I am trying to understand the charging and discharging procedure of a lithium ion cell. Eventually I would like to be able to build my own power bank, I understand that there are existing power bank chips to do this job but I would like to build this as a project.

My current understanding:

  • Power source feeds power to a charger circuit such as the TP4056 module which charges a lithium cell, say an 18650.
  • At some point after the charging phase, a DC booster steps up the voltage to a desired voltage, which is then connected to the load.

What I fail to understand:

  • What electronic circuit controls the discharging, does such a circuit exist? (discharge management)?

  • Is the load directly connected to the cell while its charging, is that safe?

  • Is the load consuming power from the cell while its charging, is that safe?

  • How do circuits that support simultaneous charge and discharge operate such as the mhcd42 module?

Any assistance is highly appreciated, could anyone please point me to a source of reading material related to designing battery chargers/power banks. I am not sure where to search for chips and modules that exist for such a task, also is there a repository with all available chips?

Diagram Of Charger I'm trying to design/build


1 Answer 1


The boost circuit is likely just made to shut down if battery cell voltage goes too low. It will likely have a safe overcurrent limit too.

Load usually cannot be connected to cell while charging. At least the simple TP4056 cannot work properly if it charges a battery and the battery has a load. It will be impossible to determine when the battery is fully charged and this usually leads to overcharging the battery which degrades and damages the battery.

And current can only go either in or out of the battery, so either the battery charges or discharges. So it is impossible that a battery simultaneously is charged and discharged. So thinking that a battery can be charged by feeding in current to battery while a load is drawing current from battery is incorrect.

There is no proper datasheet available for a MH-CD42 chip but it's likely very similar to other chips - if you have external supply coming in, it passes to output and charges the battery, otherwise without input the battery is boosted to output, there is really no other way of doing it.

  • \$\begingroup\$ Thank you for taking the time! The impossibility of simultaneously charging and discharging a battery is the dead end I reached during my research. I too wasn't able to find the datasheet of the MH-CD42 and I suspect its a clone but I found this blog post that attempts to trace its origin to a IP5306 chip :techobsessed.net/tag/mh-cd42 IP5306 datasheet: injoinic.com/wwwroot/uploads/files/20200221/… To allow switch over between the charging source and the rest of the circuit how would I go about it ? Mosfets perhaps? \$\endgroup\$
    – ZenerX
    Apr 30, 2023 at 9:37
  • 1
    \$\begingroup\$ Either design the circuit yourself, or use a chip that already handles it for you. There is no reason to stick to TP4056 if it does not do everything you want to achieve. \$\endgroup\$
    – Justme
    Apr 30, 2023 at 10:03
  • \$\begingroup\$ I am not familiar with all the existing chips/modules that perform this task simultaneously if you know where I should begin my search please let me know. My thinking was that since the TP4056 does the job of charging the cell well, I would keep it and just build on it by connecting the cell with some sort of transistor switch circuit that disables charging while the circuit is in use, any pointers on how I could achieve this? I re-edited the original post to include a diagram of what I'm trying to build. \$\endgroup\$
    – ZenerX
    Apr 30, 2023 at 11:06

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.