Can I charge Li-Ion safely by connecting PV Panels to a CC CV Buck Converter then through BMS to a 3S3P batteries?

This is basically a plan for a very small solar powered device that takes in solar power (30 Wp), simultaneously charging batteries and powering the device for a few hours during the day (as long as buck's output voltage > battery voltage) and use the battery when the sun is out (battery voltage > buck's output voltage). The device/load (3W) is to be powered continuously.

Please let me know of any of your concerns, especially in regard to hazards and references.


Please let me know if you need more information that hasn't been shown here or of whatever I am doing wrong, thank you

  • \$\begingroup\$ Data sheet links to the modules are required. Links to pee-bay (et al) means that you are considering buying from a source that is not well documented or reliable. Think again is my advice. \$\endgroup\$
    – Andy aka
    Aug 11, 2020 at 10:13
  • \$\begingroup\$ I added the datasheet to the ICs, but havent found any for the modules, but i found someone testing them on youtube, will add them here too. Will oversizing help reliability for this case? \$\endgroup\$
    – yuda
    Aug 11, 2020 at 10:20
  • \$\begingroup\$ Nobody's going to look at youtube videos. Data sheets are what are needed: What to check for when buying an electronic component or module. \$\endgroup\$
    – Andy aka
    Aug 11, 2020 at 10:39
  • \$\begingroup\$ If there is no data on the modules, how can anyone say anything about their suitability for any purpose? \$\endgroup\$
    – Justme
    Aug 11, 2020 at 10:45
  • 1
    \$\begingroup\$ Main issue (I think) is that a BMS is NOT designed to be a charger. It prevents gross excess conditions and may do balancing BUT is not designed to do proper CCCV charhing and charge termination. (SOME BMSs may have proper charging functions as well but its rare). || A "proper" charger will do CC charge to usually 12.67V, then CV charge to say Imax/4 say THEN fully remove charge voltage. A BMS will usually leave Vmax on battery indefinitely. Battery WILL die. \$\endgroup\$
    – Russell McMahon
    Aug 19, 2020 at 7:58

1 Answer 1


It's probably safe because it won't likely work as the Buck charger will probably collapse the PV voltage to very inefficient low voltages.

A MPPT controller will hunt for the MPT by matching load impedance to the PV source for any solar condition. Max Sun each PV is Zmpt=Voc/Isc=17V/0.6A.

Also, this block diagram is rather simple and leaves a lot of specs out.

  • \$\begingroup\$ Thank you! so will it not work or will it work but inefficiently? What other information would you need me to put in it? \$\endgroup\$
    – yuda
    Aug 11, 2020 at 11:27
  • \$\begingroup\$ won't likely work unless it has a soft start with MPT regulation. CC mode from a PV? impossible without impedance matching and a current source is high impedance. \$\endgroup\$ Aug 11, 2020 at 11:31
  • \$\begingroup\$ What does soft start do? In this case, I'm using CC to cap current flows, even without CC the max current the PV can possibly put is 17/12.7*0.6A= 0.8A x3= 2.4A, assuming 100% efficiency, I guess that wouldn't hurt the 3P battery. Is impedance matching mandatory for charging or will it increase efficiency? \$\endgroup\$
    – yuda
    Aug 11, 2020 at 11:57
  • \$\begingroup\$ @yuda There are two requirements: MPT (maximum power transfer) regulation and soft start. The MPT regulation is a primary requirement, once you got that taken care of, soft start should be easy. In a nutshell: buck converter is OK but it won't work by itself: it needs to be in an MPT control loop. You also seem to think like getting better efficiency is just a bonus. Not at all: in this application it's critical, since a buck converter by itself will tend to run the panels at 1-5% efficiency. And since panels are expensive, getting maximum power transfer is essential, else you're wasting $$$ \$\endgroup\$ Jan 25, 2021 at 14:59
  • \$\begingroup\$ I mentioned Match Impedance, and when done from the startup means "soft start" is guaranteed but only if sufficient power to regulate V. I didn't show how, just that \$\endgroup\$ Jan 25, 2021 at 15:09

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