LiPo battery drains down to 0V  - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: February 14, 2021, 05:45:24 pm
content: 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 TIhttps://www.ti.com/lit/ds/symlink/bq2970.pdf?ts=1613323147424&ref_url=https%253A%252F%252Fwww.google.com%252FAccording 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 batterywas 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 2.8VThanks in advanceNick

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 14, 2021, 07:41:16 pm
content: Check that the battery doesn't already have a protection circuit.

author: Nikos A.
author_rank: Regular Contributor
timestamp: February 15, 2021, 07:19:32 am
content: Thank you for your answer @NiHaoMikeIndeed, 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?

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2021, 07:30:10 am
content: 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.

author: Nikos A.
author_rank: Regular Contributor
timestamp: February 15, 2021, 09:07:32 am
content: Hi @beanflying and thank you for your answerQuote from: beanflying on February 15, 2021, 07:30:10 amThe battery inbuilt protection circuit is equipped with the DW01 IC that has Overdischarge detection voltage = 2.4VQuote from: beanflying on February 15, 2021, 07:30:10 amYes, according to the datasheet when the battery voltage is below the Voinh = 0.7V the charging process is inhibitedOk, 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

author: beanflying
author_rank: Super Contributor
timestamp: February 15, 2021, 09:16:31 am
content: 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.

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 15, 2021, 09:21:51 am
content: Put in a new cell and measure the current. No point in discussing untill you've measured there's indeed a current drain somewhere

author: Nikos A.
author_rank: Regular Contributor
timestamp: February 15, 2021, 09:42:29 am
content: 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