LiPo battery drains down to 0V

[Nikos A.]

Hi everyone,

I designed a PCB that operates with a LiPo battery and charge with wireless charging. At the very beginning of the circuit I have placed the bq29707 battery protector from TI

https://www.ti.com/lit/ds/symlink/bq2970.pdf?ts=1613323147424&ref_url=https%253A%252F%252Fwww.google.com%252F

According to the datasheet the undervoltage protection level for bq29707 model is 2.8V.



This is the sub-circuit with the battery protector. BT2 is the LiPo battery. Please ignore BT1 (this is for a coin cell battery option)



The circuit works normally and the cutoff level works as expected.. When I finished with the initial test I left the prototype laying around for around a month with the battery connected to it (this is going to be a device with integrated non-removable battery). After this period I decided to make some additional tests and I realized that the battery was drained down to 0V  . As a consequence, the wireless charging cannot charge the battery and I have to replace it. I know that batteries have some self-discharge but I am sure that is not the reason behind then drain to the button effect.

Any suggestions? I need to maintain the battery level at least to 2.8V

Thanks in advance
Nick

[NiHaoMike]

Check that the battery doesn't already have a protection circuit.

[Nikos A.]

Thank you for your answer @NiHaoMike

Indeed, the batteries have a build in protection circuit that includes a DW01 battery protector and a AM8205 dual N-Channel MOSFET. However, how this is related?

[beanflying]

If for example the inbuilt LiPo protection is 3V then it will go open circuit ( so 0V) before your 2.8V even triggers your IC to start charging. Having a quick read of the datasheet it has a zero or low voltage lockout to prevent charging commencing on batteries below a certain voltage. So look at that but better yet bypass the inbuilt Battery Protection and charge on the battery side of it to keep it out of circuit during the charge.

[Nikos A.]

Hi @beanflying and thank you for your answer

If for example the inbuilt LiPo protection is 3V then it will go open circuit ( so 0V) before your 2.8V even triggers your IC to start charging.

The battery inbuilt protection circuit is equipped with the DW01 IC that has Overdischarge detection voltage = 2.4V

Having a quick read of the datasheet it has a zero or low voltage lockout to prevent charging commencing on batteries below a certain voltage.

Yes, according to the datasheet when the battery voltage is below the Voinh = 0.7V the charging process is inhibited


Ok, so I believe that is the reason that my circuit won't charge a 0V depleted battery.

However, I still cannot figure out why my 80mAh LiPo batteries were drained down to 0V.. That's not make sense.. The bq29707 battery protector cuts off at 2.8V and after the cut off event the current consumption is reduced down to Istandby = 0.5uA

[beanflying]

Obvious maybe but are you measuring the 'cell' voltage or the output of the 'BMS'? The output of the BMS should be zero if the cell voltage goes below X.

My little indoor R/C LiPo's (20-80mAh) generally don't hold charge that well even without BMS boards fitted. For R/C use we never have them fitted but generally have low voltage cutouts on the speed controllers set at circa 3V/cell. This kicks out the motor (main current draw) and the cells then bump up to allow a bit of time to land while powering the gear.

Depending on maybe having counterfeit DW01's and a bit of stray leakage your BMS boards could also be the problem.

[Ice-Tea]

Put in a new cell and measure the current. No point in discussing untill you've measured there's indeed a current drain somewhere

[Nikos A.]

Thanks for your inputs, I will proceed with the following measurements and will return with feedback:

1. Measure the battery's protector output voltage after its cut-off threshold (2.8V). I expect close to 0V measurement.
2. Measure any leakage drain after the cut-off