LiPo battery learning cycle - charger and discharger - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: March 08, 2021, 09:50:03 am
content: Hi everyone,I am designing a prototype with an integrated LiPo battery. To monitor the battery I have included the BQ27421https://www.ti.com/lit/ds/symlink/bq27421-g1.pdf?ts=1615187477480&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ27421-G1battery fuel gauge from TI. I need to perform a learning cycle in order to obtain the real capacity (Qmax) and the impedance table (Ra) of the battery and provide this information to the battery fuel gauge for accurate SOC.According to this application reporthttps://www.ti.com/lit/an/slua903/slua903.pdf, TI recommends the following learning cycle procedure:1. Discharge Battery to Empty2. Relax for at least 5 Hours3. Charge Battery to Full (C/2 charge rate is recommended)4. Relax for at least 2 Hours5. Discharge Battery to Empty (C/5 discharge rate is recommended)6. Relax for at least 5 HoursSo, to perform the learning cycle, I need a constant current battery discharge circuit and a battery charger. Since my battery is a LiPo 80mAh, the charger should be capable to provide 40mA charging current (C/2). I found the following regarding charging and discharging LiPo batteries:Constant current load discharger circuithttps://www.instructables.com/DIY-Constant-current-load/I have to build it my own...Charger with variable charge currenthttps://learn.sparkfun.com/tutorials/lipo-usb-charger-hookup-guide/allDo you have to suggest any equipment that integrates both functionalities (constant discharge current and variable charge current)?Thanks in advanceNick

author: jiarui
author_rank: Contributor
timestamp: March 08, 2021, 03:53:33 pm
content: I have a universal RC battery charger/discharger. It will charge and discharge Lipo batteries. But the one I have has a lowest charging current at 0.1A. It won't work in your case. If you only need to do this once, you should be able to charge the battery with a bench power supply. Set the voltage limit to 4.2v and set the current limit to 40mA. This will charge the battery at 40ma, until it reaches close to 4.2V. At that point, the current will be reduced to less than 40ma until it reaches 4.2V.If you have an electronic load that has a battery mode, you can use it to discharge your lipo battery, down to a specific voltage.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 08, 2021, 04:17:50 pm
content: You apparently won't need exact discharge current; the appnote specifies C/10 must be exceeded, and C/5 is recommended.So I'd use a simple resistor as the load. Select the R so that current clearly exceeds C/10 at the lowest possible (i.e. low-voltage cutoff) voltage.Discharging current changes as a function of voltage but that shouldn't matter.Then you can use whatever Arduino to switch on the charger or load.

author: jiarui
author_rank: Contributor
timestamp: March 08, 2021, 04:22:55 pm
content: You probably don't want to just use a resistor with an unprotected lipo cell. You need to stop the discharge at some point. You can probably just put a zener diode in series that will disconnect when the voltage is lower than the zener voltage. Choose a zener diode at the right voltage and current.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 08, 2021, 06:35:34 pm
content: Obviously you need to control it.A zener won't turn the charger on.

author: Nikos A.
author_rank: Regular Contributor
timestamp: March 10, 2021, 08:43:05 am
content: Indeed a simple resistor looks like the simplest solution. Also my LiPo batteries have build in battery protection circuit that cuts-off at 2.4V. However, since this is quite low level I have included a second battery protection IC into my prototype to cut-off at 3V.However, I will design the constant current load discharger as well, since this is a usefull asset to measure easy the real capacity of the battery.