Modern Li-ion cells are typically almost 100% discharged by the time the voltage reaches 2.5 V. This leaves so little capacity left that the cell may be destroyed by a few days in cutoff, even though the PCM is only drawing a few μA. Equipment that expects 3.3 V probably can't run properly at 2.5 V anyway, so there is no point letting it go this low.

If we choose, for example, AP9101 with cut-off voltage at about 3.2 V (for example), can we make a "hardware turn-off system" to cut-off the whole circuit with the microcontroller?

Yes. The simplest way is to just substitute the existing PCM IC with a 3.2 V version of the AP9101 (eg. AP9101CAK6-ANTRG1) or equivalent device. However the MCU and other parts of the circuit will have to handle having the power disconnected without warning.

If the MCU is writing to nonvolatile storage (eg. EEPROM, SD Card) when power is removed then the data will probably be corrupted. If it is controlling some other device then it could be left in an abnormal state, and the user might not appreciate it going 'dead' without warning. In these cases the MCU should monitor the supply voltage and do an orderly shutdown before it drops below 3.2 V.

Modern Li-ion/Lipoly cells usually have less than 20% charge remaining when they reach 3.7 V at low current. For good battery lifespan you don't want to go below 20% anyway, so cutting off below 3.7 V is a good idea. If the MCU has an ADC it can probably be used to read the power supply voltage. If not then an internal comparator with voltage reference or external supervisor IC might do the job.

Would this somehow affect the pcm, which from now on would not only be used for deep discharge but for regular discharge?

If you make the PCM cut off at 3.2 V then this doesn't apply (it just switches at the 'regular' rather than 'deep' cutoff voltage). If the battery has a builtin in PCM that cannot be modified to do the job then you have two cutoff circuits in series which will drop a little more voltage, but apart from that they should work normally.

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From your question I understood that you already have a cell with a hardware circuit protection and you want to make yet another hardware protection. Of course the system has to detect a low voltage and store data (this is a shutdown) with a code. If you have nothing to store, then you could simply adjust the low voltage threshold on hardware protection.

However your question is missing some details:

  • Will the device also charge the cell? If so, you could use an IC that does both charge and protect.
  • what chemistry?

If you going to use a removable battery, then you charger shall provide overvoltage/SC protection. In that case you could simply have a device with a LDO to provide stable voltage to MCU. You do monitor the voltage with an ADC, then you do your shutdown sequence and you cut off the power with a GPIO and MOSFET transistor. A push button can be used to power on the device when you change the battery.

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